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What do we know about the locations and directions of potential Gamma ray bursts that may intersect with Earth in the future, or are we mostly blind to when such an event directed our way may occur & cause a mass extinction event?
In other words, are we able to predict to any degree at all when one might occur close enough that there may be some potential impact to life or life on Earth or infrastructure?
Although not being an expert on the field, I found a multitude of studies dealing with events which may be used as indicator that a GBR is about to happen somewhere. Research on GBR precusor events range from smaller gamma ray precursors, over gravitiational waves, to thermal signatures, and neutrinos:
- Sarp Akcay: Forecasting Gamma-Ray Bursts using Gravitational Waves (2018)
- Thomas Koshut et. al: Gamma ray precursor activity as observed with BATSE (1995)
- L. Piro et al.: Probing the Environment in Gamma-Ray Bursts: The Case of an X-Ray Precursor,… (2005)
- E. Troja, S. Rosswog, and N. Gehrels: Precursors of short gamma-ray bursts (2010)
- Frédéric Daigne, Robert Mochkovitch: The expected thermal precursors of gamma-ray bursts in the internal shock model (2002)
- Coppin, Nick van Eijndhoven for the IceCube Collaboration: IceCube search for high-energy neutrinos produced in the precursor stages of gamma-ray bursts (2019)
Gamma-ray astronomy — Future prospects
With the advent of Sigma/Granat and of the Compton Gamma Ray Observatory, the gamma-ray band has at last been opened up for astronomy. It is starting to be possible to realize some of the potential that observations in this band have for contributing to our understanding of high energy astrophysics. What, though, of prospects beyond the current generation of gamma-ray instruments?
It is ironical that in the part of the spectrum where the diffraction limit is in the micro-arcsecond range, angular resolutions available are still comparable with the angular diameter of the moon. Similarly, none of the current generation of space instruments has energy resolution better than about E ΔE ∼ 10 . Yet this is a regime where resolutions of ∼ 1000 are easily achieved in the laboratory and where the theoretical limits are many orders of magnitude better than this.
A major problem is sensitivity. The large detector masses necessary to stop high energy gamma-rays attract both high backgrounds from particle interactions and worried looks from spacecraft engineers. Ingenuity, large lift-off masses and long observations are all needed. The prospects for INTEGRAL and beyond are discussed in the context of developments over the last 20 years and the present status.
Is the universe a graveyard? This theory suggests humanity may be alone.
Imagine the birth of an entirely new ocean on the Martian surface.
There are lots of arguments for exploring space and colonizing other planets. Exploration is a natural part of our species. The knowledge we gain is bound to propel our scientific understanding and capabilities. And admittedly, there are plenty of commercial reasons too. Plus, sooner or later, the Earth is going to die out. To survive, we’ll have to become an interplanetary species.
Due to ours being a richer world today, and advances in rocketry and other technologies, a 21 st century space race is just starting to heat up. This time, it isn’t just the US and Russia competing, but India, China, the EU, and private organizations such as SpaceX and Mars One. They all want to build the first permanent colony on the Red Planet. Mars One has the swiftest timeline, placing people on the surface by 2025. NASA has a far more cautious plan, establishing a permanent colony by 2040. But there are lots of stumbling blocks to overcome.
From the surface, Mars looks like a cold and forbidding wasteland, devoid of a breathable atmosphere, running water, and virtually uninhabitable, without spacesuits and airtight shelters. It’s worse than that, however. The planet is being constantly bombarded by solar radiation. Consistent exposure is likely to cause deadly cancers and early onset Alzheimer’s among colonists. How quickly or slowly these develop however, is anyone’s guess. It depends upon shielding and lots of other factors.
Astronauts working on the international space station (ISS) encounter the same amount of radiation as workers at a nuclear power plant. But those astronauts are only up there for a limited time. The longest mission to date is 215 days. What happens if you are constantly exposed for the rest of your life? There could also be serious consequences in terms of fertility. Radiation exposure can cause mutations in the genetic code, birth defects, and even infertility. How could a colony survive?
Artist rendition of Mars being buffeted by solar radiation. By: NASA/Jim Green.
Despite terrific obstacles, the planet has potential. All the things that are needed to terraform the planet are there, minus a strong magnetic field. There is water for instance, frozen at the poles and within the soil. It once had an atmosphere, free flowing water, an ocean, and perhaps even life.
Many colonization plans suggest terraforming the planet, which is expected to take hundreds of years. Some include releasing greenhouse gasses into the atmosphere from factories, or as Elon Musk has proposed, using nuclear weapons at the poles to melt the ice caps. But with this new plan, nature actually does all the work itself, without the dangers inherent in those other options.
At a recent NASA workshop, held at its headquarters in Washington, D.C., Planetary Science Division director Jim Green, proposed a captivating alternative—encapsulate the planet in an “artificial magnetosphere.” The Planetary Science Vision 2050 Workshop is an unveiling of proposals, which could occur or at least begin, by midcentury.
Dr. Green’s presentation was entitled, "A Future Mars Environment for Science and Exploration." Green and a panel of colleagues proposed an artificial "magnetic shield" provided by a device, dubbed Mars L1. This would remain in steady orbit between the planet and the sun, shielding it from solar bombardment.
The basic idea is having an object create a large electric circuit or dipole, generating enough energy to cover the planet in an artificial magnetic field. This would be composed of two oppositely charged magnets connected to inflatable structures, placed in orbit somewhere between Mars and the sun. One important aspect according to Dr. Green, "We need to be able then to also modify that direction of the magnetic field so that it always pushes the solar wind away.”
Building an artificial magnetosphere around Mars. By: NASA/Jim Green.
Though it sounds, what the presenter called “fanciful,” experiments creating miniature magnetospheres are already ongoing. These are in hopes of devising a way to protect astronauts aboard the ISS as well as manned spacecraft. Green wants to scale up such a system to cover a whole planet. "It may be feasible that we can get up to these higher field strengths that are necessary to provide that shielding," he said.
Once stable, the “magnetotail” is expected to allow a revival of the atmosphere. Half the atmospheric pressure of our own planet could occur within just a few years. 4.2 billion years ago, something caused the Red Planet’s magnetic field to severely weaken. Since that time, highly charged solar particles have slowly stripped it of its atmosphere, causing Mars to go from a warm, wet planet, to a dry, cold one. Today, the atmosphere is 100 times thinner than ours.
Shielding from such particles would warm the surface
7 °F (4 °C). This would then melt the CO² at the poles, helping to build up the atmosphere. By creating a greenhouse effect, the ice on the planet’s surface should melt. "Perhaps one-seventh of the ancient ocean could return to Mars," Dr. Green said. At its current rate, this would take 700 million years.
Though the plan is entirely theoretical, if it worked, the planet could actually be livable in about a century or so, NASA scientists claim. That’s just a few generations. It’s vital to colonization too, as any sustainable colony will sooner or later have to start growing its own food. The distance from Earth to Mars is just too great. If it works, it could add an important tool to terraforming and help us colonize other places. “The solar system is ours, let’s take it,” Green said.
To learn more about Terraforming Mars, click here:
The present status of gamma-ray burst research is reviewed, with an emphasis on recent observations of their temporal, spectral, and global distribution properties. The observed sky distribution of weak gamma-ray bursts constrains the allowable geometrical models to sources in either a giant spherical galactic halo or to sources at cosmological distances. Observations of time dilation consistent with the latter have been reported. Extensive searches for a counterpart to gamma-ray bursts in other wavelength regions have thus far proved negative. In spite of the abundance of new observations of gamma-ray bursts, their energy source and emission mechanism remain highly speculative. New, rapid counterpart search efforts and several new space-borne experiments may provide the needed observations to make progress in the field
Annual Review of Astronomy and Astrophysics &ndash Annual Reviews
Published: Sep 1, 1995
Keywords: gamma-ray sources x-ray sources neutron stars cosmology
Astronomy: The Next Generation
In some respects, the field of astronomy has been a rapidly changing one. New advances in technology have allowed for exploration of new spectral regimes, new methods of image acquisition, new methods of simulation, and more. But in other respects, we’re still doing the same thing we were 100 years ago. We take images, look to see how they’ve changed. We break light into its different colors, looking for emission and absorption. The fact that we can do it faster and to further distances has revolutionized our understanding, but not the basal methodology.
But recently, the field has begun to change. The days of the lone astronomer at the eyepiece are already gone. Data is being taken faster than it can be processed, stored in easily accessible ways, and massive international teams of astronomers work together. At the recent International Astronomers Meeting in Rio de Janeiro, astronomer Ray Norris of Australia’s Commonwealth Scientific and Industrial Research Organization (CSIRO) discussed these changes, how far they can go, what we might learn, and what we might lose.
One of the ways astronomers have long changed the field is by collecting more light, allowing them to peer deeper into space. This has required telescopes with greater light gathering power and subsequently, larger diameters. These larger telescopes also offer the benefit of improved resolution so the benefits are clear. As such, telescopes in the planning stages have names indicative of immense sizes. The ESO’s “Over Whelmingly Large Telescope” (OWL), the “Extremely Large Array” (ELA), and “Square Kilometer Array” (SKA) are all massive telescopes costing billions of dollars and involving resources from numerous nations.
But as sizes soar, so too does the cost. Already, observatories are straining budgets, especially in the wake of a global recession. Norris states, “To build even bigger telescopes in twenty years time will cost a significant fraction of a nation’s wealth, and it is unlikely that any nation, or group of nations, will set a sufficiently high priority on astronomy to fund such an instrument. So astronomy may be reaching the maximum size of telescope that can reasonably be built.”
Thus, instead of the fixation on light gathering power and resolution, Norris suggests that astronomers will need to explore new areas of potential discovery. Historically, major discoveries have been made in this manner. The discovery of Gamma-Ray Bursts occurred when our observational regime was expanded into the high energy range. However, the spectral range is pretty well covered currently, but other domains still have a large potential for exploration. For instance, as CCDs were developed, the exposure time for images were shortened and new classes of variable stars were discovered. Even shorter duration exposures have created the field of asteroseismology. With advances in detector technology, this lower boundary could be pushed even further. On the other end, the stockpiling of images over long times can allow astronomers to explore the history of single objects in greater detail than ever before.
In recent years, many of these changes have been pushed forward by large survey programs like the 2 Micron All Sky Survey (2MASS) and the All Sky Automated Survey (ASAS) (just to name two of the numerous large scale surveys). With these large stores of pre-collected data, astronomers are able to access astronomical data in a new way. Instead of proposing telescope time and then hoping their project is approved, astronomers are having increased and unfettered access to data. Norris proposes that, should this trend continue, the next generation of astronomers may do vast amounts of work without even directly visiting an observatory or planning an observing run. Instead, data will be culled from sources like the Virtual Observatory.
Of course, there will still be a need for deeper and more specialized data. In this respect, physical observatories will still see use. Already, much of the data taken from even targeted observing runs is making it into the astronomical public domain. While the teams that design projects still get first pass on data, many observatories release the data for free use after an allotted time. In many cases, this has led to another team picking up the data and discovering something the original team had missed. As Norris puts it, “much astronomical discovery occurs after the data are released to other groups, who are able to add value to the data by combining it with data, models, or ideas which may not have been accessible to the instrument designers.”
As such, Nelson recommends encouraging astronomers to contribute data to this way. Often a research career is built on numbers of publications. However, this runs the risk of punishing those that spend large amounts of time on a single project which only produces a small amount of publication. Instead, Nelson suggests a system by which astronomers would also earn recognition by the amount of data they’ve helped release into the community as this also increases the collective knowledge.
Since there is a clear trend towards automated data taking, it is quite natural that much of the initial data processing can be as well. Before images are suitable for astronomical research, the images must be cleaned for noise and calibrated. Many techniques require further processing that is often tedious. I myself have experienced this as much of a ten week summer internship I attended, involved the repetitive task of fitting profiles to the point-spread function of stars for dozens of images, and then manually rejecting stars that were flawed in some way (such as being too near the edge of the frame and partially chopped off).
While this is often a valuable experience that teaches budding astronomers the reasoning behind processes, it can certainly be expedited by automated routines. Indeed, many techniques astronomers use for these tasks are ones they learned early in their careers and may well be out of date. As such, automated processing routines could be programmed to employ the current best practices to allow for the best possible data.
But this method is not without its own perils. In such an instance, new discoveries may be passed up. Significantly unusual results may be interpreted by an algorithm as a flaw in the instrumentation or a gamma ray strike and rejected instead of identified as a novel event that warrants further consideration. Additionally, image processing techniques can still contain artifacts from the techniques themselves. Should astronomers not be at least somewhat familiar with the techniques and their pitfalls, they may interpret artificial results as a discovery.
With the vast increase in data being generated, astronomers will need new tools to explore it. Already, there has been efforts to tag data with appropriate identifiers with programs like Galaxy Zoo. Once such data is processed and sorted, astronomers will quickly be able to compare objects of interest at their computers whereas previously observing runs would be planned. As Norris explains, “The expertise that now goes into planning an observation will instead be devoted to planning a foray into the databases.” During my undergraduate coursework (ending 2008, so still recent), astronomy majors were only required to take a single course in computer programming. If Norris’ predictions are correct, the courses students like me took in observational techniques (which still contained some work involving film photography), will likely be replaced with more programming as well as database administration.
Once organized, astronomers will be able to quickly compare populations of objects on scales never before seen. Additionally, by easily accessing observations from multiple wavelength regimes they will be able to get a more comprehensive understanding of objects. Currently, astronomers tend to concentrate in one or two ranges of spectra. But with access to so much more data, this will force astronomers to diversify further or work collaboratively.
With all the potential for advancement, Norris concludes that we may be entering a new Golden Age of astronomy. Discoveries will come faster than ever since data is so readily available. He speculates that PhD candidates will be doing cutting edge research shortly after beginning their programs. I question why advanced undergraduates and informed laymen wouldn’t as well.
Yet for all the possibilities, the easy access to data will attract the crackpots too. Already, incompetent frauds swarm journals looking for quotes to mine. How much worse will it be when they can point to the source material and their bizarre analysis to justify their nonsense? To combat this, astronomers (as all scientists) will need to improve their public outreach programs and prepare the public for the discoveries to come.
The present status of gamma-ray burst research is reviewed, with an emphasis on recent observations of their temporal, spectral, and global distribution properties. The observed sky distribution of weak gamma-ray bursts constrains the allowable geometrical models to sources in either a giant spherical galactic halo or to sources at cosmological distances. Observations of time dilation consistent with the latter have been reported. Extensive searches for a counterpart to gamma-ray bursts in other wavelength regions have thus far proved negative. In spite of the abundance of new observations of gamma-ray bursts, their energy source and emission mechanism remain highly speculative. New, rapid counterpart search efforts and several new space-borne experiments may provide the needed observations to make progress in the field
Annual Review of Astronomy and Astrophysics &ndash Annual Reviews
Published: Sep 1, 1995
Keywords: gamma-ray sources x-ray sources neutron stars cosmology
The primary mission of our department is twofold: the performance of cutting-edge research in astronomy and astrophysics, including theory, observation, and experiment, as well as the education of undergraduate and graduate students and training of postdoctoral research associates who will comprise the scientists and leaders of tomorrow.
Caltech is one of the world's preeminent centers of astronomical research. This is due to the combination of excellent human resources with premier observational facilities and computational infrastructure. Fundamental discoveries in astronomy and astrophysics are a part of Caltech's past, present, and future.
Students from either the astronomy or the physics options are best prepared to undertake research with faculty in the Cahill Center for Astronomy and Astrophysics. Students from related options such as planetary science, computer science, applied physics, and electrical engineering are also welcome.
Astronomy and astrophysics are synonymous at Caltech. Caltech scientists and students are involved in many frontier areas of research, and have been known to open new ones. Research techniques include observations, theory, numerical simulation, advanced data analysis, laboratory astrophysics, and detector development. Projects and groups often bridge these areas of inquiry.
Topics of current research interest include: observational cosmology and the nature of dark matter and dark energy studies of the cosmic microwave background galaxy formation and evolution quasars and other active galactic nuclei and radio sources studies of the dynamics and com- position of galaxies and clusters physics and evolution of the intergalactic medium interstellar matter local star and planet formation extrasolar planetary systems the structure of the galaxy globular clusters stellar abundances supernovae, gamma-ray bursts, and other types of cosmic explosions and transient phenomena neutron stars and black holes accretion disks digital sky surveys and astroinformatics numerical general relativity gravitational wave astronomy and many others.
Research in planetary and solar system astronomy is often pursued in cooperation with groups in the Division of Geological and Planetary Sciences. New types of astronomical detectors and satellites, that can revolutionize various areas of astronomical research, are developed with groups in physics and colleagues at JPL.
In addition to maintaining a leading numerical general relativity group, Caltech theorists also use high-performance computing facilities for simulations of supernova explosions, merging black holes, cosmic structure formation, etc. Caltech is leading the development of novel tools for knowledge discovery in massive and complex astronomical data sets, many obtained with Caltech facilities.
History and Current Science at Observational Facilities
Observational astronomy is pursued both from the ground-based sites and from space-based platforms. Caltech operates, or has access to an unprecedented, comprehensive set of observational facilities, spanning the entire electromagnetic spectrum. Caltech is also playing a key role in opening a new window on the universe, the gravitational wave sky.
Historically, Caltech's pioneering role in astronomy started with Palomar Observatory (about 190 km from campus), funded by the Rockefeller Foundation. The first telescope on the mountain was an 18-inch Schmidt telescope built by Fritz Zwicky and used to conduct pioneering sky surveys for supernovae, potential planetary hazard asteroids, etc. The 200-inch Hale Telescope, constructed through the 1930s and 1940s, has been used to make many historical, fundamental discoveries ever since its commissioning in 1948, including the dis- covery of quasars, and many studies of stellar populations, galaxies, intergalactic medium, etc., and it continues to produce excellent sci- ence. Novel detectors and instruments were developed there, e.g., the first astronomical CCDs and infrared detectors as well as pioneering advances in adaptive optics in addition to optical and infrared spec- troscopy. The 48-inch Samuel Oschin Telescope has made possible complete surveys of the northern sky, initially with photographic plates (including the historic POSS-I and POSS-II surveys), and now with large-format CCD array cameras. It is currently operating a uniquely wide field, high-cadence program, the Zwicky Transient Facility (ZTF).
A much larger camera for this telescope, with a 47-square-degree field, started operation in 2018, as the Zwicky Transient Facility (ZTF). The 60-inch telescope has been roboticized, and is used to monitor sources discovered by sky surveys.
In the 1990s, funded mainly by the Keck Foundation, Caltech and University of California constructed two 10-m telescopes on Mauna Kea, Hawaii. The W. M. Keck Observatory produced many recent dis- coveries in the fields of galaxy formation and evolution, intergalactic medium, extrasolar planets, cosmic gamma-ray bursts, etc. Caltech is a founding partner in the development of the Thirty-Meter Telescope (TMT), the first of the next generation of extremely large optical/infrared telescopes.
At meter to centimeter wavelengths, Caltech operates the Owens Valley Radio Observatory (OVRO) in a radio-quiet location about 400 km from Pasadena, near Big Pine, California. Its facilities include a 40-meter telescope, a growing 288 element long wavelength array which can image the entire sky every second, and a 6.1 meter tele- scope dedicated to observations of polarized radio emission from the galaxy. New radio and submm telescopes are in design and con- struction phases. From the 1980s until 2015, Caltech also operated the Caltech 10-m Submillimeter Observatory (CSO) on Mauna Kea in Hawaii, and a series of millimeter interferometers, culminating in the 23-antenna Combined Array for Research in Millimeter-wave Astronomy (CARMA) in the Inyo Mountains. These telescopes, currently being repurposed to new experiments, pioneered submm imaging and inter- ferometry and mm wave interferometry, now carried out by the interna- tional Atacama Large Millimeter/submm Array (ALMA).
In Antarctica, Caltech's BICEP2 telescope, which measures the imprint of inflation's gravitational waves on the COSMIC microwave background, has been expanded and renamed the Keck Array.
On the space observations front, Caltech hosts NASA's Spitzer Science Center (SSC) and IPAC, which are principal national archives for astronomy. Caltech scientists lead or actively participate in a num- ber of astrophysics missions, currently including the Spitzer Space Telescope, and the NuSTAR hard X-ray mission. Caltech and the Jet Propulsion Laboratory (JPL) are also leading the development of the forthcoming SPHEREx mission, that will study the early stages of the universe, galaxy formation, and formation of planetary systems. There are numerous close connections with JPL, that designs and operates a number of NASA's scientific missions. Finally, Caltech astronomers are major users of NASA's astronomical satellites, the Hubble Space Telescope, Chandra, Fermi, Herschel, Planck, etc., ALMA and the NSF's Jansky Very Large Array (JVLA).
Caltech is the headquarters for LIGO lab, which built and operates the world's most sensitive gravitational wave observatory, the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO), which in 2015 made the historic first detection of gravitational waves from a black hole binary. Several other black hole mergers have been detected since then, as well as the first gravitational wave detection of a merger of two neutron starts in 2017. Numerous other discoveries are expected as the operations continue.
Gamma-Ray Bursts: A Radio Perspective
Gamma-ray bursts (GRBs) are extremely energetic events at cosmological distances. They provide unique laboratory to investigate fundamental physical processes under extreme conditions. Due to extreme luminosities, GRBs are detectable at very high redshifts and potential tracers of cosmic star formation rate at early epoch. While the launch of Swift and Fermi has increased our understanding of GRBs tremendously, many new questions have opened up. Radio observations of GRBs uniquely probe the energetics and environments of the explosion. However, currently only 30% of the bursts are detected in radio bands. Radio observations with upcoming sensitive telescopes will potentially increase the sample size significantly and allow one to follow the individual bursts for a much longer duration and be able to answer some of the important issues related to true calorimetry, reverse shock emission, and environments around the massive stars exploding as GRBs in the early Universe.
Gamma-ray bursts (GRBs) are nonrecurring bright flashes of
-rays lasting from seconds to minutes. As we currently understand, in the standard GRB model a compact central engine is responsible for accelerating and collimating the ultra-relativistic jet-like outflows. The isotropic energy release in prompt -rays ranges from
10 54 ergs see, for example, . While the prompt emission spectrum is mostly nonthermal, presence of thermal or quasithermal components has been suggested for a handful of bursts . Since the initial discovery of GRBs  till the discovery of GRB afterglows at X-ray, optical, and radio wavelengths three decades later [4–7], the origin of GRBs remained elusive. The afterglow emission confirmed that GRBs are cosmological in origin, ruling out multiple theories proposed favouring Galactic origin of GRBs see, for example, .
In the BATSE burst population, the durations of GRBs followed bimodal distribution, short GRBs with duration less than 2 s and long GRBs lasting for more than 2 s . Long GRBs are predominantly found in star forming regions of late type galaxies , whereas short bursts are seen in all kinds of galaxies . Based on these evidences, the current understanding is that the majority of long GRBs originate in the gravitational collapse of massive stars , whereas at least a fraction of short GRBs form as a result of the merger of compact object binaries (see Berger  for a detailed review).
GRBs are detectable at very high redshifts. The highest redshift GRB is GRB 090429B with a photometric redshift of
. However, the farthest known spectroscopically confirmed GRB is GRB 090423 at a redshift of
, indicating star formation must be taking place at such early epoch in the Universe . At the same time, some GRBs at lower redshifts have revealed association with type Ib/c broad lined supernovae, for example, GRB 980425 associated with SN 1998bw .
Since the launch of the Swift satellite in November 2004 , the field of GRB has undergone a major revolution. Burst Alert Telescope (BAT)  on-board Swift has been localizing
100 GRBs per year . X-ray Telescope (XRT ) and Ultraviolet/Optical Telescope (UVOT ) on-board Swift slew towards the BAT localized position within minutes and provide uninterrupted detailed light curve at these bands. Before the launch of the Swift, due to the lack of dedicated instruments at X-ray and optical bands the afterglow coverage was sparse, which is no longer the case. Swift-XRT has revealed that central engine is capable of injecting energy into the forward shock at late times [23–25].
GRBs are collimated events. An achromatic jet break seen in all frequencies is an undisputed signature of it. However, the jet breaks are seen only in a few Swift bursts, for example, GRB 090426 , GRB 130603B , and GRB 140903A . Many of the bursts have not shown jet breaks. It could be because Swift is largely detecting fainter bursts with an average redshift of >2, much larger than the detected by previous instruments . The faintness of the bursts makes it difficult to see jet breaks. Some of the GRBs have also revealed chromatic jet breaks, for example, GRB 070125 .
An additional issue is the narrow coverage of the Swift-BAT in 15–150 keV range. Due to the narrow bandpass, the uncertainties associated in energetics are much larger since one needs to extrapolate to 1–10,000 keV bandpass to estimate the
, which is a key parameter to evaluate the total released energy and other relations. Due to this constraint, it has been possible to catch only a fraction of traditional GRBs.
The Swift drawback was overcome by the launch of Fermi in 2008, providing observation over a broad energy range of over seven decades in energy coverage (8 keV–300 GeV). Large Area Telescope (LAT ) on-board Fermi is an imaging gamma-ray detector in 20 MeV–300 GeV range with a field of view of about 20% of the sky and Gamma-ray Burst Monitor (GBM)  on-board Fermi works in 150 keV–30 MeV and can detect GRBs across the whole of the sky. The highest energy photon detected from a GRB puts a stricter lower limit on the outflow Lorentz factor. Fermi has provided useful constraints on the initial Lorentz factor owing to its high energy coverage, for example, short GRB 090510 . This is because to avoid pair production, the GRB jet must be moving towards the observer with ultra-relativistic speeds. Some of the key observations by Fermi had been (i) the delayed onset of high energy emission for both long and short GRBs [33–35], (ii) long lasting LAT emission , (iii) very high Lorentz factors (
1000) inferred for the detection of LAT high energy photons , (iv) significant detection of multiple emission components such as thermal component in several bright bursts [37–39], and (v) power-law  or spectral cut-off at high energies , in addition to the traditional band function .
While the GRB field has advanced a lot after nearly 5 decades of extensive research since the first discovery, there are many open questions about prompt emission, content of the outflow, afterglow emission, microphysics involved, detectability of the afterglow emission, and so forth. Resolving them would enable us to understand GRBs in more detail and also use them to probe the early Universe as they are detectable at very high redshifts. With the recent discoveries of gravitational waves (GWs) [42, 43], a new era of Gravitational Wave Astronomy has opened. GWs are ideal to probe short GRBs as they are the most likely candidates of GW sources with earth based interferometers.
In this paper, we aim to understand the GRBs with a radio perspective. Here we focus on limited problems which can be answered with more sensitive and extensive radio observations and modeling. By no means, this review is exhaustive in nature. In Section 2, we review the radio afterglow in general and out current understanding. In Section 3, we discuss some of the open issues in GRB radio afterglows. Section 4 lists the conclusion.
2. Afterglow Physics: A Radio Perspective and Some Milestones
In the standard afterglow emission model, the relativistic ejecta interacting with the circumburst medium gives rise to a forward shock moving into the ambient circumburst medium and a reverse shock going back into the ejecta. The jet interaction with the circumburst medium gives rise to mainly synchrotron emission in X-ray, optical, and radio bands. The peak of the spectrum moves from high to low observing frequencies over time due to the deceleration of the forward shock  (e.g., see Figure 1). Because of the relativistic nature of the ejecta, the spectral peak is typically below optical frequencies when the first observations commence, resulting in declining light curves at optical and X-ray frequencies. However, optically rising light curve has been seen in a handful of bursts after the launch of the Swift , for example, GRB 060418 .
The first radio afterglow was detected from GRB 970508 . Since then the radio studies of GRB afterglows have increased our understanding of the afterglows significantly, for example, [47–49]. A major advantage of radio afterglow emission is that, due to slow evolution, it peaks in much later time and lasts longer, for months or even years (e.g., [50–52]). Thus unlike short-lived optical or X-ray afterglows, radio observations present the possibility of following the full evolution of the fireball emission from the very beginning till the nonrelativistic phase (see, e.g., [50–52]) also see GRB 030329 [53, 54]. Therefore, the radio regime plays an important role in understanding the full broadband spectrum. This constrains both the macrophysics of the jet, that is, the energetics and the circumburst medium density, as well as the microphysics, such as energy imparted in electrons and magnetic fields necessary for synchrotron emission . Some of the phenomena routinely addressed through radio observations are interstellar scintillation, synchrotron self-absorption, forward shocks, reverse shocks, jet breaks, nonrelativistic transitions, and obscured star formation.
The inhomogeneities in the local interstellar medium manifest themselves in the form of interstellar scintillations and cause modulations in the radio flux density of a point source whose angular size is less than the characteristic angular size for scintillations . GRBs are compact objects and one can see the signatures of interstellar scintillation at early time radio observations, when the angular size of the fireball is smaller than the characteristic angular scale for interstellar scintillation. This reflects influx modulations seen in the radio observations. Eventually due to relativistic expansion, the fireball size exceeds the characteristic angular scale for scintillations and the modulations quench. This can be utilised in determining the source size and the expansion speed of the blast wave . In GRB 970508 and GRB 070125, the initial radio flux density fluctuations were interpreted as interstellar scintillations, which lead to an estimation of the upper limit on the fireball size [7, 29, 57]. In GRB 070125, the scintillation time scale and modulation intensity were consistent with those of diffractive scintillations, putting a tighter constraint on the fireball size .
Very Long Baseline Interferometry (VLBI) radio observations also play a key role by providing evidence for the relativistic expansion of the jet using for bright GRBs. This provides microarcsecond resolution and directly constrains the source size and its evolution. So far this has been possible for a nearby (
) GRB 030329 . In this case, the source size measurements were combined with its long term light curves to better constrain the physical parameters [53, 54]. In addition, GRB 030329 also provided the first spectroscopic evidence for association of a GRB with a supernova. This confirmed massive stars origin of at least a class of GRBs.
Radio observations are routinely used in broadband modeling of afterglows and used to derive blast-wave parameters [1, 29, 59–61] (also see Figure 1). Early radio emission is synchrotron self-absorbed radio observations uniquely constrain the density of the circumburst medium. Radio studies have also proven useful for inferring the opening angles of the GRB jets as their observational signature differs from those at higher wavelengths [50, 62–64]. Recently GRB 130427A, a nearby, high-luminosity event, was followed at all wavebands rigorously. It provided extremely good temporal (over 10 orders of magnitude) and spectral coverage (16 orders of magnitude in observing frequency [65, 66]). Radio observations started as early as 8 hours . One witnessed reverse shock and its peak moving from high to low radio frequencies over time [67–70]. The burst is an ideal example to show how early to late-time radio observations can contribute significantly to our understanding of the physics of both the forward and reverse shocks.
Radio afterglows can be detected at high redshifts [16, 71] owing to the negative
-correction effect . GRB 090423 at a redshift of 8.3 is the highest redshift (spectroscopically confirmed) known object in the Universe . It was detected in radio bands for several tens of days . The multiwaveband modeling indicated the
density medium and the massive star origin of the GRB. This suggested that the star formation was taking place even at a redshift of 8.3.
The radio afterglow, due to its long-lived nature, is able to probe the time when the jet expansion has become subrelativistic and geometry has become quasispherical [50, 52, 73] and thus can constrain energetics independent of geometry. This is possible only in radio bands as it lasts for months or even years (e.g., [50–52]). GRB 970508 remained bright more than a year after the discovery, when the ejecta had reached subrelativistic speeds. This gave the most accurate estimate of the kinetic energy of the burst .
Reverse shock probes the ejecta and thus can potentially put constraints on the Lorentz factor and contents of the jet (e.g., [68, 69]). The shock moving into the ejecta will result in an optical flash in the first tens of seconds after the GRB under right conditions. The radio regime is also well suited to probe the reverse shock emission as well. Short-lived radio flares, most likely due to reverse shock, have also been detected from radio observations [16, 74–76] and seem more common in radio bands than in the optical bands. GRB 990123 was the first GRB in which the reverse shock was detected in optical  as well as in radio bands .
From the radio perspective, GRB 030329 holds a very important place. It was the first high-luminosity burst at low redshift with a spectroscopic confirmation of a supernova associated with it. So far this is the only GRB for which the source size has been measured with VLBI. The radio afterglow of GRB 030329 was bright and long lasting and has been detected for almost a decade at radio frequencies [52, 78]. This enabled one to perform broadband modeling in the different phases and has led to tighter constraints on the physical parameters [53, 54]. However, the absence of a counter jet poses serious question in our understanding of GRBs .
3. Open Problems in GRB Radio Afterglows
With various high sensitivity new and refurbished telescopes, for example, Atacama Large Millimetre Array (ALMA), Karl J. Jansky Very Large Array (JVLA), upgraded Giant Metrewave Radio Telescope (uGMRT), and upcoming telescopes, for example, Square Kilometre Array (SKA), the radio afterglow physics of GRBs is entering into new era, where we can begin to answer some of the open questions in the field, answers to which are long awaited. In this section, I discuss only some of those open problems in GRB science where radio measurements can play a crucial role.
This review is not expected to be exhaustive. We concentrate on only a few major issues.
3.1. Are GRBs Intrinsically Radio Weak?
Since the launch of the Swift, the fractions of X-ray and optically detected afterglows have increased tremendously that is, almost 93% of GRBs have a detected X-ray afterglow  and
75% have detected optical afterglows [81, 82]. However, what is disconcerting is that the radio detection fraction has remained unchanged with only one-third of all GRBs being detected in radio bands [47, 48]. Chandra and Frail  attributed it to sensitivity limitation of the current telescopes (see Figure 2). This is because radio detected GRBs have flux densities typically ranging from a few tens of
Jy to a few hundreds of Jy . Even the largest radio telescopes have had the sensitivities close to a few tens of Jy, making the radio afterglow detection sensitivity limited. The newer generation radio telescopes should dramatically improve statistics of radio afterglows. For example, using numerical simulation of the forward shock, Burlon et al.  predict that the SKA-1 (SKA first phase) Mid band will be able to detect around 400–500 radio afterglows per
upper limits at 8.5 GHz frequency band for all GRBs for which no afterglow was detected. The red line shows light curve of a rare, bright event GRB 980703 and the blue line shows the light curve of a more typical event GRB 980329. The detection fraction of radio afterglows in the first 10 days certainly appears to be mainly limited by the sensitivity. The black dashed line indicates sensitivity of the JVLA in its full capacity for a 30-minute integration time. The figure is reproduced from .
The Five-hundred-meter Aperture Spherical radio Telescope (FAST) [84–86] is the largest worldwide single-dish radio telescope, being built in Guizhou province of China with an expected first light in Sep. 2016. FAST will continuously cover the radio frequencies between 70 MHz and 3 GHz. The radio afterglow of GRBs is one of the main focuses of FAST. Zhang et al.  have estimated the detectability with FAST of various GRBs like failed GRBs, low-luminosity GRBs, high-luminosity GRBs, and standard GRBs. They predict that FAST will be able to detect most of the GRBs other than subluminous ones up to a redshift of
However, Hancock et al.  used stacking of radio visibility data of many GRBs and their analysis still resulted in nondetection. Based on this they proposed a class of GRBs which will produce intrinsically faint radio afterglow emission and have black holes as their central engine. GRBs with magnetars as central engine will produce radio bright afterglow emission. This is because the magnetar driven GRBs will have lower radiative efficiency and produce radio bright GRBs, whereas the black hole driven GRBs with their high radiative efficiency will use most of their energy budget in prompt emission and will be radio-faint. This is a very important aspect and may need to be addressed. And if true, it may reflect the nature of the central engine through radio measurements. JVLA at high radio frequencies and the uGMRT at low radio frequencies test this hypothesis. SKA will eventually be the ultimate instrument to distinguish between the sensitivity limitation and the intrinsic dimness of radio bursts .
3.2. Hyperenergetic GRBs
Accurate calorimetry is very important to understand the true nature of the GRBs. This includes prompt radiation energy in the form of -rays and kinetic energy in the form of shock powering the afterglow emission. Empirical constraints from models require that all long duration GRBs have the kinetic energies ≤
ergs. GRBs are collimated events thus the jet opening angle is crucial to measure the true budget of the energies. While isotropic energies range of energies spread in four orders of magnitude (see Figure 3), the collimated nature of the jet makes the actual energies in much tighter range clustered around ergs [75, 88, 89]. However, it is becoming increasingly evident that the clustering may not be as tight as envisaged and the actual energy range may be much wider than anticipated earlier. A population of nearby GRBs have relativistic energy orders of magnitude smaller than a typical cosmological GRB these are called subluminous GRBs, for example, GRB 980425 [25, 90]. Fermi has provided evidence for a class of hyperenergetic GRBs. These GRBs have total prompt and kinetic energy release, inferred via broadband modeling [61, 91], to be at least an order of magnitude above the canonical value of erg [1, 29, 48, 92]. The total energy budget of these hyperenergetic GRBs poses a significant challenge for some accepted progenitor models. The maximum energy release in magnetar models  is
erg, set by the rotational energy of a maximally rotating stable neutron star [94, 95].
, in rest frame 1 keV–10 MeV bandpass) of GRBs with measured redshift. One can see a large range of . Reproduced from Cenko et al. .
It has been very difficult to constrain the true prompt energy budget of the GRBs, mainly, for the following reasons. So far, Swift has been instrumental in detecting majority of the GRBs. However, peaks of the emission for various GRBs lie outside the narrow energy coverage of Swift-BAT (15–150 keV). In addition, extrapolation of 15–150 keV to 1–10,000 keV bandpass causes big uncertainties in the determination of prompt isotropic energies. With its huge energy coverage (8 keV–300 GeV), Fermi has overcome some of these limitations and provided unparalleled constraints on the spectral properties of the prompt emission. Fermi has been able to distinguish the true hyperenergetic bursts (such as GRB 090323, GRB 090902B, and GRB 090926A  also see Figure 3). While Swift sample is biased towards faint bursts, Fermi sample is biased towards GRBs with very large isotropic energy releases (
erg), which even after collimation correction reach very high energies, for example, [1, 96], and provide some of the strongest constraints on possible progenitor models.
The uncertainty in jet structure in GRBs pose additional difficulty in constraining the energy budget of GRBs. Even after a jet break is seen, to convert it into opening angle, one needs density to convert it into the collimation angle. While some optical light curves can be used to constrain the circumburst density (e.g., Liang et al. ), radio SSA peak is easier to detect due to slow evolution in radio bands. With only one-third of sample being radio bright, this has been possible for only a handful of bursts. A larger radio sample at lower frequencies, at early times when synchrotron self-absorption (SSA) is still playing a major role, could be very useful. The uGMRT after upgrade will be able to probe this regime as SSA will be affecting the radio emission at longer wavelength for a longer time. However, the this works on the assumption that the entire relativistic outflow is collimated into a single uniform jet. While the proposed double-jet models for GRB 030329 [97, 98] and GRB 080319B  ease out the extreme efficiency requirements, it has caused additional concerns.
The ALMA also has an important role to play since GRB spectrum at early times peak at mm wavelengths, when it is the brightest. ALMA with its high sensitivity can detect such events at early times and give better estimation of the kinetic energy of the burst.
While X-ray and optical afterglows stay above detection limits only for weeks or months, radio afterglows of nearby bursts can be detected up to years [50, 100]. The longevity of radio afterglows also makes them interesting laboratories to study the dynamics and evolution of relativistic shocks. At late stages, the fireball would have expanded sideways so much that it would essentially make transition into nonrelativistic regime and become quasispherical and independent of the jet geometry calorimetry can be employed to obtain the burst energetics [50, 52]. These estimates will be free of relativistic effects and collimation corrections. This regime is largely unexplored due to limited number of bursts staying above detection limit beyond subrelativistic regime. Several numerical calculations exist for the afterglow evolution starting from the relativistic phase and ending in the deep nonrelativistic phase [79, 101]. SKA with its Jy level sensitivity will be able to extend the current limits of afterglow longevity. This will provide us with an unprecedented opportunity to study the nonrelativistic regime of afterglow dynamics and thereby will be able to refine our understanding of relativistic to nonrelativistic transition of the blast-wave and changing shock microphysics and calorimetry in the GRBs. Burlon et al.  have computed that SKA1-MID will be able to observe 2% afterglows till the nonrelativistic (NR) transition but that the full SKA will routinely observe 15% of the whole GRB afterglow population at the NR transition.
3.3. Can Jet Breaks Be Chromatic?
After the launch of Swift, one obtained a far better sampled optical and X-ray light curves, thus expected to witness achromatic jet breaks across the electromagnetic spectrum, a robust signature associated with a collimated outflow. Several groups conducted a comprehensive analysis of a large sample of light curves of Swift bursts in the X-rays [102–105] and optical  bands. Surprisingly fewer Swift bursts have shown this unambiguous signature of the jet collimation. Without these collimation angles, the true energy release from Swift events has remained highly uncertain. A natural explanation for absence of the jet breaks can be attributed to the high sensitivity of Swift. Due to its high sensitivity Swift is preferentially selecting GRBs with smaller isotropic gamma-ray energies and larger redshifts. This dictates that typical Swift events will have large opening angles, thus causing jet breaks to occur at much time than those of pre-Swift events. Since afterglow is already weak at later times, making jet break measurements is quite difficult [103, 107].
There have been some cases where chromatic jet breaks are also seen. For example, in GRB 070125, the X-ray jet break occurred around day 10, whereas the optical jet break occurred on day 3. Chandra et al.  attributed it to inverse Compton (IC) effect, which does not affect the photons at low energies but shifts the X-ray jet break at a later time (see Figure 4, ). As IC effects are dominant in high density medium, radio observations are an important indicator of the effectiveness of the IC effect. Chandra et al.  showed that, for a given density of GRB 070125, the estimated delay in X-ray jet break due to the IC effect is consistent with the observed delay. However, this area needs to be explored further for other GRBs. While high density bursts are likely to be brighter in radio bands, it may cause a burst to be a dark one in optical wavelength (Xin et al.  and references therein), which then make it difficult to detect the jet break simultaneously in several wavelengths. uGMRT and JVLA will be ideal instruments to probe IC effect and will potentially be able to explain the cause of chromaticity in some of the Swift bursts.
(joint) is the joint fit to optical and X-ray data and grey solid line (X-ray) is the independent fit. The independent fit shifts the jet break to
9-10 days, which was found to be day 3 for optical bands. (b) Contribution of IC in the synchrotron model for the X-ray light curve of GRB 070125. The thin line represents the broadband model with the synchrotron component only. The thick line represents the IC light curve. One can see that IC effect can delay the jet breaks in X-ray bands .
3.4. High- GRBs and PoP III Stars
One of the major challenges of the observational cosmology is to understand the reionization of the Universe, when the first luminous sources were formed. So far quasar studies of the Gunn-Peterson absorption trough, the luminosity evolution of Lyman galaxies, and the polarization isotropy of the cosmic microwave background have been used as diagnostics. But they have revealed a complicated picture in which reionization took place over a range of redshifts.
The ultraviolet emission from young, massive stars (see Fan et al.  and references therein) appears to be the dominant source of reionization. However, none of these massive stars have been detected so far. Long GRBs, which are explosions of massive stars, are detectable out to large distances due to their extreme luminosities and thus are the potential signposts of the early massive stars. GRBs are predicted to occur at redshifts beyond those where quasars are expected thus they could be used to study both the reionization history and the metal enrichment of the early Universe . They could potentially reveal the stars that form from the first dark matter halos through the epoch of reionization [72, 111, 112]. The radio, infrared, and X-ray afterglow emission from GRBs are in principle observable out to [72, 111–114]. Thus GRB afterglows make ideal sources to probe the intergalactic medium as well as the interstellar medium in their host galaxies at high .
The fraction of detectable GRBs that lie at high redshift (
) is, however, expected to be less than 10% [115, 116]. So far there are only 3 GRBs with confirmed measured redshifts higher than 6. These are GRB 050904 , GRB 080913 , and GRB 090423 . Radio bands are ideal to probe GRB circumburst environments at high redshift because radio flux density show only a weak dependence on the redshift, due to the negative -correction effect  (also see  and Figure 5). In k-correction effect, the afterglow flux density remains high because of the dual effects of spectral and temporal redshift, offsetting the dimming due to the increase in distance  (see Figure 5). GRB 050904 and GRB 090423 were detected in radio bands and radio observations of these bursts allowed us to put constraints on the density of the GRB environments at such high redshifts. While the density of GRB 090423 was
 (Figure 5), the density of GRB 050904 was
, indicating dense molecular cloud surrounding the GRB 050904 . This revealed that these two high- GRBs exploded in a very different environment.
) plot for radio afterglows with known redshifts. Blue diamonds are GRBs associated with supernovae, while the grey circles denote cosmological GRBs. The green dashed line indicates if the flux density scales as the inverse square of the luminosity distance. The red thick line is the flux density scaling in the canonical afterglow model which includes a negative -correction effect, offsetting the diminution in distance (reproduced from ). (b) Multiwaveband afterglow modeling of highest redshift GRB 090423 at (reproduced from ).
ALMA will be a potential tool for selecting potential high- bursts that would be suitable for intense follow-up across the electromagnetic spectrum. With an order of magnitude enhanced sensitivity the JLA will be able to study a high- GRB for a longer timescale. For example, VLA can detect GRB 090423-like burst for almost 2 years. The uGMRT can also detect bright bursts up to a redshift of . These measurements will therefore obtain better density measurements and reveal the environments where massive stars were forming in the early Universe.
3.5. Reverse Shock
In a GRB explosion, there is a forward shock moving forward into the circumburst medium, as well as a reverse shock moving backwards into the ejecta . The nearly self-similar behavior of a forward shock means that little information is preserved about the central engine properties that gave rise to the GRB. In contrast, the brightness of the short-lived reverse shock depends on the initial Lorentz factor and the magnetization of the ejecta. Thus, multifrequency observations of reverse shocks tell about the acceleration, the composition, and the strength and orientation of any magnetic fields in the relativistic outflows from GRBs [68, 69, 121–123]. In general, the reverse shock is expected to result in an optical flash in the first tens of seconds after the GRB , which makes it difficult to detect as robotic telescopes are required for fast triggers.
The discovery of a bright optical flash from GRB 990123  leads to extensive searches for reverse shocks [124–127] in optical bands. One expected to see more evidences of reverse shocks in optical bands due to Swift-UVOOT however, based on these efforts it seems that the incidence of optical reserve shocks is low. Since the peak of this emission moves to lower frequencies over time and can be probed at radio frequencies on a time scale of hours to days , the radio regime is well suited for studying early time reverse shock phenomena.
There have been several observational as well as theoretical studies of radio reverse shock emission in the literature after the first reverse shock detection in GRB 990123 . Gao et al. , Kopač et al. , and Resmi and Zhang  have done comprehensive analytical and numerical calculations of radio reverse shock emissions and about their detectability. It has been shown [48, 67] that deep and fast monitoring campaigns of radio reverse shock emission could be achieved with the VLA for a number of bursts. JVLA radio frequencies are well suited as reverse shock emission is brighter in higher radio frequencies where self-absorption effects are relatively lesser. Radio afterglow monitoring campaigns in higher SKA bands (e.g., SKA1-Mid Band-4 and Band-5) will definitely be useful in exploring reverse shock characteristics .
Reverse shock is detectable in high redshift GRBs (
) as well. Inoue et al.  have predicted that at mm bands the effects of time dilation almost compensate for frequency redshift, thus resulting in a near-constant observed peak frequency at a few hours after event and a flux density at this frequency that is almost independent of redshift. Thus ALMA mm band is ideal to look for reverse shock signatures at high redshifts. Burlon et al.  predict that SKA1-Mid will be able to detect a reverse shock from a GRB990123 like GRB at a redshift of
3.6. Connecting Prompt and Afterglow Physics
Swift is an ideal instrument for quick localization of GRBs and rapid follow-up and consequently redshift measurement [20, 132] and Fermi for the wideband spectral measurement during the prompt emission. However, good spectral and timing measurement covering early prompt to late afterglow phase is available for a few sources and rarely available for the short GRBs. Some of the key problems that can be addressed by the observation of the radio afterglows in connection with the prompt emission are (i) comparing the Lorentz factor estimation with both LAT detected GeV photons as well as from the reverse shock [133, 134] (ii) comparison between nonthermal emission of both the prompt and afterglow emission, which would enable one to constrain the microphysics of the shocks accelerating electrons to ultra-relativistic energies eventually producing the observed radiation (iii) detailed modeling of the afterglow observation of both long and short GRBs, which will enhance our knowledge about the circumburst medium surrounding the progenitors (iv) current refurbished and upcoming radio telescopes with their finer sensitivity, which would play a key role in constraining the energetics of GRBs which is crucial in estimating the radiation efficiency of the prompt emission of GRBs. This would strengthen the understanding of the hardness-intensity correlation .
The recently launched AstroSAT satellite  carries several instruments enabling multiwavelength studies. The Cadmium Zinc Telluride Imager (CZTI) on-board AstroSAT can provide time resolved polarization measurements for bright GRBs and can act as a monitor above 80 keV [137, 138]. So far no other instrument has such capability to detect polarization. Hence, for a few selected bright GRBs, CZTI, in conjunction with ground based observatories like uGMRT and JVLA, and other space based facilities can provide a complete observational picture of a few bright GRBs from early prompt phase to late afterglow. This will provide us with a comprehensive picture of GRBs, thus enabling a good understanding of the emission mechanisms.
3.7. Some Other Unresolved Issues
So far I have discussed only that small fraction of on-axis GRBs, in which the jet is oriented along our line of sight. Due to large Lorentz factors, small opening angles of the collimated jets, we only detect a small fraction of GRBs . Ghirlanda et al.  have estimated that, for every GRB detected, there must be 260 GRBs which one is not able to detect. However, their existence can be witnessed as “orphan afterglow” at late times when the GRB jet is decelerated and spread laterally to come into our line of sight. At such late times, the emission is expected to come only in radio bands. So far attempts to find such orphan radio afterglows have been unsuccessful [75, 141, 142]. Even if detected, disentangling the orphan afterglow emission from other classes will be very challenging. Soderberg et al.  carried out a survey towards the direction of 68 Type Ib/c supernovae looking for the orphan afterglows and put limit on GRB opening angles,
d. The detection of population of orphan afterglows with upcoming sensitive radio facilities is promising. This will give a very good handle on jet opening angles and on the total GRB rate whether beamed towards us or not.
The inspiral and merger of binary systems with black holes or neutron stars have been speculated as primary source of gravitational waves (GWs) for the ground based GW interferometers [143, 144]. The discovery of GWs from GW 150914  and GW 151226  with the Advanced LIGO detectors have provided the first observational evidence of the binary black hole systems inspiraling and merging. At least some of the compact binaries involving a neutron star are expected to give rise to radio afterglows of short GRBs. Electromagnetic counterparts of GW source, including emission in the radio bands, are highly awaited as they will, for the first time, confirm the hypothesis of binary merger scenario for GW waves. If localized at high energies, targeted radio observations can be carried out to study these events at late epochs.
Short GRBs arising from mergers of two neutron stars eject significant amount of mass in several components, including subrelativistic dynamical ejecta, mildly relativistic shock-breakout, and a relativistic jet . Hotokezaka and Piran  have calculated the expected radio signals produced between the different components of the ejecta and the surrounding medium. The nature of radio emission years after GRB will provide invaluable information on the merger process  and the central products . Fong et al.  have predicted that the formation of stable magnetar of energy erg during merger process will give rise to a radio transient a year later. They carried out search for radio emission from 9 short GRBs in rest frame times of 1–8 years and concluded that such a magnetar formation can be ruled out in at least half their sample.
In addition, radio observations can also probe the star formation and the metallicity of the GRB host galaxies when optical emissions are obscured by dust [147, 148].
In this article, I have reviewed the current status of the Swift/Fermi GRBs in context of their radio emission. With improved sensitivity of the refurbished radio telescopes, such as JVLA and uGMRT and upcoming telescopes like SKA, it will be possible to answer many open questions. The most crucial of them is the accurate calorimetry of the GRBs. Even after observing a jet break in the GRB afterglow light curves, which is an unambiguous signature of the jet collimation, one needs density estimation to convert the jet break epoch to collimation angle. The density information can be more effectively provided by the early radio measurements when the GRBs are still synchrotron self-absorbed. So far it has been possible for very limited cases because only one-third of the total GRBs have been detected in radio bands . Sensitive radio measurements are needed to understand whether the low detection rate of radio afterglows is intrinsic to GRBs or the sensitivity limitations of the current telescopes are playing a major role. In the era of JVLA, uGMRT, ALMA, and upcoming SKA, this issue should be resolved. In addition, these sensitive radio telescopes will be crucial to detect radio afterglows at very high redshifts and provide unique constraints on the environments of the exploding massive stars in the early Universe. If GRBs are not intrinsically dim in radio bands and the sample is indeed sensitivity limited, then SKA is expected to detect almost 100% GRBs . SKA will be able to study the individual bursts in great detail. This will also allow us to carry out various statistical analyses of the radio sample and drastically increase our overall understanding of the afterglow evolution from very early time to nonrelativistic regime. Detection of the orphan afterglow is due any time and will be novel in itself.
The author declared that there are no competing interests.
The author thanks L. Resmi, Shabnam Iyyanni, A. R. Rao, Kuntal Misra, and D. Frail for many useful discussions in the past, which helped shape this article. The author acknowledges support from the Department of Science and Technology via SwarnaJayanti Fellowship Award (File no. DST/SJF/PSA-01/2014-15). The author also acknowledges SKA Italy handbook (http://pos.sissa.it/cgi-bin/reader/conf.cgi?confid=215), where many of the SKA numbers on sensitivity, GRB detection rates, and so forth are taken.
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Copyright © 2016 Poonam Chandra. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ESA Science & Technology - The Gamma-Ray Lens
Diagnosis of gamma-ray line emission tells astronomers a great deal about the most violent and energetic phenomena in the Universe. One particularly interesting application of a focusing gamma-ray telescope would be fine spectroscopy of the emission from Type Ia supernovae. Type Ia supernovae are an important cosmological tool as they are used as standard candles and have the potential to be used as a tool for measuring the extent of Dark Energy.
Another area of specific interest is the 511 keV emission line produced by the annihilation of electrons and positions. This distinct line is expected to be present in a range of astronomical objects including: gamma-ray bursts, accretion disks around compact objects, the galactic centre, galactic binaries, active galactic nuclei and supernovae. Annihilation radiation will reveal a great deal about the nature of these objects and, in particular, the interstellar material surrounding them.
To date the limiting problem for gamma-ray telescopes, particularly at the soft end of the spectrum at around a few hundred keV, has been instrument sensitivity and effective area. The low photon flux and high background levels at these energies result in a very low signal-to-noise ratio. The effective area of current gamma-ray instruments are, at best, equivalent to the area of the detector. As high-energy astrophysics is photon limited, increasing the detector size to improve effective area results in larger background readings and sensitivity reduction. At best, sensitivity increases as the square root of detector surface area.
A focusing gamma-ray telescope would overcome some of these problems by allowing the creation of a smaller detector plane as well as offering lower intrinsic noise levels. A 1 million second point source observation at 511 keV would return an instrument sensitivity around 100 times higher than currently obtainable with INTEGRAL.
Gamma-Ray Focussing Technologies
There are two main gamma-ray focusing technologies that were investigated with the Gamma-Ray Lens in mind - Multilayer coatings applied to Silicon Pore Optics and Laue crystal diffraction. Both of these techniques are utilised in the mission profile to combine and provide a broader science capability.
Multilayer Coatings and Silicon Pore Optics
Multilayer mirror technology is an extension of classical high-energy optics, using the principle of reflection at grazing incidences. The multilayers are, in fact, a series of coatings, or bilayers, made up of alternating materials. Many hundreds of bilayers are used in a multilayer coating, where varying the thickness of the bilayers will affect the energy response of the optic. It is expected that the maximum energy focused will increase from
10 keV, as from classical coatings such as gold, to
300 keV using multilayer coatings.
Silicon Micropore Optics are novel, lightweight, high-resolution optics currently being investigated for IXO, the International X-ray Observatory. They are expected to provide a much greater effective area per unit mass than previous optic technology and will, therefore, be crucial to higher energy missions where the number of photons is very limited. A multilayer coating designed for the Gamma-Ray Lens would be applied to the Silicon Micropore Optic in order to maximise the effective area at the relevant energies of interest.
Laue Crystals and the Gamma-Ray Lens
For years, X-ray diffraction has been used to establish the structure of crystals. A high-energy ray will diffract through a crystal, changing direction depending on its energyਊnd the crystal plane it hits. The equation that governs this is known as Bragg&aposs law. Using this equation, a &aposlens&apos can be constructed that changes the direction of gamma rays passing through it so they converge at a point - a focus. Crystals with a large atomic density such as copper work best as X-ray and gamma-ray diffractors. The focal length of such a lens can be very long - a lens with an effective area of about 1 square meter could have a focal length of five hundred meters! This obviously implies the need for two spacecraft flying in formation: one spacecraft carrying the lens and another holding the gamma-ray detector.
To achieve optimum performance with the large focal length of
500m two spacecraft are required. These are described below:
The Detector Spacecraft (DSC)
The detector spacecraft is summarised as follows:
- One primary payload - a SPI-INTEGRAL-style, high resolution, Germanium detector array with an advanced inorganic scintillator anticoincidence shield
- The detector is actively cooled to 80 K
- It is the smaller of the two spacecraft - approximately 1940 kg with margin
- The DSC is the more active of the two spacecraft as it performs formation flying manoeuvres
- All science data to Earth
The Optic Spacecraft (OSC)
Two views of the Optic Spacecraft. (Credit: ESA)
The Optic spacecraft has two main payloads:
- A Multilayer Silicon Pore Optic for the 50-200 keV energy bands, and
- A Crystal Laue Lens for the 425 - 522 keV and 825 - 910 keV bands.
The OSC has the following main characteristics:
- Requires a deployment mechanism for the 9m-diameter Laue optic, which is significantly larger than the launcher fairing size
- It is the larger of the two spacecraft - approximately 3500 kg with margin
- The spacecraft is more passive than the DSC with regards to formation flying, metrology, data handling and communications
Launch configuration (Credit: ESA)
- The two spacecraft are designed to be launched on a single dedicated Ariane 5, the spacecraft stacked with the OSC on top of the DSC.
- The spacecraft will be launched to L2, the second libration point, where formation flying can benefit from very low gravity gradients.
- The cruise phase to L2 will be performed as a stack, with the spacecraft separating and deploying after the final injection manoeuvre to L2 is made.
- At L2, the spacecraft will be in Halo orbit where very small manoeuvres will ensure zero eclipse.
- Once at L2 the two spacecraft enter observation mode, flying in rigid formation approximately 500 meters apart.
- The nominal lifetime of the spacecraft is 10 years, extendable to 15 years.
Separation and deployment of the two spacecraft (Credit: ESA)
Many challenges have been identified during this study, requiring either further investigation or technology development. Some key mission drivers are:
- Formation flying
- Laue Lens
- Gradient crystal development
- Crystal alignment
- Crystal growth and fabrication
- Deployment mechanism
- Multilayer Optics
- Multilayer design and fabrication
- Silicon Pore Optics
- Background rejection techniques
- Detector development
- Advanced inorganic scintillator detectors
Sensitivity analysis of the Gamma-Ray Lens has shown that a large improvement on previous missions is achievable - approximately one hundred times better than ESA&aposs current gamma-ray mission, INTEGRAL.
This study has been performed by SRE-PAM with support from SRE-PAT and was completed in 2005.
For further information about this study please contact:
Head of Science Missions section (SRE-PAM)
Advanced Studies and Technology Preparation Division
ESA - ESTEC, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
Gamma Ray Bursts, Gravity Waves, and Earthquakes
On December 26, 2004 a magnitude 9.3 earthquake occurred in the Indian Ocean off the coast of Sumatra in Malaysia. It caused a powerful tsunami which devastated coastal regions of many countries leaving over 240,000 people either dead or missing. It was the worst tsunami to affect this area since the 1883 explosion of Krakatao. The earthquake that produced it was so strong that it exceeded by a factor of 10 the next most powerful earthquake to occur anywhere in the past 25 years.
It is then with some alarm that we learn that just 44.6 hours later gamma ray telescopes orbiting the Earth picked up the arrival of the brightest gamma ray burst ever recorded!
This gamma ray blast was 100 times more intense than any burst that had been previously recorded, equaling the brightness of the full Moon, but radiating most of its energy at gamma ray wavelengths. Gamma ray counts spiked to a maximum in 1.5 seconds and then declined over a 5 minute period with 7.57 second pulsations. The blast temporarily changed the shape the Earth’s ionosphere, distorting the transmission of long-wavelength radio signals. See stories on Space.com, BBC News, NY TImes.
Artists conception, courtesy of NASA
It was determined that the burst originated from the soft gamma ray repeater star, SGR 1806-20, a neutron star 20 kilometers in diameter which rotates once every 7.5 seconds, matching the GRB pulsation period. SGR 1806-20 is located about 10 degrees northeast of the Galactic center and about 20,000 to 32,000 light years from us, or about as far away as the Galactic center. (Originally, it had been thought to be 45,000 light years from us. but new results place it closer.) The outburst released more energy in a tenth of a second than the Sun emits in 100,000 years. Other gamma ray bursts have been detected whose explosions were intrinsically more powerful than this one at the source of the explosion, but since those explosions originated in other galaxies tens of thousands of times more distant, the bursts were not nearly as bright when they reached our solar system. What makes the December 27th gamma ray burst unique is that it is the first time that a burst this bright has been observed, one that also happens to originate from within our own Galaxy.
Astronomers have theorized that gamma ray bursts might travel in association with gravity wave bursts. In the course of their flight through space, gamma rays would be deflected by gravitational fields and would be scattered by dust and cosmic ray particles they encountered, so they would be expected to travel slightly slower than their associated gravity wave burst which would pass through space unimpeded. After a 45,000 year light-speed journey, a gamma ray burst arrival delay of 44.6 hours would not be unexpected. It amounts to a delay of just one part in 9 million. So if the gravity wave traveled at the speed of light (c), the gamma ray burst would have averaged a speed of 0.99999989 c, just 0.11 millionths slower. There is also the possibility that at the beginning of its journey the gravity wave may have had a superluminal speed see textbox below.
Artist’s conception, courtesy of NASA
The 9.3 Richter earthquake was ten times stronger than any other earthquake during the past 25 years, and was followed just 44.6 hours later on December 27th by a very intense gamma ray burst, which was 100 fold brighter than any other in the past 25 year history of gamma ray observation. It seems difficult to pass off the temporal proximity of these two Class I events as being just a matter of coincidence. A time period of 25 years compared to a time separation of 44.6 hours amounts to a time ratio of about 5000:1. For two such unique events to have such a close time proximity is highly improbable if they are not somehow related. But, as mentioned above, gravity waves would very likely be associated with gamma ray bursts, and they would be expected to precede them.
Many have inquired if there might be a connection between these two events (e.g., see the Space.com article). Not thinking of the gravity wave connection, astronomers have been reluctant to admit there might be a connection since they know of no mechanism by which gamma rays by themselves could trigger earthquakes. They admit that the December 27th gamma ray burst had slightly affected the ionization state of the Earth’s atmosphere, but this by itself should not have caused earthquakes. However, if a longitudinal gravity potential wave pulse were to accompany a gamma ray burst, the mystery becomes resolved. The connection between earthquakes and gamma ray bursts now becomes plausible.
In his 1983 Ph.D. dissertation, Paul LaViolette called attention to terrestrial dangers of Galactic core explosions, pointing out that the arrival of the cosmic ray superwave they produced would be signaled by a high intensity gamma ray burst which would also generate EMP effects (e.g., see Page 3). He also noted that a strong gravity wave might be expected to travel forward at the forefront of this superwave and might be the first indication of a superwave’s arrival. He pointed out that such gravity waves could induce substantial tidal forces on the Earth during their passage which could induce earthquakes and cause polar axis torquing effects.
In his book Earth Under Fire (as well as in his dissertation), LaViolette presents evidence showing that the superwave that passed through the solar system around 14,200 years ago had triggered supernova explosions as it swept through the Galaxy. Among these were the Vela and Crab supernova explosions whose explosion dates align with this superwave event horizon. He points out that these explosions could be explained if a gravity wave accompanied this superwave, it could have produced tidal forces which could have triggered unstable stars to explode as it passed through.
He wrote at a time when gamma ray bursts had just begun to be discovered, and when no one was concerned with them as potential terrestrial hazards. In recent years scientific opinion has come around to adopt LaViolette’s concern, as can be seen in news articles discussing the SGR 1806-20 gamma ray outburst, e.g., see Space.com news story. They note that if this gamma ray burst had been as close as 10 light years it would have completely destroyed the ozone layer. By comparison, the Galactic superwaves LaViolette has postulated to have been generated as a result of an outburst of our Galaxy’s core and to have impacted the Solar system during the last ice age would have impacted the solar system with a cosmic ray electron volley having an energy intensity 100 times greater than this hypothetical 10 light year distant stellar gamma ray burst. In comparision, SGR 1806-20 has been estimated to have a stellar progenitor mass of 150 solar masses, whereas our Galactic core has a mass of 2.6 million solar masses. In its present active phase, SGR 1806-20 is estimated to have a luminosity 40 million times that of the Sun, whereas during its active phase the Galactic center could reach luminosities of 400 trillion times that of the Sun. So it is understandable that if the Galactic center were to erupt, it would produce a gamma ray burst and a gravity wave far more intense than the outburst from this star.
If anything, the December 27, 2004 gamma ray burst shows us that we do not live in a peaceful celestial environment. And if the December 26th earthquake was in fact part of this same celestial event, we see that this stellar eruption has claimed many lives. For this reason, it is important that we prepare for the possibility of even stronger events in the future, the arrival of superwaves issuing from the core of our Galaxy. Like the December 26th earthquake and the December 27th gamma ray burst, the next superwave will arrive unexpectedly. It will take us by surprise.
It would have been possible to determine whether a Galactic gravity wave had indeed immediately preceded the December 26th earthquake by examining data from gravity wave telescopes. Since seismic waves from the Indonesian earthquake would have taken some time to propagate through the Earth to these gravity wave antenna, their signature could be distinguished from the gravity wave coming from SGR 1806-20. However, the major gravity wave telescopes were unfortunately not on line at that time. LIGO (Laser Interferometer Gravity Wave Observatory), which consists of two correlated telescopes, one in Washington state and one in Louisiana, each having a four kilometer long laser interferometer beam path, was in the process of being made operational and unfortunately was not collecting data at that time. We sent an email to the staff of the TAMA gravity wave antenna in Japan. Dr. Takahashi, who is responsible for the detector, replied that their telescope was unfortunately not operating during that week since they were making modifications at that time. So at present the gravity wave hypothesis remains neither confirmed nor disproven.
Superwave Monitoring Center
Those interested in monitoring earthquake, gamma ray burst, cosmic ray background activity, and gravity wave bursts may try the following websites:
- Current earthquakes: https://earthquake.usgs.gov/recenteqsww/Quakes/quakes_all.html
- Past earthquakes: https://earthquake.usgs.gov/activity/past.htmlpast.html
- Gamma ray bursts: https://grad40.as.utexas.edu/grblog.php?author=D.%20Gotz
- Cosmic ray radiation intensity: https://cr0.izmiran.rssi.ru/mosc/main.htm
- Gravity wave bursts (LIGO site: no posted data, just posted papers):
- https://www.ligo.caltech.edu/ and https://www.ligo.org/results/
- Listing of various relevant events: https://www.earthchangestv.com
The December 27th GRB was not accompanied by any rise in the cosmic ray background, indicating that if it was accompanied by cosmic rays their intensity was unable to exceed the relatively constant extragalactic background flux arriving from distant galaxies. A Galactic superwave, on the other hand, would most likely produce a substantial rise in these levels.
Note that almost two months passed before the December 27th gamma ray burst found its way into news media stories. If unusually intense activity were to occur in the near future as the beginning stages of a superwave arrival, it is hoped that scientists will not keep this knowledge to themselves but rather allow the global news media to disseminate the story quickly to inform the world.
A Superluminal Gravity Wave?
Experiments carried out by Eugene Podkletnov show that a shock front outburst produces a longitudinal gravitational wave that travels forward with the burst. He has found that this gravity wave pulse has a speed in excess of 64 times the speed of light (personal communication). Also Guy Obolensky has produced spark discharge electric potential shock fronts and observed them to propagate forward at speeds as high as 10 times the speed of light. Observations suggest that the gravity wave from an expanding stellar explosion will decrease its superluminal speed and eventually approach the speed of light as the shock front expands. But meanwhile, the gravity wave will have obtained a headstart over the electromagnetic wave radiation component traveling in its wake (light waves, gamma rays, etc.). So one would expect that the gravity wave from such an outburst (and its resultant earthquake activity) would precede the gamma ray burst component.
Others Coming to the Same Conclusion
Dr. LaViolette posted the above page on February 20, 2005 having learned about the December 2004 gamma ray burst on the day before and at that time realized that it must have been associated with the Malaysian tsunami. On February 21st Lazarus Long also posted a similar idea on a news group.
Posted: Feb 21 2005-21:23
I just thought I should put this on this thread for the record.
A very interesting event occurred almost simultaneously with the Tsunami event but it appears few have noticed but this gamma ray burst almost coincided with the tectonic event (it was slightly afterward) here on Earth. I am curious if what we witnessed was a gravitational bow wave effect that preceded the actual gamma radiation as a shock-wave. Just coincidence?
Remember the distance traveled was 50,000 lt yrs. This could be evidence to verify the critical aspects of quantum gravitational theory as a mountain of data was collected. My suspicion is that a type of Huygens Gravitational Wave effect may have preceded the actual EM *flash* by many hours or it might simply reflect the period of compression and gravitational displacement within the Neutron Stars’ power surge that preceded the actual EM emission.
Apparently the entire solar system was shaking from the event, in fact the entire galaxy is apparently experiencing a kind of gravitational oscillation (vibration) as this wave propagates. What if we are seeing an event that can be quantified to be slightly faster than light and being propagated just ahead of it?
While the flash was noticed after the tectonic event locally it is entirely conceivable that the actual gravitation shock wave struck earlier and while no one to date has related the two events I am curious if we may also be seeing evidence of how cosmic scale events could interfere with planetary tectonics by introducing a trigger force sufficient to release the pent up energy that already exists internally within the planet.
I am curious if the relationship of gravity and the event can be measured precisely in relation to the actual EM radiation emitted that we may have sufficient data and measures to test quantum gravitational theory against, which are analogous to how the Michaelson Morley experiment was used to verify Relativity Theory.