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As follow-up question for my previous question I'm thinking about the search for the electromagnetic (EM) counterpart of a gravitational wave (GW). Is it possible to search for those events with 'normal' telescope, the ones (professional) amateur astronomers have? What I mean with a (professional) amateur astronomer is: a telescope with an aperture of 10" or more (remote) and the ability to do long exposures.
Is that equipment capable of detecting the EM counterpart of a gravitational wave (in theory)? Thanks in advance!
EDIT: I'm not sure if there is an EM counterpart in the visible light at all. Is this the case? Can such an event, that triggers a detectable GW, been seen in the visible light (by extension: with 'amateur' telescopes as stated above)?
The Neutron star merger GW170817 had an optical counterpart (SSS 17a) The optical magnitude peaked at about +18, this is beyond the abilities of most amateur set ups.
Black hole mergers may be intrinsically more energetic, but less bright in the electromagnetic spectrum, as the black holes don't have a surface to interact electromagnetically.
However each gravitational wave detection is examined to see if there is a corresponding gamma ray burst, or optical transient. Combining results of gravitational, neutrino and optical observations has been called "multi messenger astronomy".
Seeing the light of neutron star collisions
When two neutron stars collided on Aug. 17, a widespread search for electromagnetic radiation from the event led to observations of light from the afterglow of the explosion, finally connecting a gravitational-wave-producing event with conventional astronomy using light, according to an international team of astronomers.
Previous gravitational-wave detections by LIGO (Laser Interferometer Gravitational-Wave Observatory) and Virgo, a European observatory based in Pisa, Italy, were caused by collisions of two black holes. Black hole collisions generally are not expected to result in electromagnetic emissions and none were detected.
"A complete picture of compact object mergers, however, requires the detection of an electromagnetic counterpart," the researchers report online today (Oct. 16) in Science.
The August 17 detection of a gravitational wave from the collision of two neutron stars by gravitational wave observatories in the U.S. and Europe initiated a rapid cascade of observations by a variety of orbiting and ground-based telescopes in search of an electromagnetic counterpart.
Two seconds after detection of the gravitational wave, the Gamma Ray Burst monitor on NASA's Fermi spacecraft detected a short gamma ray burst in the area of the gravitational wave's origin.
While the Swift Gamma Ray Burst Explorer -- a NASA satellite in low Earth orbit containing three instruments -- the Burst Alert Telescope, the X-ray Telescope and the Ultraviolet/Optical Telescope -- can view one-sixth of the sky at a time, it did not see the gamma ray burst because that portion of the sky was not then visible to Swift. Penn State is in charge of the Mission Operations Center for Swift orbits the Earth every 96 minutes and can maneuver to observe a target in as little as 90 seconds.
Once the Swift team knew the appropriate area to search, it put the satellite's instruments into action. Swift is especially valuable in this type of event because it can reposition to a target very quickly. In this case, the telescope was retargeted approximately 16 minutes after being notified by LIGO/Virgo, and began to search for an electromagnetic counterpart.
Initially, because of the predictions of theoretical models, the researchers thought that the electromagnetic radiation they would see would be X-rays. This is why NASA's NuSTAR, (Nuclear Spectroscopic Telescope Array), which looks at X-rays, also searched the sky for electromagnetic signals. Neither Swift nor NuSTAR detected any X-rays.
"For gamma ray bursts, models predict that an early X-ray emission would be seen," said Aaron Tohuvavohu, Swift science operations and research assistant, Penn State. "But there were none detectable from this event until 9 days post-merger."
Instead, Swift identified a rapidly fading ultraviolet afterglow.
"The early UV emission was unexpected and very exciting," Tohuvavohu added.
Gamma ray bursts appear as a directional burst of energy from collapsed massive stars. Any type of detector must be within a certain arc of the burst to see it. The afterglow of the explosion, is however, more omnidirectional.
"Whatever we thought was going to happen, wasn't what actually happened," said Jamie A Kennea, head, Swift Science Operations team and associate research professor of astronomy and astrophysics, Penn State. "The next neutron star-neutron star merger event could look very different."
The combination of location data from the various observations of the event presented a good estimate of where the two stars were in the universe.
"Swift tiled the entire field in the area identified and did not find anything else that could have caused the emission," said Michael H. Siegel, associate research professor and head of the Ultraviolet Optical Telescope team, Penn State. "We are confident that this is the counterpart to the detected gravitational wave that LIGO saw."
The Swift discovery is spectacular because it is associated with a gravitational wave event which makes this a bona-fide double neutron star merger, said Peter Mészáros, Eberly Chair of Astronomy and Astrophysics and professor of physics, Penn State, who has studied gamma ray bursts and gravitational waves extensively.
"The thing that is surprising is that we now have only optical but not X-ray emissions," said Mészáros. "Typically, a neutron star-neutron star merger should have X-rays for a long time with optical emissions fading relatively faster. The only thing one can infer from this, based on the models that I and others have developed, is that the X-ray beam is narrower and not directed straight at us."
In this case, the merger would have produced X-rays, but they would have been pointed in a direction away from the Earth, preventing Swift and NuSTAR from detecting the initial X-ray emissions.
Mészáros notes that the gravitational waves looked like they came from objects smaller in mass than black holes, which pointed to neutron stars, and that the electromagnetic emissions separately correlated to the event provide two ways to show proof-positive that this is a neutron star merger.
The neutron star-neutron star collision occurred 130 million light years away in another galaxy. A light year is the distance light can travel in one year, which is almost 6 trillion miles.
According to the researchers, this event was close to our solar system by astronomical standards. The black hole-black hole collisions originally detected by LIGO, in contrast, were billions of light years away.
"A neutron star-neutron star collision was our best hope for an electromagnetic signature," said Kennea. "But it is still surprising that we got one on our first neutron star-neutron star collision."
Other Penn State researchers on the project were D.N. Burrows, professor C. Gronwall, research professor J.A. Nousek, professor and B. Sbarufatti, assistant research professor, all in the Department of Astronomy and Astrophysics.
Other researchers on this paper come from 27 different institutions. NASA supported this research.
ESO telescopes observe first light from gravitational wave source
For the first time ever, astronomers have observed both gravitational waves and light (electromagnetic radiation) from the same event, thanks to a global collaborative effort and the quick reactions of both ESO's facilities and others around the world.
On 17 August 2017 the NSF 's Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States, working with the Virgo Interferometer in Italy, detected gravitational waves passing the Earth. This event, the fifth ever detected, was named GW170817. About two seconds later, two space observatories, NASA's Fermi Gamma-ray Space Telescope and ESA's INTErnational Gamma Ray Astrophysics Laboratory (INTEGRAL), detected a short gamma-ray burst from the same area of the sky.
The LIGO-Virgo observatory network positioned the source within a large region of the southern sky, the size of several hundred full Moons and containing millions of stars . As night fell in Chile many telescopes peered at this patch of sky, searching for new sources. These included ESO's Visible and Infrared Survey Telescope for Astronomy (VISTA) and VLT Survey Telescope (VST) at the Paranal Observatory, the Italian Rapid Eye Mount (REM) telescope at ESO's La Silla Observatory , the LCO 0.4-meter telescope at Las Cumbres Observatory, and the American DECam at Cerro Tololo Inter-American Observatory. The Swope 1-metre telescope was the first to announce a new point of light. It appeared very close to NGC 4993, a lenticular galaxy in the constellation of Hydra (constellation), and VISTA observations pinpointed this source at infrared wavelengths almost at the same time. As night marched west across the globe, the Hawaiian island telescopes Pan-STARRS and Subaru also picked it up and watched it evolve rapidly.
"There are rare occasions when a scientist has the chance to witness a new era at its beginning," said Elena Pian, astronomer with INAF, Italy, and lead author of one of the Nature papers. "This is one such time!"
ESO launched one of the biggest ever "target of opportunity" observing campaigns and many ESO and ESO-partnered telescopes observed the object over the weeks following the detection . ESO's Very Large Telescope (VLT), New Technology Telescope, VST, the MPG/ESO 2.2-metre telescope, and the Atacama Large Millimeter/submillimeter Array (ALMA)  all observed the event and its after-effects over a wide range of wavelengths. About 70 observatories around the world also observed the event, including the NASA/ESA Hubble Space Telescope.
Distance estimates from both the gravitational wave data and other observations agree that GW170817 was at the same distance as NGC 4993, about 130 million light-years from Earth. This makes the source both the closest gravitational wave event detected so far and also one of the closest gamma-ray burst sources ever seen .
The ripples in spacetime known as gravitational waves are created by moving masses, but only the most intense, created by rapid changes in the speed of very massive objects, can currently be detected. One such event is the merging of neutron stars , the extremely dense, collapsed cores of high-mass stars left behind after supernovae . These mergers have so far been the leading hypothesis to explain short gamma-ray bursts. An explosive event 1000 times brighter than a typical nova -- known as a kilonova -- is expected to follow this type of event.
The almost simultaneous detections of both gravitational waves and gamma rays from GW170817 raised hopes that this object was indeed a long-sought kilonova and observations with ESO facilities have revealed properties remarkably close to theoretical predictions. Kilonovae were suggested more than 30 years ago but this marks the first confirmed observation.
Following the merger of the two neutron stars, a burst of rapidly expanding radioactive heavy chemical elements left the kilonova, moving as fast as one-fifth of the speed of light. The colour of the kilonova shifted from very blue to very red over the next few days, a faster change than that seen in any other observed stellar explosion.
"When the spectrum appeared on our screens I realised that this was the most unusual transient event I'd ever seen," remarked Stephen Smartt, who led observations with ESO's NTT as part of the extended Public ESO Spectroscopic Survey of Transient Objects (ePESSTO) observing programme. "I had never seen anything like it. Our data, along with data from other groups, proved to everyone that this was not a supernova or a foreground variable star, but was something quite remarkable."
Spectra from ePESSTO and the VLT's X-shooter instrument suggest the presence of caesium and tellurium ejected from the merging neutron stars. These and other heavy elements, produced during the neutron star merger, would be blown into space by the subsequent kilonova. These observations pin down the formation of elements heavier than iron through nuclear reactions within high-density stellar objects, known as r-process nucleosynthesis, something which was only theorised before.
"The data we have so far are an amazingly close match to theory. It is a triumph for the theorists, a confirmation that the LIGO-VIRGO events are absolutely real, and an achievement for ESO to have gathered such an astonishing data set on the kilonova," adds Stefano Covino, lead author of one of the Nature Astronomy papers.
"ESO's great strength is that it has a wide range of telescopes and instruments to tackle big and complex astronomical projects, and at short notice. We have entered a new era of multi-messenger astronomy!" concludes Andrew Levan, lead author of one of the papers.
 The LIGO-Virgo detection localised the source to an area on the sky of about 35 square degrees.
[2 The galaxy was only observable in the evening in August and then was too close to the Sun in the sky to be observed by September.
 On the VLT, observations were taken with: the X-shooter spectrograph located on Unit Telescope 2 (UT2) the FOcal Reducer and low dispersion Spectrograph 2 (FORS2 and Nasmyth Adaptive Optics System (NAOS) -- Near-Infrared Imager and Spectrograph (CONICA) (NACO) on Unit Telescope 1 (UT1) VIsible Multi-Object Spectrograph (VIMOS) and VLT Imager and Spectrometer for mid-Infrared (VISIR) located on Unit Telescope 3 (UT3) and the Multi Unit Spectroscopic Explorer (MUSE and High Acuity Wide-field K-band Imager (HAWK-I -- http://eso.org/public/teles-instr/paranal-observatory/vlt/vlt-instr/hawk-i/) on Unit Telescope 4 (UT4). The VST observed using the OmegaCAM and VISTA observed with the VISTA InfraRed CAMera (VIRCAM). Through the ePESSTO programme, the NTT collected visible spectra with the ESO Faint Object Spectrograph and Camera 2 (EFOSC2 ) spectrograph and infrared spectra with the Son of ISAAC (SOFI) spectrograph. The MPG/ESO 2.2-metre telescope observed using the Gamma-Ray burst Optical/Near-infrared Detector (GROND ) instrument.
 The comparatively small distance between Earth and the neutron star merger, 130 million light-years, made the observations possible, since merging neutron stars create weaker gravitational waves than merging black holes, which were the likely case of the first four gravitational wave detections.
 When neutron stars orbit one another in a binary system, they lose energy by emitting gravitational waves. They get closer together until, when they finally meet, some of the mass of the stellar remnants is converted into energy in a violent burst of gravitational waves, as described by Einstein's famous equation E=mc2.
This research was presented in a series of papers to appear in Nature, Nature Astronomy and Astrophysical Journal Letters.
First Cosmic Event Observed in Both Gravitational Waves and Light
About 130 million years ago, in a galaxy far away, two neutron stars collided. The cataclysmic crash produced gravitational waves, ripples in the fabric of space and time. This event is now the 5th observation of gravitational waves by the Laser Interferometer Gravitational wave Observatory (LIGO) and Virgo collaboration, and the first detected that was not caused by the collision of two black holes.
But this event — called a kilonova — produced something else too: light, across multiple wavelengths.
For the first time in history, an astronomical phenomenon has been first observed through gravitational waves and then seen with telescopes. In an incredibly collaborative effort, over 3,500 astronomers using 100 instruments on over 70 telescopes around the world and in space worked with physicists from the LIGO and Virgo collaboration.
Scientists call this “multimessenger astronomy.”
“Together, all these observations are bigger than the sum of their parts,” said Laura Cadonati, LIGO’s Deputy Spokesperson at a briefing today. “We are now learning about the physics of the universe, about the elements we are made of, in a way that no one has ever done before.”
Observations of the kilonova. Credit: P.K. Blanchard/ E. Berger/ Pan-STARRS/DECam.
“It will give us insight into how supernova explosions work, how gold and other heavy elements are created, how the nuclei in our body works and even how fast the universe is expanding,” said Manuela Campanelli, from the Rochester Institute of Technology. “Multimessenger astronomy demonstrates how we can combine the old way with the new. It has changed the way astronomy is done.”
Neutron stars are the crushed leftover cores of massive stars that long ago exploded as supernovae. The two stars, located near each other in a galaxy called NGC 4993, started out between 8-20 times the mass of our sun. Then with their supernovas, each condensed down to about 10 miles in diameter, the size of a city. These are stars composed entirely of neutrons and are in-between normal stars and black holes in size and density — just a teaspoon of neutron star material would weigh 1 billion tons.
They spun around each other in a cosmic dance until their mutual gravity caused them to collide. That collision produced a fireball of astronomical proportions and the repercussions of that event arrived at Earth 130 million years later.
“While this event took place 130 million years ago, we only found out about this on Earth on August 17, 2017, just before the solar eclipse,” said Andy Howell from the Las Cumbres Observatory, speaking at a press briefing today. “We’ve been keeping this secret the whole time and we’re about to bust!”
At 8:41 am EDT, LIGO and Virgo felt the early tremors of the ripples of spacetime, gravitational waves. Just two seconds later, a bright flash of gamma rays was detected by NASA’s Fermi space telescope. This allowed researchers to quickly pinpoint the direction from which the waves were coming.
Alerted by an Astronomers Telegram, thousands of astronomers around the world scrambled to make observations and begin collecting additional data from the neutron star merger.
This animation shows how LIGO, Virgo, and space- and ground-based telescopes zoomed in on the location of gravitational waves detected August 17, 2017 by LIGO and Virgo. By combining data from the Fermi and Integral space missions with data from LIGO and Virgo, scientists were able to confine the source of the waves to a 30-square-degree sky patch. Visible-light telescopes searched a large number of galaxies in that region, ultimately revealing NGC 4993 to be the source of gravitational waves. (This event was later designated as GW170817.)
“This event has the most precise sky localization of all detected gravitational waves so far,” Jo van den Brand, spokesperson for the Virgo collaboration, said in a statement. “This record precision enabled astronomers to perform follow-up observations that led to a plethora of breathtaking results.”
This provides the first real evidence that light and gravitational waves travel at the same speeds – near the speed of light — as Einstein predicted.
The kilonova became red and faded by a factor of over 20 in just a few days. This rapid change was captured by Las Cumbres Observatory telescopes as night time moved around the globe. Credit: Sarah Wilkinson / LCO.
Observatories from the very small to the most well-known were involved, quickly making observations. While bright at first, the event faded in less than 6 days. Howell said the observed light was 2 million times brighter than the Sun over the course of the first few hours, but it then faded over a few days.
The Dark Energy Camera (DECam), which is mounted on the Blanco 4-meter Telescope at the Cerro Tololo Inter-American Observatory in the Chilean Andes was one of the instruments that helped localize the source of the event.
“The challenge that we face every time that the LIGO collaboration issues a new observational trigger is how do we search for a source that is rapidly fading, was possibly faint to begin with, and is located somewhere over there,” said Marcelle Soares-Santos, from Brandeis University at the briefing. She is the first author on the paper describing the optical signal associated with the gravitational waves. “It’s the classical challenge of finding a needle in a haystack with the added complication that the needle is far away and haystack is moving.”
With the DECam, they were quickly able to determine the source galaxy, and rule out 1,500 other candidates that were present in that haystack.
“Things that look like these ‘needles’ are very common, so we need to make sure we have the right one. Today, we are certain we have,” Soares-Santos added.
In the very small department, a small robotic 16-inch telescope called PROMPT (Panchromatic Robotic Optical Monitoring and Polarimetry Telescope) — which astronomer David Sand from the University of Arizona described at “basically a souped-up amateur telescope,” — also helped determine the source. Sand said this proves that even small telescopes can play a roll in multimessenger astronomy.
The well known is led by Hubble and several other NASA and ESA space observatories, such as the Swift, Chandra and Spitzer missions. Hubble captured images of the galaxy in visible and infrared light, witnessing a new bright object within NGC 4993 that was brighter than a nova but fainter than a supernova. The images showed that the object faded noticeably over the six days of the Hubble observations. Using Hubble’s spectroscopic capabilities the teams also found indications of material being ejected by the kilonova as fast as one-fifth of the speed of light.
“This is a game-changer for astrophysics,” said Howell. “A hundred years after Einstein theorized gravitational waves, we’ve seen them and traced them back to their source to find an explosion with new physics of the kind we only dreamed about before.”
Here are just a few of insights this single event created, using multimessenger astronomy:
* Gamma rays: These flashes of light are now definitively associated with merging neutron stars and will help scientists figure out how supernova explosions work, explained Richard O’Shaughnessy, also from Rochester Institute of Technology and a member of the LIGO team. “The initial gamma-ray measurements, combined with the gravitational-wave detection, further confirm Einstein’s general theory of relativity, which predicts that gravitational waves should travel at the speed of light,” he said.
* The source of gold and platinum: “These observations reveal the direct fingerprints of the heaviest elements in the periodic table,” said Edo Berger, from the Harvard Smithsonian Center for Astrophysics, speaking at the briefing. “The collision of the two neutron stars produced 10 times of mass of Earth in gold and platinum alone. Think about how as these materials are flying out of this event, they eventually combine with other elements to form stars, planets, life … and jewelry.”
Berger added something else to think about: the original supernova explosions of these stars produced all the heavy elements up to iron and nickel. Then in the kilonova in this one system, we can see the complete history of how the periodocial table of the heavy elements came into being.
Howell said that when you split the signatures of the heavy elements into a spectrum, you create a rainbow. “So there really was a pot of gold at the end of the rainbow, at least a kilonova rainbow,” he joked.
* Nuclear physics astronomy: “Eventually, more observations like this discovery will tell us how the nuclei in our body works,” O’Shaughnessy said. “The effects of gravity on neutron stars will tell us how big balls of neutrons behave, and, by inference, little balls of neutrons and protons — the stuff inside of our body that makes up most of our mass” and
* Cosmology:- “Scientists now can independently measure how fast the universe is expanding by comparing the distance to the galaxy containing the bright flare of light and distance inferred from our gravitational wave observation,” said O’Shaughnessy.
“The ability to study the same event with both gravitational waves and light is a real revolution in astronomy,” said astronomer Tony Piro from the CfA. “We can now study the universe with completely different probes, which teaches things we could never know with only one or the other.”
“For me, what made this event so amazing is that not only did we detect gravitational waves, but we saw light across the electromagnetic spectrum, seen by 70 observatories around the world,” said David Reitz, scientific spokesman for LIGO, at today’s press briefing. “This is the first time the cosmos has provided to us the equivalent of movies with sound. The video is the observational astronomy across various wavelengths and the sound is gravitational waves.”
Ripples in the fabric of space-time
Einstein predicted the existence of gravitational waves in 1916 in his general theory of relativity, but until recently they were impossible to observe. LIGO’s extraordinarily sensitive detectors achieved the first direct detection of gravitational waves, from the collision of two black holes, in 2015. Gravitational waves are created by any massive accelerating object, but the strongest waves (and the only ones we have any chance of detecting) are produced by the most extreme phenomena.
Two massive compact objects—such as black holes, neutron stars, or white dwarfs—orbiting around each other faster and faster as they draw closer together are just the kind of system that should radiate strong gravitational waves. Like ripples spreading in a pond, the waves get smaller as they spread outward from the source. By the time they reached Earth, the ripples detected by LIGO caused distortions of space-time thousands of times smaller than the nucleus of an atom.
The rarefied signals recorded by LIGO’s detectors not only prove the existence of gravitational waves, they also provide crucial information about the events that produced them. Combined with the telescope observations of the neutron star merger, it’s an incredibly rich set of data.
LIGO can tell scientists the masses of the merging objects and the mass of the new object created in the merger, which reveals whether the merger produced another neutron star or a more massive object that collapsed into a black hole. To calculate how much mass was ejected in the explosion, and how much mass was converted to energy, scientists also need the optical observations from telescopes. That’s especially important for quantifying the nucleosynthesis of heavy elements during the merger.
LIGO can also provide a measure of the distance to the merging neutron stars, which can now be compared with the distance measurement based on the light from the merger. That’s important to cosmologists studying the expansion of the universe, because the two measurements are based on different fundamental forces (gravity and electromagnetism), giving completely independent results.
“This is a huge step forward in astronomy,” Foley said. “Having done it once, we now know we can do it again, and it opens up a whole new world of what we call ‘multi-messenger’ astronomy, viewing the universe through different fundamental forces.”
What We’re Learning
In the few years since the direct detection of gravitational waves, these barely perceptible bends in spacetime have taught us a lot about our Universe.
- How Old is the Universe? In the case of the neutron star collision, by measuring the strength of gravitational waves, we’re able to compute a distance to the event and its host galaxy, NGC 4993. We know that the further a galaxy is, the faster it moves away. When we measure how the the light from NGC 4993 is stretched, or redshifted, we know how fast it is moving. With these values, we can work backward and calculate the age of the Universe. This novel way of dating the Universe agrees with the currently accepted age of 13.8 billion years.
- Where Gamma-Ray Bursts Come From. Since the late 60’s, scientists have observed short bursts of high-energy gamma-ray radiation but could not pinpoint their origin. After detecting a gamma-ray burst and the gravitational wave event almost simultaneously and in the same area of the sky, it was determined that neutron star mergers must be the source.
- Origin of Heavy Elements. Heavy elements like gold and platinum were thought to be created in hot radioactive events, like supernovae explosions. But the amount of these elements observed in supernova remnants was less than sufficient to explain the abundance we see in the Universe. After the 2017 neutron star merger, astronomers saw the radioactive aftermath suggesting that neutron star collisions are the perfect factories for heavy elements. That one collision alone formed several Earth masses of gold and platinum. We now know that these events are responsible for most of the heavy elements in the Universe.
- Test of Dark Matter. Some theories have attempted to explain the peculiar motion of galaxies and clusters of galaxies without invoking dark matter, the invisible material that makes up 80% of the matter in the Universe. This involved altering the current model of gravity to fit the observations. While the theory of general relativity says that light and gravity travel at the same speed, many of these adjusted models require them to be different. But after traveling 130 million light years, the 2017 gravitational wave arrived 1.7 seconds before the corresponding electromagnetic radiation. This means that the speeds couldn’t differ by more than 1 in 1,000,000,000,000,000. In other words, they’re pretty much equal.
Of course, we’re not done learning from gravitational waves. By continuing to study these flickers in spacetime, we may be rewarded with the discovery of new particles, new models for what happens to matter at extreme densities, and a deeper understanding of gravity itself. Gravitational waves have opened a new realm of astronomy.
Counterparts to Gravitational Wave Events: Very Important Needles in a Very Large Haystack
First Author’s Institutions: Universita degli Studi di Milano-Bicocca, Milano, Italy INAF – Osservatorio Astronomico di Brera Merate, Merate, Italy INFN – Sezione di Milano-Bicocca, Milano, Italy
Status: Submitted to ApJ [ open access ]
The LIGO Scientific Collaboration’s historic direct detection of gravitational waves (GWs) brought with it the promise of answers to long-standing astrophysical puzzles that were unsolvable with traditional electromagnetic (EM) observations. In previous astrobites, we’ve mentioned that an observational approach that involves both the EM and GW windows into the Universe can help shed light on mysteries such as the neutron star (NS) equation of state, and can serve as a unique test of general relativity. Today’s paper highlights the biggest hinderance to EM follow-up of GW events: the detection process doesn’t localize the black hole (BH) and NS mergers well enough to inform a targeted observing campaign with radio, optical, and higher-frequency observatories. While EM counterparts to GW-producing mergers are a needle that’s likely worth searching an entire haystack for, the reality is that telescope time is precious, and everyone needs a chance to use these instruments for widely varying scientific endeavors.
The first GW detection by LIGO, GW150914, was followed up by many observatories that agreed ahead of time to look for EM counterparts to LIGO triggers. The authors of this study propose to improve upon the near-aimless searches in swaths of hundreds of degrees that have been necessary following the first few GW candidate events (see Figure 1). Luckily, there are two key pieces of information we have a priori (in advance): information about the source of the GW signal that can be pulled out of the LIGO data, and an understanding of the EM signal that will be emitted during significant GW-producing events.
Figure 1: Simplified skymaps for the two likely and one candidate (LVT151012) GW detections as 3-D projections onto the Milky Way. The largest contours are 90-percent confidence intervals, while the innermost are 10-percent contours. From the LIGO Scientific Collaboration.
What are we even looking for?
Mergers that produce strong GW signals include BH-BH, BH-NS, and NS-NS binary inspirals. GW150914 was a BH-BH merger, which is less likely to produce a strong EM counterpart due to a lack of circumbinary material. The authors of this work therefore focus on the two most likely signals following a BH-NS or NS-NS merger. The first is a short gamma-ray burst (sGRB), which would produce an immediate (“prompt”) gamma-ray signal and a longer-lived “afterglow” in a large range of frequencies. Due to relativistic beaming, it’s rare that prompt sGRB emission is detected, as jets must be pointing in our direction to be seen. GRB afterglows are more easily caught, however. The second is “macronova” emission from material ejected during the merger, which contains heavy nuclei that decay and produce a signal in the optical and infrared shortly after coalescence. One advantage to macronova events is that they’re thought to be isotropic (observable in all directions), so they’ll be more easily detected than the beamed, single-direction sGRBs.
(Efficiently) searching through the haystack
LIGO’s direct GW detection method yields a map showing the probability of the merger’s location on the sky (more technically, the posterior probability density for sky position, or “skymap”). The uncertainty in source position is partly so large because many parameters gleaned from the received GW signal, like distance, inclination, and merger mass, are degenerate. In other words, many different combinations of various parameters can produce the same received signal.
An important dimension that’s missing from the LIGO skymap is time. No information can be provided about the most intelligent time to start looking for the EM counterpart after receiving the GW signal unless the search is informed by information about the progenitor system. In order to produce a so-called “detectability map” showing not only where the merger is possibly located but also when we’re most likely to observe the resulting EM signal at a given frequency, the authors follow an (albeit simplified) procedure to inform their searches.
The first available pieces of information are the probability that the EM event, at some frequency, will be detectable by a certain telescope, and the time evolution of the signal strength. This information is available a priori given a model of the sGRB or macronova. Then, LIGO will detect a GW signal, from which information about the binary inspiral will arise. These parameters are combined with the aforementioned progenitor information to create a map that helps inform not only where the source will most likely be, but also when various observatories should look during the EM follow-up period. Such event-based, time-dependent detection maps will be created after each GW event, allowing for a much more responsive search for EM counterparts.
Figure 2: The suggested radio telescope campaign for injection 28840, the LIGO signal used to exemplify a more refined observing strategy. Instead of blindly searching this entire swath of sky, observations are prioritized by signal detectability as a function of time (see color gradient for the scheduled observation times). Figure 8 in the paper.
Using these detectability maps to schedule follow-up observations with various telescopes (and therefore at different frequencies) is complicated to say the least. The authors present a potential strategy for follow-up using a real LIGO injection (a fake signal fed into data to test their detection pipelines) of a NS-NS merger with an associated afterglow. Detectability maps are constructed and observing strategies are presented for an optical, radio, and infrared follow-up search (see Figure 2 as an example). Optimizing the search for an EM counterpart greatly increased the efficiency of follow-up searches for the chosen injection event for example, the example radio search would have found the progenitor in 4.7 hours, whereas an unprioritized search could have taken up to 47 hours.
The process of refining an efficient method for EM follow-up is distressingly complicated. Myriad unknowns, like EM signal strength, LIGO instrumental noise, observatory availability, and progenitor visibility on the sky all present a strategic puzzle that needs to be solved in the new era of multimessenger astronomy. This work proves that improvements in efficiency are readily available, and that follow-up searches for EM counterparts to GW events will likely be more fruitful as the process is refined.
Narrowing the Search After Gravitational-Wave Detections
Now that we’re able to detect gravitational waves, the next challenge is to spot electromagnetic signatures associated with gravitational-wave events. A team of scientists has proposed a new algorithm that might narrow the search.
Artist’s illustrations of the stellar-merger model for short gamma-ray bursts. In the model, 1) two neutron stars inspiral, 2) they merge and produce a gamma-ray burst, 3) a small fraction of their mass is flung out and radiates as a kilonova, 4) a massive neutron star or black hole with a disk remains after the event. [NASA, ESA, and A. Feild (STScI)]
Light from Neutron-Star Mergers
Just over a year ago, LIGO detected its first gravitational-wave signal: GW150914, produced when two black holes merged. While we didn’t expect to see any sort of light-based signal from this merger, we could expect to see transient electromagnetic signatures in the case of a neutron star–black hole merger or a neutron star–neutron star merger — in the form of a kilonova or a short gamma-ray burst.
While we haven’t yet detected any mergers involving neutron stars, LIGO has the sensitivity to make these detections in the local universe, and we hope to start seeing them soon! Finding the electromagnetic companions to gravitational-wave signals would be the best way to probe the evolution history of the universe and learn what happens when evolved stars collide. So how do we hunt them down?
2D localization maps for LIGO’s detection of GW150914 (black contours), as well as the footprints of follow-up observations (red for radio, green for optical/IR, blue for X-ray). [Abbott et al. 2016]
Pinpointing a Volume
The two LIGO detectors can already provide rough 2D localization of where the gravitational-wave signal came from, but the region predicted for GW150914 still covered 600 square degrees, which is a pretty hefty patch of sky! In light of this, the simplest follow-up strategy of tiling large survey observations of the entire predicted region is somewhat impractical and time-consuming. Could we possibly take a more targeted approach?
The key, say a team of scientists led by Leo Singer (NASA Goddard SFC), is in using 3D estimates of the source location, rather than 2D sky maps: we need to produce distance estimates for the gravitational-wave source as well. Singer and collaborators have developed an algorithm that, from a gravitational-wave signal, can produce a fast full-volume estimate of the probability distribution for its source’s location.
Volume rendering of the 90% credible region for a simulated gravitational-wave event, superimposed over a galaxy map for the region. Green crosshairs represent the true location of the source the most massive galaxies inside the credible region are highlighted. Searching only these galaxies could significantly reduce the observing time needed to detect an electromagnetic counterpart. [Singer et al. 2016]
Singer and collaborators’ approach would make searching for electromagnetic counterparts to gravitational-wave events a much more efficient process. One particular advantage would be in reducing the number of false positives: for a typical wide-field follow-up campaign searching
100 square degrees, hundreds of contaminating supernovae would be in the field. Targeting only 10’ x 10’ patches around 100 nearby galaxies, however, reduces the background to fewer than 10 contaminating supernovae.
An additional benefit is that this targeted strategy opens the door of gravitational-wave follow-up to many small-field-of-view, large-aperture telescopes, instead of limiting the task to broad synoptic surveys. This permits the involvement of many more campaigns in the hunt for the important electromagnetic counterparts to gravitational waves.
Note: Want to check out the team’s data? It’s publicly available here!
Leo P. Singer et al 2016 ApJL 829 L15. doi:10.3847/2041-8205/829/1/L15
Using ISS telescopes for electromagnetic follow-up of gravitational wave detections of NS-NS and NS-BH mergers
The International Space Station offers a unique platform for rapid and inexpensive deployment of space telescopes. A scientific opportunity of great potential later this decade is the use of telescopes for the electromagnetic follow-up of ground-based gravitational wave detections of neutron star and black hole mergers. We describe this possibility for OpTIIX, an ISS technology demonstration of a 1.5 m diffraction limited optical telescope assembled in space, and ISS-Lobster, a wide-field imaging X-ray telescope now under study as a potential NASA mission. Both telescopes will be mounted on pointing platforms, allowing rapid positioning to the source of a gravitational wave event. Electromagnetic follow-up rates of several per year appear likely, offering a wealth of complementary science on the mergers of black holes and neutron stars.
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GW170817 Update: Surprises From First Gravitational Wave Observed Independently
“This is quite literally a physics gold mine!” said Masao Sako, with the University of Pennsylvania.
For over a week now, the astronomy and astrophysics communities have been buzzing with the news of the latest gravitational wave discovery. And this discovery has been big.
Four days before the Great American Solar Eclipse on August 21, a newly discovered gravitational wave caused more astronomers (8,223+), using more telescopes (70), to publish more papers (100 — see the list below) in less time than for any other astronomical event in history. The sixth gravitational wave (GW) to be discovered by the Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo GW observatories, which occurred on August 17, 2017 at 12:41:04 UTC, was surprising in two ways already reported.
GW event six, designated GW170817, did not result from the collision and subsequent explosion of two black holes. All previous GW events, including the first ever discovered in 2015, had involved the collision of black holes with typically 40 times the mass of the Sun between them. Here however, the GW was evidently triggered by the collision and explosion of two neutron stars, having only 3 times the Sun’s mass in total.
Afterglow of GW170817 is shown in close-ups captured by the NASA Hubble Space Telescope, showing it dimming in brightness over days and weeks. CREDIT: NASA and ESA: A. Levan (U. Warwick), N. Tanvir (U. Leicester), and A. Fruchter and O. Fox (STScI)
Crucially, GW170817 occurred ten times closer to Earth than all earlier GW events. Earlier GWs involved black hole collisions at more than 1.3 billion light-years (400 million parsecs or Mpc). GW170817, in comparison, was known within hours of its discovery to lie within only 130 million light-years (40 Mpc). That vastly improved astronomer’s odds of detecting the event independently, because in cosmological terms, it occurred within less than 1% of the universe’s Hubble length of 14 billion light-years (4,300 Mpc).
Not widely reported is that our current astronomical theory regarding GW170817 still depends significantly on observations yet to be made. In brief, astronomers currently believe that GW170817 was triggered by the merger of two neutron stars, which triggered the explosion of a Short Gamma-Ray Burst (SGRB), which emitted only a fraction of the gamma-ray energy in our direction normally associated with SGRBs, because it was the first SGRB observed at such a large angle away from the direction of its focused jets of gamma-rays. The SGRB associated with GW170817 emitted its jet at roughly 30 degrees away from our line-of-sight. All other SGRBs have been observed at only a few degrees from alignment with their jets. The exact angle of the newly discovered SGRB’s jet is important in understanding how its afterglow compares with other SGRB afterglows. Significant properties reported for the GW, including its distance, depend on the angle at which the two neutron stars collided relative to Earth.
The collision angle determined roughly based on the GW itself is probably OK. Only radio maps of the SGRB region at 100 days however, will provide astronomers with the most precise measurements of the resulting explosion’s velocities and directions over time to date. Only then will astronomers learn more about the exact angle of the SGRB’s jet, providing potentially a more accurate estimate of the angle at which the neutron stars collided. More surprises could be in store as a result, including refinements of the properties reported.
ANIMATION (you may have to click image for animation in some browsers): This time-lapse image of the afterglow of GW170817 shows it continuing to increase in radio wavelength brightness over the first month, and was provided by the National Radio Astronomy Observatory Very Large Array radio telescope. CREDIT: NRAO/VLA
Unlike previous events, GW170817 was close enough that within 1.74 seconds of its initial detection by LIGO, it’s gamma radiation was detected by the Fermi Gamma-Ray space telescope. The INTEGRAL Gamma-Ray space observatory detected it too, and it was later designated SGRB 170817A. As an SGRB alone, the event would have triggered alerts to observatories worldwide and aloft, each aiming to detect the explosion’s faint optical afterglow. SGRB optical afterglows have been used to pinpoint the exact positions of SGRBs, not only on the sky, but also in terms of their distance from Earth.
Astronomers in this case had the first GW ever to coincide with, and be independently corroborated by, any observable counterpart, and alerts became a call to astronomical arms. Even though its exact position on the sky was uncertain by many degrees, GW170817 was so close that astronomers were able to quickly narrow down its exact location.
“With a previously-compiled list of nearby galaxies having positions and distances culled from the massive on-line archive of the NASA/IPAC Extragalactic Database (NED), our team rapidly zeroed in on the host galaxy of the event,” said Barry Madore, of Carnegie Observatories.
Precisely because GW170817 occurred at only 130 million light-years, the number of candidate galaxies to observe was only several dozen. In contrast, for previous GW discoveries occurring at billions of light-years, thousands of galaxies would have to be observed. Within 11 hours of the explosion, its afterglow was discovered in the lenticular galaxy NGC 4993, by the Swope 1-m telescope in Chile. They obtained the first-ever visual image of an event associated with a GW.
“Where observation is concerned, chance favors only the prepared mind,” added Madore, quoting Louis Pasteur from 150 years ago. Madore is also a researcher with the Swope team and a co-author on six papers reporting Swope’s discovery of the afterglow and some of its implications. “When alerts were sent out to the LIGO/VIRGO gravity wave detection consortium on the night of August 17, 2017, our team of astronomers was indeed prepared.”
New images of the afterglow of GW170817, aka SGRB 170817A, initially designated as Swope Supernova Survey SSS17a, revealed a bright blue astronomical transient, later designated as AT2017gfo by the International Astronomical Union (IAU).
“There will be more such events, no doubt but this image taken at the Henrietta Swope 1m telescope at the Las Campanas Observatory in Chile was the first in history, and it truly ushered in the Era of Multi-Messenger Astronomy,” said Madore.
Radio observatories joined the hunt, including the Karl G. Jansky Very Large Array (VLA), the Australia Telescope Compact Array (ATCA) and the Giant Metrewave Radio Telescope (GMRT). So did the Swift ultraviolet and Chandra X-ray space observatory satellites. By day one after the explosion, all frequencies of the electromagnetic spectrum were being observed in the direction of NGC 4993. On multiple wavelengths, multiple “messengers” of GW170817’s existence began to reveal more than the sum of their parts.Change in brightness of GW170817’s afterglow over time since explosion (merger), is shown in these light-curves. Brightness in 14 different optical wavelengths is shown, including invisible ultraviolet, and visible blue, green, and yellow, and invisible infrared wavelengths in orange and red. Afterglow fades quickly in all wavelengths, except infrared. In infrared, afterglow continues to brighten until
3 days after explosion, before beginning to fade. CREDIT: Las Campanas Observatory, Carnegie Institution of Washington (Swope + Magellan)
AT2017gfo brightened over the next few days after explosion, in near infrared observations continued by Swope. Their light-curves show the changes in the afterglow’s brightness over time. At three days post explosion, the near-infrared afterglow stops brightening and begins to fade. As with other SGRB afterglows, AT2017gfo faded completely from visual observation over the course of days to weeks, but observations in X-rays and radio continue. Radio observations at 100 days post explosion, which will not occur until November 25, are crucial as said. Although a month away, planned radio observations will determine more than just the long-term evolution of the afterglow over 3 months. Indeed, our astronomical theory accounting for the event’s first three weeks, as already observed, analyzed, and reported, still depends to a surprising degree on an exact number of degrees. The number of degrees relative to Earth for this SGRB based on radio data however, will not be known for at least a month.
“With GW170817 we have for the first time truly independent verification of a gravitational wave source,” said Robert Quimby, of the Kavli Institute for the Physics and Mathematics of the Universe at the University of Tokyo, and coauthor of a paper regarding the event’s implications. “The electromagnetic signature of this event can be uniquely matched to the predictions of binary neutron star mergers, and it is actually quite amazing how well the theory matches the data considering how few observational constraints were available to guide the model.”
“With GW170817, we can learn about nuclear physics, relativity, stellar evolution, and cosmology all in one shot,” added Sako, who is also a co-author on ten papers regarding the event. “Plus we now know how all of the heaviest elements in the Universe are created.”
T = 0 sec.: GW170817 detected by LIGO/VIRGO [1, 82]
T = 1.74 sec.: SGRB 170817A detected by Fermi Gamma-Ray Burst Monitor satellite immediately after GW170817 
T = 28 min.: Gamma-ray Coordinates Network (GCN) Notice 
T = 40 min.: GCN Circular 
T = 5.63 hr.: First sky map locating GW170817 to within several degrees 
T = 10.9 hr.: Swope 1-m observatory discovers explosion’s afterglow, AT 2017gfo, in galaxy NGC 4993 [18, 24, 64, 75, 77]
T = 11.09 hr.: PROMPT 0.4m observatory detects AT 2017gfo 
T = 11.3 hr.: Hubble Space Telescope images AT 2017gfo 
T = 12-24 hr.: Magellan Las Campanas Observatory W. M. Keck Observatory Blanco 4-m Cerro Tololo Inter-American Observatory Gemini South European Southern Observatory VISTA Subaru among 6 Japanese telescopes Pan-STARRS1 Very Large Telescope 14 Australian telescopes and Antarctic Survey Telescope optical observatories, and VLA, VLITE, ATCA, GMRT, and ALMA radio observatories, as well as Swift and NuSTAR ultraviolet satellite observatories
Position: Right Ascension 13h09m48.085s ± 0.018s Declination -23d22m53.343s ± 0.218s (J2000 equinox) 10.6s or 7,000 light-years (2.0 kiloparsecs or kpc) from the nucleus of lenticular galaxy NGC 4993 
Distance: 140 ± 40 million light-years (41 ± 13 Mpc), with 30% scatter based on 3 GW-based estimates [1, 25, 82], and 131 ± 9 million light-years (39.3 ± 2.7 Mpc), with 7% scatter based on 3 distance indicators, including GW-based as well as new Fundamental Plane relation-based distances for NGC 4993 [41, 43], and Tully-Fisher relation-based distances for galaxies in the group of galaxies including NGC 4993 from the NASA/IPAC Extragalactic Database (NED)
Mass: Neutron stars total 2.82 +0.47 -0.09 Sun’s mass  mass ejected in elements heavier than iron is 0.03 ± 0.01 Sun’s mass or 10,000 Earth masses, based on 4 estimates [24, 59, 82, 93], including gold amounting to 150 ± 50 Earth masses 
Luminosity: Peaks at 0.5 days after explosion, at
10 42 erg/s, equivalent to 260 million Suns 
SGRB jet angle: 31 ± 3 degrees away from line-of-sight to Earth, based on 9 estimates [2, 25, 34, 35, 36, 44, 58, 62, 82]
SGRB jet speed: 30% speed of light, based on 4 estimates [20, 42, 59, 75]
Names: GW170817, SGRB 170817A, AT 2017gfo = IAU designation for SGRB afterglow, aka SSS17a, DLT17ck, J-GEM17btc, and MASTER OTJ130948.10-232253.3
Astronomy (1): Confirms binary neutron star collisions as a source for GW and SGRB events [1, 82]
Astronomy (2): GWs provide a new way of measuring neutron star diameters 
Astronomy (3): Gives universal expansion rate, or Hubble constant, as H0 = 71 ± 10 km -1 Mpc -1 , with 14% accuracy, based on 6 GW-based estimates for GW170817 ranging from 69 to 74 km -1 Mpc -1 , bridging current estimates [1, 22, 36, 60, 74, 82] accuracy will improve to 4% with future similar events 
General Relativity (1): Confirms GW velocity equals speed of light to within 1 part per 1,000,000,000,000,000 or 1/10 15 [7, 21, 70, 91]
General Relativity (2): Confirms equivalence of gravitational energy and inertial energy, or Weak Equivalence Principle, to within 1 part per 1,000,000,000 or 1/10 9 [7, 11, 91, 92]
Physics: Confirms binary neutron star collisions are significant production sites for elements heavier than iron, including gold, platinum, and uranium [17, 69]
Life on Earth: Indicates a higher deadly rate of gamma-rays for extraterrestrial life 
GW170817 (1): Predicted one binary neutron star collision per year similar to GW170817 within a distance from Earth of 130 million light-years [40 Mpc] 
GW170817 (2): Predicted to produce a 10 Giga-Hertz afterglow that peaks at
100 days with a radio magnitude of
10 milli-Janskys 
GW170817 (3): Predicted to remain visible in radio for 5-10 years, and for decades with next-generation radio observatories