# Why are there no gamma-ray bursts detected in our galaxy?

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I found from Wikipedia and other sites that there are no GRBs detected in the Milky Way. Can someone give a feasible reason for that? Why are there no GRBs detected in the Milky Way galaxy?

All models of gamma-ray bursts involve extremely energetic phenomena: particular types of supernovae, the coalescence of binary compact objects, strong magnetar flares, or tidal disruption events. It turns out that these events are quite rare - so rare, in fact, that GRBs would be expected to occur in a low-redshift Milky Way-like galaxy at a rate of only one every few million or tens of millions of years (Zhang & Meszaros 2003). If the Milky Way was undergoing a period of intense star formation, yielding more massive stars and therefore more supernovae, then this rate would go up, but still not significantly. Bear in mind that the Galactic supernova rate, for example, is believed to be only several supernovae per century, and the vast majority of supernovae don't lead to gamma-ray bursts.

We know of many extragalactic GRBs for several reasons. It helps that we can observe large numbers of galaxies (thanks to how bright the bursts are), and if we could look at millions of Milky Way-like galaxies, it wouldn't be surprising if we could detect $$sim$$1 event per year. (There's also the advantage that star formation peaked at a redshift $$zsim2$$, and so high-redshift objects would be more likely to produce more GRBs!)

## National Aeronautics and Space Administration

What causes gamma-ray bursts? The first burst was detected nearly 50 years ago and the mystery that surrounds their origin continues to exist. We do know that gamma-ray bursts are the most energetic events to occur in the Universe!

In order to understand what a gamma-ray burst (or GRB) is, you must first realize that gamma-rays are a type of light. In fact, gamma-rays are the most energetic form of light known. Light is a form of energy called electromagnetic radiation. Electromagnetic radiation comes in tiny packets of energy called photons. Photons come in a wide range of energies. Electromagnetic radiation can be placed in an arrangement according to the energy amount of the photons. This orderly arrangement is known as the electromagnetic spectrum.

At the low-energy end of the spectrum we find radio waves. They have a very long wavelength. At the high-energy end of the spectrum we find gamma-rays. They possess a very short wavelength. For electromagnetic waves, the relationship between wavelength and energy is an inverse relationship. The shorter the wavelength, the greater the energy the longer the wavelength, the less the energy. Humans cannot see the light forms at the low and high-energy ends of the spectrum. We can only see light that falls in the visible range of the spectrum. Visible light is in the middle of the spectrum and accounts for a very small percentage of the energy range on the whole spectrum.

If an astronomer were to study the Universe only in the visible range of the spectrum, the large majority of events would go unobserved. Cosmological events such as star birth and star death emit photons that occur across the entire electromagnetic spectrum. Thanks to considerable technological advances, astronomers now have the ability to view the Universe in radio waves, gamma-rays, and all energies in between. Distant quasars were first discovered by the radio waves they emit. Galactic dust can be observed in the infrared range while light from ordinary stars such as the Sun can be observed in the visible and ultraviolet range. Extremely hot gas can be observed by the X-rays that it emits. Observations in the gamma-ray range of the spectrum reveal a very energetic Universe. Such energetic phenomena as a blazar (which consist of a supermassive black hole with jets of particles blasting away from near the event horizon), solar flares, and the radioactive decay of atomic nuclei created in supernova explosions all produce gamma-rays.

So what exactly is a gamma-ray burst? At least once a day, the sky lights up with a spectacular flash of gamma-rays coming from deep space (remember: gamma-rays are not in the visible range of the electromagnetic spectrum so we consequently are not aware of the phenomena). The brightness of this flash of gamma-rays can temporarily overwhelm all other gamma-ray sources in the Universe. Gamma-ray bursts can release more energy in 10 seconds than the Sun will emit in its entire 10 billion-year lifetime. The burst can last from a fraction of a second to over a thousand seconds. The time that the burst occurs and the direction from which it will come cannot be predicted.

When this booklet was first published in 2000, the exact cause of these flashes was unknown. At the time, astronomers had determined that the observed bursts came from outside the Milky Way Galaxy, and they believed that a gamma-ray burst would occur about once every few million years here in the Milky Way.

The first gamma-ray bursts were detected while scientists were looking for violations of the Nuclear Test Ban Treaty during the Cold War Era of the 1960s. Several satellites employed to monitor treaty compliance detected a large increase in the number of gamma-rays they counted each second. It was determined that the gamma-rays were coming from outer space and not from a nuclear bomb exploding in the Earth’s atmosphere. Although Ray Klebesadel and his colleagues at the Los Alamos National Laboratory in New Mexico found these bursts in data going back to 1967, their discovery was not reported to the world until 1973.

Satellites, such as NASA’s Compton Gamma-Ray Observatory and Hubble Space Telescope, and ESA’s BeppoSAX, gave us valuable data in our quest to solve the mystery of GRBs. Those satellites had limitations, however. One limitation was that once a burst was detected, it took too long to reposition the satellite in order to face the burst and collect data. The satellites were also limited as to the range of the electromagnetic spectrum in which they can make observations. In 1999, scientists were able to observe an optical counterpart to a burst as the burst was occurring, which occurred only through a great deal of planning, cooperation, and luck. On January 23, 1999, a network of scientists was notified within 4 seconds of the start of a burst that a burst was in progress. Thanks to the Compton Gamma-Ray Observatory, BeppoSAX, the Internet, and a special robotic ground-based telescope, scientists were able to monitor the burst from start to finish at multiple wavelengths. It had the optical brightness of 10 million billion Suns, which was only one-thousandth of its gamma-ray brightness!

A few leading theories were developed that addressed the possible causes of gamma-ray bursts. One explanation proposed that they are the result of colliding neutron stars. Neutron stars are the corpses of massive stars (5 to 10 times the mass of our Sun) that have come to the ends of their life cycles. They are extremely dense. Although their diameter may only be 20 kilometers, their mass is about 1.4 times that of the Sun. A second theory proposed that gamma-ray bursts are the result of a merging between a neutron star and a black hole or between two black holes. Black holes result when supermassive (greater than 20 times the mass of our Sun) stars die. A third theory stated that gamma-ray bursts occur as the result of material shooting towards Earth at almost the speed of light as the result of a hypernova. A hypernova explosion can occur when the largest of the supermassive stars come to the end of their lives and collapse to form black holes. Hypernova explosions can be at least 100 times more powerful than supernova explosions.

Swift, a satellite with the capacity to study the Universe in a multitude of wavelengths, was launched in 2004. The satellite is aptly named because once a burst is detected, it can be repositioned to face the gamma ray source within 50 seconds. Through simultaneous observations of the burst in the optical, ultraviolet, X-ray, and gamma-ray ranges of the electromagnetic spectrum, scientists began to answer the many questions surrounding gamma-ray bursts. In 2008, the Fermi Gamma-Ray Space Telescope was launched and provided scientists with additional insight into the gamma-ray burst mystery.

Until recently, gamma-ray bursts could arguably have been called the biggest mystery in high-energy astronomy. Today, however, evidence from recent satellites like Swift and Fermi indicate that the energy behind a gamma-ray burst comes from the collapse of matter into a black hole. However, the type of collapse depends on the type of gamma-ray burst.

When astronomers looked at the number of bursts versus how long they lasted, they found two different classes of bursts: long-duration and short-duration. These two classes are likely created by different processes, but the end result in both cases is a brand new black hole.

Graph of the time versus number of bursts for the gamma-ray bursts observed by the BATSE instrument on the Compton Gamma-ray Telescope.

Long-duration bursts last anywhere from 2 seconds to a few hundreds of seconds (several minutes), with an average time of about 30 seconds. They are associated with the deaths of massive stars in supernovas though not every supernova produces a gamma-ray burst.

Short duration bursts are those that last less then 2 seconds lasting anywhere from a few milliseconds to 2 seconds with an average duration of about 0.3 seconds (or 300 milliseconds). These bursts appear to be associated with the merger of two neutron stars into a new black hole or a neutron star with a black hole to form a larger black hole.

This modern science mystery that plagued scientist for the past 50 years is now nearly solved.

## Why are there no gamma-ray bursts detected in our galaxy? - Astronomy

One of astronomy's most baffling mysteries is the undiscovered source of sudden, intense bursts of gamma rays. The Compton Observatory typically detects one burst a day. The bursts differ greatly in duration and appear randomly from any direction. They are not known to repeat.

Amazingly, once a burst fades away, no trace of it remains. No known object has yet been linked with any gamma-ray burst.

### A Serendipitous Discovery

Gamma-ray bursts were first recorded accidently in 1967 by the Vela satellites. These satellites were developed to look for clandestine nuclear tests by searching for telltale x-rays or gamma rays emitted from nuclear explosions. But the gamma-ray bursts the satellite discovered came from deep space.

Astronomers expected to associate the locations of the bursts with known stars or galaxies. But despite many observations by many different telescopes, no such identification has ever been made.

### Strange Stars or Cosmic Collisions?

Many theories have been advanced to explain the distribution and origin of the bursts, but none answers all the questions the bursts pose. Some scientists say the sources of the bursts are relatively close, and others say they are some of the most distant objects in the known universe.

Gamma-ray bursts may come from relatively nearby, perhaps from a spherical "cloud" of neutron stars that could surround our own Milky Way galaxy.

(Courtesy of the National Air and Space Museum)

Other theories propose that the bursts emanate from the outer reaches of the observable universe, perhaps from violent collisions between neutron stars and black holes.

(Courtesy of R. Windhorst (ASU) and NASA)

## Why are there no gamma-ray bursts detected in our galaxy? - Astronomy

University scientists have found new evidence that cosmic gamma ray bursts originate from within the Milky Way -- our own galaxy -- and not from the edge of the universe, as many astronomers had believed. Jean Quashnock, Research Scientist in the Enrico Fermi Institute, and Don Lamb, Professor in Astronomy & Astrophysics, announced their results in two papers published in the Monthly Notices of the Royal Astronomical Society on Dec. 15.

For 25 years scientists have observed these mysterious bursts of extremely high energy radiation. But because the bursts are so brief -- from less than a second to a few minutes in duration -- and because they appear randomly, no one has been able to determine what causes them, or even how far away they are. They never seemed to appear in the same place twice, Lamb said. "You didn't know when they were going to happen or where they were going to happen, and you didn't get a second chance."

Scientists have hotly debated whether the bursts are cosmological -- from the edge of the universe -- or galactic, from within the Milky Way. "Now we have strong evidence that most gamma ray bursts come from within our galaxy," Lamb said. "The 25-year-old question about where the bursts originate may finally be answered."

Lamb said the evidence that the bursts are galactic in origin was a surprise, "and certainly not something I was confident of initially."

Quashnock said gamma ray bursts have been one of the most tantalizing problems in astronomy. "You've got these incredibly high energy bursts if they were cosmological, these would have been the biggest events in the universe," he said. "And it's gamma rays and nothing else -- here and there and then gone again."

Quashnock said that when he began studying the problem, he approached it from a decidedly cosmological perspective. But when he saw the patterns fall into place and realized the bursts were galactic, he said, "It was very exciting. For a small part of the day, I knew something that no one else in the world knew."

Prior to the launch of NASA's Compton Gamma Ray Observatory (CGRO) satellite in April 1991, many astronomers believed that the bursts came from within the galaxy. But when astronomers looked at the data the satellite had collected, they saw a random distribution of bursts on the sky. Scientists had expected to see a concentration of bursts in the plane of the Milky Way, which would have confirmed the galactic hypothesis. When the distribution of the bursts suggested sources far beyond the realm of our galaxy, scientific opinion shifted toward a cosmological explanation.

Quashnock and Lamb started looking more closely at the CGRO data to try to make sense of it. At first, the distribution of the bursts did appear to be totally random. But when the researchers took into account the statistical uncertainties inherent in determining the exact positions of the bursts in the sky, they began to see clumps of bursts. In fact, some were clustered so closely together they could not be distinguished from each other. In most cases, there was one bright burst with one or more fainter bursts around it.

Lamb said these bursts are almost certainly coming from the same sources. About a third of the 38 brightest bursts the researchers studied appeared to repeat within the 10 months of observations they analyzed.

"No one in 25 years had ever seen anything like this," Lamb said. Lamb and Quashnock analyzed data produced by the CGRO's Burst and Transient Source Experiment (BATSE), an instrument that can "see" bursts up to 10 times fainter than those seen with any previous detector.

Lamb said that in order for the energies seen in the bursts to reach the earth, much greater energies must be generated at the source. If the bursts were cosmological, the explosions would have been so large -- on the order of 1052 ergs -- they would have completely consumed the mass of any known compact object, such as a neutron star or a black hole, and thus eliminated the possibility of a repeat burst of energy from the same source. Galactic sources would require explosions on the order of 1038 ergs, making it quite plausible that mass could be left over to generate one or more additional bursts from the same source. "The fact that sources repeat knocks out all of the cosmological models discussed to date," Lamb said.

To probe the question further, Quashnock said he and Lamb divided the bursts into three categories based on brightness. They found that the brightest bursts were uniformly distributed across the sky. The very faintest bursts appeared to be repeats of the bright ones. But bursts of medium brightness were concentrated in the plane of, and toward the center of, the galaxy.

Quashnock likened the distribution to that of the stars in the night sky. The ones that are nearest to the earth show up brightest and most evenly distributed on the sky. "You look at the night sky and you see stars all around you. But on a dark night you also see the Milky Way, which is made up of the rest of the stars in the plane of the galaxy, only they are much farther away," he said. When Quashnock saw the same distribution pattern show up with the gamma ray bursts, he said, "I never considered the cosmological model again after that."

The medium-bright bursts didn't appear to repeat, Quashnock said, simply because they are bright bursts that are farther away. The instrument can't see the very faint gamma ray bursts that might be expected to be the repeats of these distant, but still galactic, bursts, he said.

The most plausible source of the bursts, said Lamb, are galactic neutron stars. These are dense, highly evolved stars, in which 1.4 to 2 times the mass of the sun is compressed into a sphere only about 15 or 20 kilometers across. No one knows exactly how the gamma ray bursts might be generated, but one theory suggests that they might come from material (such as a comet) crashing to the surface of a neutron star. The gravitational pull of neutron stars is so great that a tremendous amount of energy would be released when the material collided with the star's surface.

Quashnock said no one had seen the burst patterns before because the positions of the bursts were so poorly known. "If there were no ambiguity in the positions, the repeating would have been obvious," he said.

The method of measuring of the bursts makes the exact origin of the bursts difficult to pinpoint. A burst's position is determined by measuring the ratio of photons to each of BATSE's eight detectors, which are arrayed on the corners of the satellite. To further complicate these measurements, some gamma ray bursts are reflected into BATSE's detectors by the earth's atmosphere. To correct for this effect, scientists at the Marshall Space Flight Center, BATSE's command center, use sophisticated modeling techniques. But even with these corrections, the locations of the brightest bursts can be determined only to within about four degrees.

Lamb and Quashnock performed their analyses on 260 bursts cataloged by BATSE between April 1991 and March 1992.

Quashnock said the next step will be to try to pin down more precisely where the burst sources are located and to try to find radiation of lower energy that may be emanating from the same sources. In early 1995, the High Energy Transient Experiment (HETE) will be launched. This may allow scientists to correlate gamma ray bursts with ultraviolet or visible radiation, for example. "HETE has a good chance of finally linking up gamma ray bursts with the rest of astronomy," Lamb said.

## 23.6 The Mystery of the Gamma-Ray Bursts

Everybody loves a good mystery, and astronomers are no exception. The mystery we will discuss in this section was first discovered in the mid-1960s, not via astronomical research, but as a result of a search for the tell-tale signs of nuclear weapon explosions. The US Defense Department launched a series of Vela satellites to make sure that no country was violating a treaty that banned the detonation of nuclear weapons in space.

Since nuclear explosions produce the most energetic form of electromagnetic waves called gamma rays (see Radiation and Spectra), the Vela satellites contained detectors to search for this type of radiation. The satellites did not detect any confirmed events from human activities, but they did—to everyone’s surprise—detect short bursts of gamma rays coming from random directions in the sky. News of the discovery was first published in 1973 however, the origin of the bursts remained a mystery. No one knew what produced the brief flashes of gamma rays or how far away the sources were.

### From a Few Bursts to Thousands

With the launch of the Compton Gamma-Ray Observatory by NASA in 1991, astronomers began to identify many more bursts and to learn more about them (Figure 23.19). Approximately once per day, the NASA satellite detected a flash of gamma rays somewhere in the sky that lasted from a fraction of a second to several hundred seconds. Before the Compton measurements, astronomers had expected that the most likely place for the bursts to come from was the main disk of our own (pancake-shaped) Galaxy. If this had been the case, however, more bursts would have been seen in the crowded plane of the Milky Way than above or below it. Instead, the sources of the bursts were distributed isotropically that is, they could appear anywhere in the sky with no preference for one region over another. Almost never did a second burst come from the same location.

To get a good visual sense of the degree to which the bursts come from all over the sky, watch this short animated NASA video showing the location of the first 500 bursts found by the later Swift satellite.

For several years, astronomers actively debated whether the burst sources were relatively nearby or very far away—the two possibilities for bursts that are isotropically distributed. Nearby locations might include the cloud of comets that surrounds the solar system or the halo of our Galaxy, which is large and spherical, and also surrounds us in all directions. If, on the other hand, the bursts occurred at very large distances, they could come from faraway galaxies, which are also distributed uniformly in all directions.

Both the very local and the very distant hypotheses required something strange to be going on. If the bursts were coming from the cold outer reaches of our own solar system or from the halo of our Galaxy, then astronomers had to hypothesize some new kind of physical process that could produce unpredictable flashes of high-energy gamma rays in these otherwise-quiet regions of space. And if the bursts came from galaxies millions or billions of light-years away, then they must be extremely powerful to be observable at such large distances indeed they had to be the among the biggest explosions in the universe.

### The First Afterglows

The problem with trying to figure out the source of the gamma-ray bursts was that our instruments for detecting gamma rays could not pinpoint the exact place in the sky where the burst was happening. Early gamma-ray telescopes did not have sufficient resolution. This was frustrating because astronomers suspected that if they could pinpoint the exact position of one of these rapid bursts, then they would be able to identify a counterpart (such as a star or galaxy) at other wavelengths and learn much more about the burst, including where it came from. This would, however, require either major improvements in gamma-ray detector technology to provide better resolution or detection of the burst at some other wavelength. In the end, both techniques played a role.

The breakthrough came with the launch of the Italian Dutch BeppoSAX satellite in 1996. BeppoSAX included a new type of gamma-ray telescope capable of identifying the position of a source much more accurately than previous instruments, to within a few minutes of arc on the sky. By itself, however, it was still not sophisticated enough to determine the exact source of the gamma-ray burst. After all, a box a few minutes of arc on a side could still contain many stars or other celestial objects.

However, the angular resolution of BeppoSAX was good enough to tell astronomers where to point other, more precise telescopes in the hopes of detecting longer-lived electromagnetic emission from the bursts at other wavelengths. Detection of a burst at visible-light or radio wavelengths could provide a position accurate to a few seconds of arc and allow the position to be pinpointed to an individual star or galaxy. BeppoSAX carried its own X-ray telescope onboard the spacecraft to look for such a counterpart, and astronomers using visible-light and radio facilities on the ground were eager to search those wavelengths as well.

Two crucial BeppoSAX burst observations in 1997 helped to resolve the mystery of the gamma-ray bursts. The first burst came in February from the direction of the constellation Orion. Within 8 hours, astronomers working with the satellite had identified the position of the burst, and reoriented the spacecraft to focus BeppoSAX’s X-ray detector on the source. To their excitement, they detected a slowly fading X-ray source 8 hours after the event—the first successful detection of an afterglow from a gamma-ray burst. This provided an even-better location of the burst (accurate to about 40 seconds of arc), which was then distributed to astronomers across the world to try to detect it at even longer wavelengths.

That very night, the 4.2-meter William Herschel Telescope on the Canary Islands found a fading visible-light source at the same position as the X-ray afterglow, confirming that such an afterglow could be detected in visible light as well. Eventually, the afterglow faded away, but left behind at the location of the original gamma-ray burst was a faint, fuzzy source right where the fading point of light had been—a distant galaxy (Figure 23.20). This was the first piece of evidence that gamma-ray bursts were indeed very energetic objects from very far away. However, it also remained possible that the burst source was much closer to us and just happened to align with a more distant galaxy, so this one observation alone was not a conclusive demonstration of the extragalactic origin of gamma-ray bursts.

On May 8 of the same year, a burst came from the direction of the constellation Camelopardalis. In a coordinated international effort, BeppoSAX again fixed a reasonably precise position, and almost immediately a telescope on Kitt Peak in Arizona was able to catch the visible-light afterglow. Within 2 days, the largest telescope in the world (the Keck in Hawaii) collected enough light to record a spectrum of the burst. The May gamma-ray burst afterglow spectrum showed absorption features from a fuzzy object that was 4 billion light-years from the Sun, meaning that the location of the burst had to be at least this far away—and possibly even farther. (How astronomers can get the distance of such an object from the Doppler shift in the spectrum is something we will discuss in Galaxies.) What that spectrum showed was clear evidence that the gamma-ray burst had taken place in a distant galaxy.

### Networking to Catch More Bursts

After initial observations showed that the precise locations and afterglows of gamma-ray bursts could be found, astronomers set up a system to catch and pinpoint bursts on a regular basis. But to respond as quickly as needed to obtain usable results, astronomers realized that they needed to rely on automated systems rather than human observers happening to be in the right place at the right time.

Now, when an orbiting high-energy telescope discovers a burst, its rough location is immediately transmitted to a Gamma-Ray Coordinates Network based at NASA’s Goddard Space Flight Center, alerting observers on the ground within a few seconds to look for the visible-light afterglow.

The first major success with this system was achieved by a team of astronomers from the University of Michigan, Lawrence Livermore National Laboratory, and Los Alamos National Laboratories, who designed an automated device they called the Robotic Optical Transient Search Experiment ( ROTSE ), which detected a very bright visible-light counterpart in 1999. At peak, the burst was almost as bright as Neptune—despite a distance (measured later by spectra from larger telescopes) of 9 billion light-years.

More recently, astronomers have been able to take this a step further, using wide-field-of-view telescopes to stare at large fractions of the sky in the hope that a gamma-ray burst will occur at the right place and time, and be recorded by the telescope’s camera. These wide-field telescopes are not sensitive to faint sources, but ROTSE showed that gamma-ray burst afterglows could sometimes be very bright.

Astronomers’ hopes were vindicated in March 2008, when an extremely bright gamma-ray burst occurred and its light was captured by two wide-field camera systems in Chile: the Polish “Pi of the Sky” and the Russian-Italian TORTORA [Telescopio Ottimizzato per la Ricerca dei Transienti Ottici Rapidi (Italian for Telescope Optimized for the Research of Rapid Optical Transients)] (see Figure 23.21). According to the data taken by these telescopes, for a period of about 30 seconds, the light from the gamma-ray burst was bright enough that it could have been seen by the unaided eye had a person been looking in the right place at the right time. Adding to our amazement, later observations by larger telescopes demonstrated that the burst occurred at a distance of 8 billion light-years from Earth!

### To Beam or Not to Beam

The enormous distances to these events meant they had to have been astoundingly energetic to appear as bright as they were across such an enormous distance. In fact, they required so much energy that it posed a problem for gamma-ray burst models: if the source was radiating energy in all directions, then the energy released in gamma rays alone during a bright burst (such as the 1999 or 2008 events) would have been equivalent to the energy produced if the entire mass of a Sun-like star were suddenly converted into pure radiation.

For a source to produce this much energy this quickly (in a burst) is a real challenge. Even if the star producing the gamma-ray burst was much more massive than the Sun (as is probably the case), there is no known means of converting so much mass into radiation within a matter of seconds. However, there is one way to reduce the power required of the “mechanism” that makes gamma-ray bursts . So far, our discussion has assumed that the source of the gamma rays gives off the same amount of energy in all directions, like an incandescent light bulb.

But as we discuss in Pulsars and the Discovery of Neutron Stars, not all sources of radiation in the universe are like this. Some produce thin beams of radiation that are concentrated into only one or two directions. A laser pointer and a lighthouse on the ocean are examples of such beamed sources on Earth (Figure 23.22). If, when a burst occurs, the gamma rays come out in only one or two narrow beams, then our estimates of the luminosity of the source can be reduced, and the bursts may be easier to explain. In that case, however, the beam has to point toward Earth for us to be able to see the burst. This, in turn, would imply that for every burst we see from Earth, there are probably many others that we never detect because their beams point in other directions.

### Long-Duration Gamma-Ray Bursts: Exploding Stars

After identifying and following large numbers of gamma-ray bursts, astronomers began to piece together clues about what kind of event is thought to be responsible for producing the gamma-ray burst. Or, rather, what kind of events, because there are at least two distinct types of gamma-ray burst s. The two—like the different types of supernovae—are produced in completely different ways.

Observationally, the crucial distinction is how long the burst lasts. Astronomers now divide gamma-ray bursts into two categories: short-duration ones (defined as lasting less than 2 seconds, but typically a fraction of a second) and long-duration ones (defined as lasting more than 2 seconds, but typically about a minute).

All of the examples we have discussed so far concern the long-duration gamma-ray burst s. These constitute most of the gamma-ray bursts that our satellites detect, and they are also brighter and easier to pinpoint. Many hundreds of long-duration gamma-ray bursts, and the properties of the galaxies in which they occurred, have now been studied in detail. Long-duration gamma-ray bursts are universally observed to come from distant galaxies that are still actively making stars. They are usually found to be located in regions of the galaxy with strong star-formation activity (such as spiral arms). Recall that the more massive a star is, the less time it spends in each stage of its life. This suggests that the bursts come from a young and short-lived, and therefore massive type of star.

Furthermore, in several cases when a burst has occurred in a galaxy relatively close to Earth (within a few billion light-years), it has been possible to search for a supernova at the same position—and in nearly all of these cases, astronomers have found evidence of a supernova of type Ic going off. A type Ic is a particular type of supernova, which we did not discuss in the earlier parts of this chapter these are produced by a massive star that has been stripped of its outer hydrogen layer. However, only a tiny fraction of type Ic supernova e produce gamma-ray bursts.

Why would a massive star with its outer layers missing sometimes produce a gamma-ray burst at the same time that it explodes as a supernova? The explanation astronomers have in mind for the extra energy is the collapse of the star’s core to form a spinning, magnetic black hole or neutron star . Because the star corpse is both magnetic and spinning rapidly, its sudden collapse is complex and can produce swirling jets of particles and powerful beams of radiation—just like in a quasar or active galactic nucleus (objects you will learn about Active Galaxies, Quasars, and Supermassive Black Holes), but on a much faster timescale. A small amount of the infalling mass is ejected in a narrow beam, moving at speeds close to that of light. Collisions among the particles in the beam can produce intense bursts of energy that we see as a gamma-ray burst.

Within a few minutes, the expanding blast from the fireball plows into the interstellar matter in the dying star’s neighborhood. This matter might have been ejected from the star itself at earlier stages in its evolution. Alternatively, it could be the gas out of which the massive star and its neighbors formed.

As the high-speed particles from the blast are slowed, they transfer their energy to the surrounding matter in the form of a shock wave. That shocked material emits radiation at longer wavelengths. This accounts for the afterglow of X-rays, visible light, and radio waves—the glow comes at longer and longer wavelengths as the blast continues to lose energy.

### Short-Duration Gamma-Ray Bursts: Colliding Stellar Corpses

What about the shorter gamma-ray bursts? The gamma-ray emission from these events lasts less than 2 seconds, and in some cases may last only milliseconds—an amazingly short time. Such a timescale is difficult to achieve if they are produced in the same way as long-duration gamma-ray bursts, since the collapse of the stellar interior onto the black hole should take at least a few seconds.

Astronomers looked fruitlessly for afterglows from short-duration gamma-ray burst s found by BeppoSAX and other satellites. Evidently, the afterglows fade away too quickly. Fast-responding visible-light telescopes like ROTSE were not helpful either: no matter how fast these telescopes responded, the bursts were not bright enough at visible wavelengths to be detected by these small telescopes.

Once again, it took a new satellite to clear up the mystery. In this case, it was the Swift Gamma-Ray Burst Satellite, launched in 2004 by a collaboration between NASA and the Italian and UK space agencies (Figure 23.23). The design of Swift is similar to that of BeppoSAX. However, Swift is much more agile and flexible: after a gamma-ray burst occurs, the X-ray and UV telescopes can be repointed automatically within a few minutes (rather than a few hours). Thus, astronomers can observe the afterglow much earlier, when it is expected to be much brighter. Furthermore, the X-ray telescope is far more sensitive and can provide positions that are 30 times more precise than those provided by BeppoSAX, allowing bursts to be identified even without visible-light or radio observations.

On May 9, 2005, Swift detected a flash of gamma rays lasting 0.13 seconds in duration, originating from the constellation Coma Berenices. Remarkably, the galaxy at the X-ray position looked completely different from any galaxy in which a long-duration burst had been seen to occur. The afterglow originated from the halo of a giant elliptical galaxy 2.7 billion light-years away, with no signs of any young, massive stars in its spectrum. Furthermore, no supernova was ever detected after the burst, despite extensive searching.

What could produce a burst less than a second long, originating from a region with no star formation? The leading model involves the merger of two compact stellar corpses: two neutron stars, or perhaps a neutron star and a black hole . Since many stars come in binary or multiple systems, it’s possible to have systems where two such star corpses orbit one another. According to general relativity (which will be discussed in Black Holes and Curved Spacetime), the orbits of a binary star system composed of such objects should slowly decay with time, eventually (after millions or billions of years) causing the two objects to slam together in a violent but brief explosion. Because the decay of the binary orbit is so slow, we would expect more of these mergers to occur in old galaxies in which star formation has long since stopped.

To learn more about the merger of two neutron stars and how they can produce a burst that lasts less than a second, check out this computer simulation by NASA.

While it was impossible to be sure of this model based on only a single event (it is possible this burst actually came from a background galaxy and lined up with the giant elliptical only by chance), several dozen more short-duration gamma-ray bursts have since been located by Swift, many of which also originate from galaxies with very low star-formation rates. This has given astronomers greater confidence that this model is the correct one. Still, to be fully convinced, astronomers are searching for a “smoking gun” signature for the merger of two ultra-dense stellar remnants.

Astronomers identified two observations that would provide more direct evidence. Theoretical calculations indicate that when two neutron stars collide there will be a very special kind of explosion neutrons stripped from the neutron stars during the violent final phase of the merger will fuse together into heavy elements and then release heat due to radioactivity, producing a short-lived but red supernova sometimes called a kilonova. (The term is used because it is about a thousand times brighter than an ordinary nova, but not quite as “super” as a traditional supernova.) Hubble observations of one short-duration gamma-ray burst in 2013 showed suggestive evidence of such a signature, but needed to be confirmed by future observations.

The second “smoking gun” is the detection of gravitational waves. As will be discussed in Black Holes and Curved Spacetime, gravitational waves are ripples in the fabric of spacetime that general relativity predicts should be produced by the acceleration of extremely massive and dense objects—such as two neutron stars or black holes spiraling toward each other and colliding. The construction of instruments to detect gravitational waves is very challenging technically, and gravitational wave astronomy became feasible only in 2015. The first few detected gravitational wave events were produced by mergers of black holes. In 2017, however, gravitational waves were observed from a source that was coincident in time and space with a gamma-ray burst. The source consisted of two objects with the masses of neutron stars. A red supernova was also observed at this location, and the ejected material was rich in heavy elements. This observation not only confirms the theory of the origin of short gamma-ray bursts, but also is a spectacular demonstration of the validity of Einstein’s theory of general relativity.

### Probing the Universe with Gamma-Ray Bursts

The story of how astronomers came to explain the origin of the different kinds of bursts is a good example of how the scientific process sometimes resembles good detective work. While the mystery of short-duration gamma-ray bursts is still being unraveled, the focus of studies for long-duration gamma-ray bursts has begun to change from understanding the origin of the bursts themselves (which is now fairly well-established) to using them as tools to understand the broader universe.

The reason that long-duration gamma-ray bursts are useful has to do with their extreme luminosities, if only for a short time. In fact, long-duration gamma-ray bursts are so bright that they could easily be seen at distances that correspond to a few hundred million years after the expansion of the universe began, which is when theorists think that the first generation of stars formed. Some theories predict that the first stars are likely to be massive and complete their evolution in only a million years or so. If this turns out to be the case, then gamma-ray bursts (which signal the death of some of these stars) may provide us with the best way of probing the universe when stars and galaxies first began to form.

So far, the most distant gamma-ray burst found (on April 29, 2009) was in a galaxy with a redshift that corresponds to a remarkable 13.2 billion light years—meaning it happened only 600 million years after the Big Bang itself. This is comparable to the earliest and most distant galaxies found by the Hubble Space Telescope. It is not quite old enough to expect that it formed from the first generation of stars, but its appearance at this distance still gives us useful information about the production of stars in the early universe. Astronomers continue to scan the skies, looking for even more distant events signaling the deaths of stars from even further back in time.

## Astronomers Detect Record-Breaking Gamma Ray Bursts From Colossal Explosion in Space

On the night of January 14, 2019, astronomer Razmik Mirzoyan got a call at his home in Germany. The observers on shift at the Major Atmospheric Gamma Imaging Cherenkov Telescope (MAGIC) in the Canary Islands were on the other line. Alerted by two space telescopes—the Neil Gehrels Swift Observatory and Fermi Gamma-ray Space Telescope—the two MAGIC telescopes were pointed in the direction of emissions from an immensely powerful cosmic outburst that were arriving at Earth. Within the first 20 minutes of observation, the telescopes detected a strong and increasing signal that seemed to be from a gamma ray burst, the most energetic type of explosion known to occur in the universe.

Mirzoyan told the observers to keep measuring.

That night Mirzoyan, who is a researcher at the Max Planck Institute for Physics in Munich, dashed off a short note on the Astronomer's Telegram, hoping other telescope operators would turn their machines toward the signal. He described how the MAGIC telescopes saw the highest energy emissions ever measured from a gamma ray burst (GRB), with photon energies of up to 1,000 billion electronvolts, or 1 teraelectronvolt (TeV). These were also the first observations of a gamma ray burst (GRB) by MAGIC or any other ground-based telescope.

Without any sleep, Mirzoyan headed to Arizona the next day to celebrate the inauguration of a next-generation gamma ray telescope at Whipple Observatory. By the time he arrived, word had spread about the detection. Everyone in the room was eager to shake Mirzoyan's hand and congratulate the MAGIC team, says Jamie Holder, an astronomer from the University of Delaware who was there. "Almost every conversation I had that week centered around the discovery," he says. "What have they seen? What does it mean? Can we see it, too?"

GRB 190114C, located about 4.5 billion light-years away in the constellation Fornax. (NASA / ESA / V. Acciari et al. 2019)

A few months later, another group of scientists went through their archived observations and found that they, too, detected GRB emissions from the ground. In July 2018, the High Energy Stereoscopic System (HESS) array of telescopes in Namibia detected the faint afterglow emission of another GRB 10 hours after the initial explosion. Even after nearly half a day, the afterglow still had photons with energies of 100 to 440 gigaelectronvolts. Both teams published their results in separate papers the journal Nature today.

"These ground-based telescopes have been operating for more than a decade, and GRBs have been one of their main targets, and this is the first time they actually detected them," says astrophysicist Bing Zhang of University of Nevada, Las Vegas, who was not involved in the research but wrote an editorial about the new papers for Nature.

Gamma rays are the highest-energy form of radiation, with wavelengths that can be smaller than the nucleus of an atom. (Radio waves, for comparison, have wavelengths ranging between about a millimeter to hundreds of kilometers.) Gamma ray bursts are phenomena that occur in distant galaxies, and astronomers believe the violent outbursts can happen when a massive star dies and collapses in on itself, resulting in a supernova. In one second, a GRB can release as much energy as the sun will produce in its lifetime. The light arrives at Earth as a prompt "flash" of gamma rays. This flash is associated with the highly energetic jets of plasma that form as the core of a dying star becomes a black hole or a neutron star, Holder says, and the afterglow that follows comes from the shock waves as this jet plows into in the surrounding region.

Compared to space-based telescopes, which have been observing GRBs for years, ground-based telescopes have much larger surfaces for detection, but they have the disadvantage of being beneath Earth's atmosphere, which absorbs gamma radiation. Until now, detecting a GRB from Earth's surface has proven elusive.

"Now we know that it is possible to observe GRBs from the ground, to high energies, long after the burst occurred," says Holder. "This will allow us to tune our search strategies to discover more bursts, and to study them as a population."

One of the telescopes at the MAGIC observatory that recently detected emissions from a powerful gamma ray burst. (Pachango / Wikicommons via CC BY-SA 3.0)

Both of the GRBs that were observed are believed to be the result of supernovas. The burst seen by MAGIC, called GRB 190114C, came from about 4.5 billion light-years away, and the one seen by HESS, named GRB 180720B, came from 6 billion light-years away.

The observations show that GRBs produce even more energetic emissions than previously known. Konstancja Satalecka, a scientist at the German Electron Synchrotron (DESY) who was part of the MAGIC collaboration, said in a statement that researchers were missing about half of the energy budget of GRBs until now. "Our measurements show that the energy released in very-high-energy gamma-rays is comparable to the amount radiated at all lower energies taken together," she said. "That is remarkable!”

Now scientists also know that GRBs are able to accelerate particles within the explosion ejecta. After ruling out other theoretical explanations, both teams of scientists have suggested that the very-high-energy gamma ray photons had been scattered by electrons while traveling through space, boosting their energy in a process known as inverse Compton scattering.

"These results are very exciting," Dan Hooper, head of the Theoretical Astrophysics Group at the Fermi National Accelerator Laboratory, says in an email. "Astrophysicists have long expected gamma-ray bursts to emit photons in this energy range (the teraelectronvolt range), but until now this had never been observed." Hooper was also surprised by how high-energy emissions were able to persist in the long afterglow of GRB 180720B. "Considering that the initial burst is measured in tens of seconds, a 10-hour afterglow at such high energies is a remarkable feature."

The findings from MAGIC and HESS have scientists even more excited for the next generation of gamma ray telescopes. The new telescope that Mirzoyan was celebrating in Arizona is a prototype for the Cherenkov Telescope Array (CTA) Observatory, which will consist of 118 telescopes being built in Chile and the Canary Islands. Once in operation, these telescopes will be able to detect gamma rays in the range of 20 GeV to 300 TeV, with about ten times better sensitivity than other current observatories.

Edna Ruiz-Velasco, a researcher at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, who is part of the HESS team, says these new observatories will be able to detect GRBs several days after the initial burst, covering longer timescales of the total emissions. Better detections might also help scientists investigate the possible connection between gamma ray bursts and gravitational waves, or the ripples in spacetime that scientists have only recently observed directly.

After decades of waiting, Mirzoyan says he thinks that observations of GRBs from the ground will become much more routine. Already, the HESS team posted another notice on the Astronomer's Telegram that they spotted another burst in August. With so much more data pouring in, astronomers may soon unravel the mysteries of the most immense explosions in the universe.

## Cosmic Jets

Among the oldest and brightest entities in the universe, quasars eject jets of very bright light that can be seen from lightyears away. It was initially believed that different events were being seen when quasars were observed, but it was later established that our line of sight affected the appearance of the quasar, for example a blazar is a quasar with jets that are pointing towards Earth. Image credit: ESO/M. Kornmesser Cosmic jets are seen in all kinds of objects (including GRBs , active galactic nuclei, quasars, and radio galaxies), but how they're formed is not yet understood. The phenomenon is defined as streams of matter being emitted along the axis of rotation of a compact object, and are a staple component of most artists' impressions of such objects.

Although researchers are not clear as to exactly what causes these jets, their focus tends to be on the central body – such as a black hole – or the surrounding accretion disc.

Fast radio bursts that lasted for 4 milliseconds have been detected from 5.5-10.4 billion light-years away. Weirdly, no associated gamma, X-ray, optical or gravitational wave signatures were detected with the burst, and there were no repeat events. Scientists think this points to a cataclysmic source, which could be soft gamma-ray repeaters or core-collapse supernova (ccSN) orbiting neutron stars.

High-energy neutrinos may originate from high energy cosmic events and, as such, could provide information on objects such as black holes and gamma-ray bursts. They interact only weakly with matter, and so pass straight through bodies such as the earth and are unperturbed by gravity. The IceCube Neutrino Observatory recently claimed to have detected such neutrinos, although their results ares unconfirmed and, if so, what they can tell us.

## Why are there no gamma-ray bursts detected in our galaxy? - Astronomy

A number of satellites have been built to observe GRBs. The Compton Gamma Ray Observatory determined that GRBs were from outside our Galaxy. (There is a class of gamma-ray objects within our galaxy, but not with the extreme power of GRBs). Some have speculated that GRBs are possibly at the edge of the early Universe and the death throes of extremely massive stars, which only lasted about 1 million years. These stars appear to eject Gamma Rays after a hypernova event , an extreme supernova which produces gamma radiation. GRBs also appear to come from within stellar nurseries.

Public Domain | Image courtesy of NASA, ESA, N. Tanvir (University of Leicester), A. Fruchter (STScI), and A. Levan (University of Warwick).

## We may have seen a huge explosion in the oldest galaxy in the universe

An explosion of high-energy radiation may have been spotted coming from a galaxy in the distant universe. If confirmed, it would be the oldest known gamma-ray burst, occurring about 400 million years after the big bang.

Linhua Jiang at Peking University in Beijing, China, and his colleagues were using the Keck Observatory in Hawaii to study the faintest and oldest known galaxy in the universe, GN-z11, when they saw the galaxy appear to grow hundreds of times brighter for just under 3 minutes.

The researchers think this could have been a gamma-ray burst, a type of extremely luminous event that has been seen in other galaxies and is thought to occur when certain giant stars explode in a supernova.

We see GN-z11 as it looked 13.4 billion years ago, meaning it was one of the first galaxies to form after the big bang. However, it is actually located about 32 billion light years from Earth owing to the universe’s expansion. This expansion also stretched the duration of the event Jiang and his colleagues saw, which would probably have lasted for only around 20 seconds.

Previously, the most distant known gamma-ray burst occurred about 100 million years later, roughly 500 million years after the big bang, which would make GN-z11’s event the oldest yet spotted, suggesting galaxies in the early universe were more active than thought.

#### Read more: We’ve found the oldest ever galaxy that looks like our own

This is because gamma-ray bursts should be extremely rare, says Jiang. “The probability to detect a gamma-ray burst [in a particular galaxy] is near to zero,” he says. “If you observed a galaxy for a million years, you’d probably find [only] a few gamma-ray bursts. That’s why it’s so surprising.”

What isn’t clear is whether a rogue signal from a satellite or something in our solar system like an asteroid could have been the cause of the event. Although it is unlikely, Jiang says there will be no way to ever know for sure, given the event has passed.

However, the brightness and duration of the event point to a gamma-ray burst and it is possible more could be found from this era. “Either we were so lucky or the gamma-ray burst rate is higher than what we expected,” says Jiang.

Journal reference: Nature Astronomy, DOI: 10.1038/s41550-020-01275-y

## Sky survey reveals first 'orphan' gamma ray burst

Astronomers comparing data from an ongoing major survey of the sky using the National Science Foundation's Karl G. Jansky Very Large Array (VLA) to data from earlier surveys likely have made the first discovery of the afterglow of a powerful gamma ray burst that produced no gamma rays detectable at Earth. The unprecedented discovery of this "orphan" gamma ray burst (GRB) offers key clues to understanding the aftermath of these highly energetic events.

"GRBs emit their gamma rays in narrowly focused beams. In this case, we believe the beams were pointed away from Earth, so gamma ray telescopes did not see this event. What we found is the radio emission from the explosion's aftermath, acting over time much as we expect for a GRB," said Casey Law, of the University of California, Berkeley.

While searching through data from the first epoch of observing for the VLA Sky Survey (VLASS) in late 2017, the astronomers noted that an object that appeared in images from an earlier VLA survey in 1994 did not appear in the VLASS images. They then searched for additional data from the VLA and other radio telescopes. They found that observations of the object's location in the sky dating back as far as 1975 had not detected it until it first appeared in a VLA image from 1993.

The object then appeared in several images made with the VLA and the Westerbork telescope in the Netherlands from 1993 through 2015. The object, dubbed FIRST J1419+3940, is in the outskirts of a galaxy more than 280 million light-years from Earth.

"This is a small galaxy with active star formation, similar to others in which we have seen the type of GRBs that result when a very massive star explodes," Law said.

The strength of the radio emission from J1419+3940 and the fact that it slowly evolved over time support the idea that it is the afterglow of such a GRB, the scientists said. They suggested that the explosion and burst of gamma rays should have been seen sometime in 1992 or 1993.

However, after searching databases from gamma ray observatories, "We could find no convincing candidate for a detected GRB from this galaxy," Law said.

While there are other possible explanations for the object's behavior, the scientists said that a GRB is the most likely.

"This is exciting, and not just because it probably is the first 'orphan' GRB to be discovered. It also is the oldest well-localized GRB, and the long time period during which it has been observed means it can give us valuable new information about GRB afterglows," Law said.

"Until now, we've never seen how the afterglows of GRBs behave at such late times," noted Brian Metzger of Columbia University, co-author of the study. "If a neutron star is responsible for powering the GRB and is still active, this might give us an unprecedented opportunity to view this activity as the expanding ejecta from the supernova explosion finally becomes transparent."

"I'm delighted to see this discovery, which I expect will be the first of many to come from the unique investment the National Radio Astronomy Observatory (NRAO) and the National Science Foundation are making in VLASS," said NRAO Director Tony Beasley.

VLASS is the largest observing project in the history of the VLA. Begun in 2017, the survey will use 5,500 hours of observing time over seven years. The survey will make three complete scans of the sky visible from the VLA, roughly 80 percent of the sky. Initial images from the first round of observations now are available to astronomers.

VLASS follows two earlier sky surveys done with the VLA. The NRAO VLA Sky Survey (NVSS), like VLASS, was an all-sky survey done from 1993 to 1996, and the FIRST (Faint Images of the Radio Sky at Twenty centimeters) survey studied a smaller portion of the sky in more detail from 1993 to 2002. The astronomers discovered FIRST J1419+3940 by comparing a 1994 image from the FIRST survey to the VLASS 2017 data.

From 2001 to 2012, the VLA underwent a major upgrade, greatly increasing its sensitivity, or ability to image faint objects. The upgrade made possible a new, improved survey offering a rich scientific payoff. The earlier surveys have been cited more than 4,500 times in scientific papers, and scientists expect VLASS to be a valuable resource for research in the coming years.

Law and his colleagues are publishing their findings in the Astrophysical Journal Letters.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.