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Tag Archives: Kilonova
Astronomers Find Rare Star System That Will Trigger a Kilonova
The universe has no shortage of oddities, and researchers at the National Science Foundation’s NOIRLab have observed another one in the form of a particular binary star system. The system, called CPD-29 2176, will eventually trigger a kilonova, a celestial event in which two neutron stars collide in a massive explosion that forms heavy elements, including gold and platinum.
CPD-29 2176 is located around 11,400 light-years from Earth and was found by researchers using NASA’s Neil Gehrels Swift Observatory. Astronomers then conducted more observations at NOIRLab’s Cerro Tololo Inter-American Observatory in Chile. CPD-29 2176 is home to one neutron star and one massive star that is in the process of going supernova, only to become a second neutron star in the future. Eventually, the two neutron stars will collide, producing a kilonova, an explosion that is thought to produce bursts of gamma rays and large amounts of gold and platinum. The paper documenting the research team’s find is published today in Nature.
“We know that the Milky Way contains at least 100 billion stars and likely hundreds of billions more. This remarkable binary system is essentially a one-in-ten-billion system,” said André-Nicolas Chené in a NOIRLab press release. Chené is a NOIRLab astronomer and an author on the study. “Prior to our study, the estimate was that only one or two such systems should exist in a spiral galaxy like the Milky Way.”
While many stars implode was a powerful supernova when they die, the dying star in CPD-29 2176 is becoming an ultra-stripped supernova. An ultra-stripped supernova lacks the vast amount of force that a typical supernova has, since the dying star has had much of its mass stripped by its companion. The researchers think that the neutron star in the system was also formed with an ultra-stripped supernova and argue that this is the reason that CPD-29 2176 is able to remain as a binary—a typical supernova would have enough power to kick a companion star out of its orbit.
“The current neutron star would have to form without ejecting its companion from the system. An ultra-stripped supernova is the best explanation for why these companion stars are in such a tight orbit,” said lead author Noel D. Richardson, a physics and astronomy professor at Embry-Riddle Aeronautical University, in the NOIRLab release. “To one day create a kilonova, the other star would also need to explode as an ultra-stripped supernova so the two neutron stars could eventually collide and merge.”
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It will take around one million years for the star undergoing ultra-stripped supernova to turn into a neutron star. It is then when the two stars will begin to spiral into each other, eventually resulting in the metal-producing kilonova, according to the research. In these dramatic cosmic endings, we can look forward to the creation of the same elements that make life possible.
More: Watch Four Planets Spin Around a Star 130 Million Light-Years Away
Astronomers identify 1st twin stars doomed to collide in kilonova explosion
Although massive stars usually die with spectacular explosions, a handful fizzle out like dud firecrackers.
Astronomers have identified the remnants of one such dud firecracker in SGR 0755-2933, a neutron star about 11,400 light-years from Earth in the southern constellation of Puppis. In new research, scientists say that earlier in its lifetime, this star transferred abnormally high amounts of mass to its binary companion — so much so that it was not left with enough material for an explosive death. Instead, it ended in a quiet “ultra-stripped” supernova, a rare cosmic event that leaves a super-dense remnant called a neutron star in its wake.
“This remarkable binary system is essentially a one-in-10-billion system,” André-Nicolas Chené, an astronomer at the National Science Foundation’s NOIRLab research center and a co-author of the new study, said in a statement.
Related: Right place, right time: Hubble telescope captured a supernova as it exploded
The neutron star and its closely orbiting binary companion — a star that the researchers also predict will someday collapse to become a neutron star — mark the first clear example of a star system that will ultimately trigger a kilonova, a cosmic explosion during which two neutron stars merge.
Although a kilonova was first detected in 2017, astronomers then recorded only the aftermath of the event, thanks to observations of light and gravitational waves. The new research is the first time scientists have identified a binary star system that they know will end in a kilonova explosion.
Moreover, astronomers previously thought that only one or two such systems would exist in spiral galaxies like our Milky Way. Researchers of the latest study have now increased that estimate to 10, noting that these observations help them better understand the history, evolution and atypically calm deaths of stars in such systems.
“For quite some time, astronomers speculated about the exact conditions that could eventually lead to a kilonova,” Chené said in the statement. “These new results demonstrate that, in at least some cases, two sibling neutron stars can merge when one of them was created without a classical supernova explosion.”
The sibling star is massive, orbits the primary neutron star every 60 days, and has a name like a license plate: CPD-29 2176. Scientists behind the latest research studied this sibling star to understand the formation of the current star system, as well as what might unfold in its future.
“This is not just a simple binary system”
Clarissa Pavao, an undergraduate student at the Embry-Riddle Aeronautical University in Arizona, found the system while scouring data captured by the Cerro Tololo Inter-American Observatory in Chile. In particular, she was plotting the spectra of the sibling star, an analysis of how much light a star emits at particular wavelengths. After cleaning noise from the data, she noticed one simple line in the spectra that suggested the massive star had a highly circular orbit — an unusual feature in binary star systems.
This was a key finding that helped the team conclude that the primary neutron star ended as a dud supernova, the astronomers said.
Usually, when one of the stars in a binary system burns through its hydrogen and nears the end of its main-sequence stage, it begins transferring mass to its companion star. The resulting end-of-life explosion often kicks companion stars out of the systems and into highly elliptical orbits.
But this did not seem to have occurred in the intriguing system. To better understand what might have happened at the end of SGR 0755-2933’s life, astronomers waded through thousands of models that described binary star systems resembling the one they were studying. They only found two that matched.
The team then traced the star’s history and concluded it behaved, for the most part, like any other massive star running out of fuel: Toward the end of its life, the star began transferring mass to its companion and dwindled into a low-mass star with a helium core, as scientists expected. In this process, however, the star lost so much mass that its end-of-life supernova “didn’t even have enough energy to kick the orbit into the more typical elliptical shape seen in similar binaries,” Noel Richardson, an astronomer at Embry-Riddle and lead author of the new study, said in a statement.
The dying star also did not have enough energy to kick its companion out of the system, which is why the two stars continue to have tight orbits, according to the study.
In addition to learning more about kilonova events, the new research will help astronomers better understand the origins of some of the heaviest elements in our universe.
The quiet supernova occurred only a few million years ago, and astronomers expect the CPD-29 2176 system to remain as it is for at least one million years more. Their models show that, much like the primary neutron star, the sibling star too will then become an ultra-stripped supernova and eventually collapse into a neutron star.
Millions of years from now, the team predicts that the two neutron stars will spiral slowly toward each other in a cosmic dance, ultimately colliding in a kilonova explosion. Such explosions are known to be a source of immense quantities of heavy elements like platinum, xenon, uranium and gold “that get hurled into the universe,” Richardson said.
Astronomers have long suspected that heavy metals released during such events hovered in the interstellar medium until they coalesced into asteroids, which then bombarded Earth as it formed and deposited the precious metals we see today. The 2017 kilonova event alone sent at least 100 Earth’s worth of precious heavy metals out there, so it looks like a failed supernova isn’t such a loss to the universe after all.
The research is described in a paper (opens in new tab) published Wednesday (Feb. 1) in the journal Nature.
Follow Sharmila Kuthunur on Twitter @Sharmilakg. Follow us on Twitter @Spacedotcom and on Facebook.
New kilonova has astronomers rethinking what we know about gamma-ray bursts
Enlarge / Artist’s impression of GRB 211211A. The kilonova and gamma-ray burst is on the right.
Aaron M. Geller/Northwestern/CIERA
A year ago, astronomers discovered a powerful gamma-ray burst (GRB) lasting nearly two minutes, dubbed GRB 211211A. Now that unusual event is upending the long-standing assumption that longer GRBs are the distinctive signature of a massive star going supernova. Instead, two independent teams of scientists identified the source as a so-called “kilonova,” triggered by the merger of two neutron stars, according to a new paper published in the journal Nature. Because neutron star mergers were assumed to only produce short GRBs, the discovery of a hybrid event involving a kilonova with a long GBR is quite surprising.
“This detection breaks our standard idea of gamma-ray bursts,” said co-author Eve Chase, a postdoc at Los Alamos National Laboratory. “We can no longer assume that all short-duration bursts come from neutron-star mergers, while long-duration bursts come from supernovae. We now realize that gamma-ray bursts are much harder to classify. This detection pushes our understanding of gamma-ray bursts to the limits.”
As we’ve reported previously, gamma-ray bursts are extremely high-energy explosions in distant galaxies lasting between mere milliseconds to several hours. The first gamma-ray bursts were observed in the late 1960s, thanks to the launching of the Vela satellites by the US. They were meant to detect telltale gamma-ray signatures of nuclear weapons tests in the wake of the 1963 Nuclear Test Ban Treaty with the Soviet Union. The US feared that the Soviets were conducting secret nuclear tests, violating the treaty. In July 1967, two of those satellites picked up a flash of gamma radiation that was clearly not the signature of a nuclear weapons test.
Just a couple of months ago, multiple space-based detectors picked up a powerful gamma-ray burst passing through our solar system, sending astronomers worldwide scrambling to train their telescopes on that part of the sky to collect vital data on the event and its afterglow. Dubbed GRB 221009A, it was the most powerful gamma-ray burst yet recorded and likely could be the “birth cry” of a new black hole.
There are two types of gamma-ray bursts: short and long. Classic short-term GRBs last less than two seconds, and they were previously thought to only occur from the merging of two ultra-dense objects, like binary neutron stars, producing an accompanying kilonova. Long GRBs can last anywhere from a few minutes to several hours and are thought to occur when a massive star goes supernova.
Enlarge / This Gemini North image, superimposed on an image taken with the Hubble Space Telescope, shows the telltale near-infrared afterglow of a kilonova produced by a long GRB.
Int’l Gemini Observatory/NOIRLab/NSF/AURA/NASA/ESA
Astronomers at the Fermi and Swift telescopes simultaneously detected this latest gamma-ray burst last December and pinpointed the location in the constellation Boötes. That quick identification allowed other telescopes around the world to turn their attention to that sector, enabling them to catch the kilonova in its earliest stages. And it was remarkably nearby for a gamma-ray burst: about 1 billion light-years from Earth, compared to around 6 billion years for the average gamma-ray burst detected to date. (Light from the most distant GRB yet recorded traveled for some 13 billion years.)
“It was something we had never seen before,” said co-author Simone Dichiara, an astronomer at Penn State University and a member of the Swift team. “We knew it wasn’t associated with a supernova, the death of a massive star, because it was too close. It was a completely different kind of optical signal, one that we associate with a kilonova, the explosion triggered by colliding neutron stars.”
As two binary neutron stars begin circling into their death spiral, they send out powerful gravitational waves and strip neutron-rich matter from each other. Then the stars collide and merge, producing a hot cloud of debris that glows with light of multiple wavelengths. It’s the neutron-rich debris that astronomers believe creates a kilonova’s visible and infrared light—the glow is brighter in the infrared than in the visible spectrum, a distinctive signature of such an event that results from heavy elements in the ejecta which block visible light but lets the infrared through.
Enlarge / When neutron stars merge, they can produce radioactive ejecta that powers a kilonova signal. A recently observed gamma-ray burst turned out to signal a previously undetected hybrid event involving a kilonova.
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That signature is what subsequent analysis of GRB211211A revealed. And since the subsequent decay of a neutron star merger produces heavy elements like gold and platinum, astronomers now have a new means of studying how these heavy elements form in our universe.
Several years ago, the late astrophysicist Neil Gehrels suggested that longer gamma-ray bursts could be produced by neutron star mergers. It seems only fitting that NASA’s Swift Observatory, which is named in his honor, played a key role in the discovery of GRB 211211A and the first direct evidence for that connection.
“This discovery is a clear reminder that the Universe is never fully figured out,” said co-author Jillian Rastinejad, a Ph.D. student at Northwestern University. “Astronomers often take it for granted that the origins of GRBs can be identified by how long the GRBs are, but this discovery shows us there’s still much more to understand about these amazing events.”
DOI: Nature, 2022. 10.1038/s41550-022-01819-4 (About DOIs).
Radioactive Kilonova Glow Suggests Rapid Spin Delayed Collapse of Neutron Stars Into Black Hole
In this artist’s representation, the merger of two neutron stars to form a black hole (hidden within bright bulge at center of image) generated opposing, high-energy jets of particles (blue) that heated up material around the stars, making it emit X-rays (reddish clouds). The Chandra X-ray Observatory is still detecting X-rays from the event today. They could be produced by a shock wave in the material around the black hole, or by material falling violently into the black hole (yellowish disk around central bulge). Credit: X-ray data from NASA, CXC and Northwestern Univ./A. Hajela; visual by NASA/CXC/M. Weiss
Excess X-ray emissions from remnant four years after merger hint at bounce from delayed collapse.
When two neutron stars spiral into one another and merge to form a 
X-ray sources captured by Chandra, including, at top, the black hole that formed from the merger of two neutron stars and was first observed in 2017. Credit: NASA, CXC and Northwestern Univ./A. Hajela
Chandra, too, pivoted to observe GW170817, but saw no X-rays until nine days later, suggesting that the merger also produced a narrow jet of material that, upon colliding with the material around the neutron stars, emitted a cone of X-rays that initially missed Earth. Only later did the head of the jet expand and begin emitting X-rays in a broader jet visible from Earth.
The X-ray emissions from the jet increased for 160 days after the merger, after which they steadily grew fainter as the jet slowed down and expanded. But Hajela and her team noticed that from March 2020 — about 900 days after the merger — until the end of 2020, the decline stopped, and the X-ray emissions remained approximately constant in brightness.
“The fact that the X-rays stopped fading quickly was our best evidence yet that something in addition to a jet is being detected in X-rays in this source,” Margutti said. “A completely different source of X-rays appears to be needed to explain what we’re seeing.”
The researchers suggest that the excess X-rays are produced by a shock wave distinct from the jets produced by the merger. This shock was a result of the delayed collapse of the merged neutron stars, likely because its rapid spin very briefly counteracted the gravitational collapse. By sticking around for an extra second, the material around the neutron stars got an extra bounce that produced a very fast tail of kilonova ejecta that created the shock.
“We think the kilonova afterglow emission is produced by shocked material in the circumbinary medium,” Margutti said. “It is material that was in the environment of the two neutron stars that was shocked and heated up by the fastest edge of the kilonova ejecta, which is driving the shock wave.”
The merger of two neutron stars produced a black hole (center, white) and a burst of gamma-rays generated by a narrow jet or beam of high-energy particles, depicted in red. Initially, the jet was narrow and undetectable by Chandra, but as time passed the material in the jet slowed down and widened (blue) as it slammed into surrounding material, causing the X-ray emission to rise as the jet came into direct view by Chandra. This jet and its oppositely directed counterpart were likely generated by material falling onto the black hole after it formed. Credit: NASA/CXC/K. DiVona
The radiation is reaching us only now because it took time for the heavy kilonova ejecta to be decelerated in the low-density environment and for the kinetic energy of the ejecta to be converted into heat by shocks, she said. This is the same process that produces radio and X-rays for the jet, but because the jet is much, much lighter, it is immediately decelerated by the environment and shines in the X-ray and radio from the very earliest times.
An alternative explanation, the researchers note, is that the X-rays come from material falling towards the black hole that formed after the neutron stars merged.
“This would either be the first time we’ve seen a kilonova afterglow or the first time we’ve seen material falling onto a black hole after a
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Mysterious X-rays could be kilonova “afterglow” from 2017 neutron star merger
Enlarge / Artist’s representation of the merger of two neutron stars to form a black hole (hidden within bright bulge at center of image). The merger generates opposing, high-energy jets of material (blue) that heat up material around the stars, making it emit X-rays (reddish clouds).
NASA/CXC/M. Weiss
Back in 2017, astronomers detected a phenomenon known as a “kilonova”: the merger of two neutron stars accompanied by powerful gamma-ray bursts. Three and a half years later, astrophysicists spotted mysterious X-rays they believe could be the very first detection of a kilonova “afterglow,” according to a new paper published in The Astrophysical Journal Letters. Alternatively, what the astrophysicists saw could be the first observation of matter falling into the black hole that formed after the merger.
As we’ve reported previously, LIGO detects gravitational waves via laser interferometry. This method uses high-powered lasers to measure tiny changes in the distance between two objects positioned kilometers apart. (LIGO has detectors in Hanford, Washington, and in Livingston, Louisiana. A third detector in Italy, known as Advanced VIRGO, came online in 2016.) Having three detectors means scientists can triangulate and better pinpoint where in the night sky any telltale chirps are coming from.
In addition to seven more binary black hole mergers, LIGO’s second run, from November 30, 2016, to August 25, 2017, detected a binary neutron-star merger with a simultaneous gamma-ray burst and signals in the rest of the electromagnetic spectrum. The event is now known as GW170817. These signals included the telltale signatures of heavy elements—notably gold, platinum and uranium—created by the collision. Most lighter elements are forged in the death-throe explosions of massive stars known as supernovas, but astronomers have long theorized that the heavier elements might originate in kilonovas produced when two neutron stars collide.
The 2017 detection of the kilonova provided evidence that those astronomers were right. Recording this kind of celestial event was unprecedented, and it officially marked the dawn of a new era in so-called “multi-messenger astronomy.”
Ever since, astronomers have been looking for a corresponding optical signature whenever LIGO/VIRGO picks up a gravitational wave signal for neutron star mergers or possible neutron star-black hole mergers. The assumption had been that black hole-black hole mergers would not produce any optical signature, so there was no point even looking for one—until 2020. That’s when astronomers found the first evidence of just such a phenomenon. The astronomers made the discovery by combining gravitational wave data with data collected during a robotic sky survey.
But the 2017 kilonova remains unique, according to Aprajita Hajela, the lead author of the new paper and a graduate student at Northwestern University. Hajela calls the kilonova “the only event of its kind” and “a treasure chest of several first observations our field.” Along with other astronomers at Northwestern and the University of California, Berkeley, she has been monitoring the evolution of GW170817 since LIGO/Virgo first detected it by using the space-based Chandra X-ray Observatory.
Enlarge / Illustration of the space-based Chandra X-ray Observatory, the most sensitive X-ray telescope ever built.
NASA/CXC/NGST (Public domain)
Chandra first detected X-ray and radio emissions from GW170817 a couple of weeks after the merger, which persisted for 900 days. But those initial X-rays, powered by a jet resulting from the merger moving close to the speed of light, started to fade in early 2018. However, from March 2020 through the end of that year, the steep decline in brightness stopped, and the X-ray emission became fairly constant in terms of brightness.
To help resolve the mystery, Hajela and her team collected additional observational data with both Chandra and the Very Large Array (VLA) in December 2020, 3.5 years after the merger. It was Hajela who was awakened at 4 am by a notification of surprisingly strong and bright X-ray emissions—four times higher than would be expected at this point if the emission were powered solely by the jet. (The VLA didn’t pick up any radio emissions.) These new emissions have remained at a constant level for 700 days.
That means a completely different source of X-rays must be powering them. One likely explanation is that expanding debris from the merger generated a shock wave, akin to a sonic boom, in addition to the jets. In that case, the merged neutron stars could not have immediately collapsed into a black hole. Instead, the stars spun down rapidly for a second. That rapid spin would have briefly counteracted the gravitational collapse just long enough to produce a fast tail of heavy kilonova ejecta, which drove the shock wave. As that heavy ejecta decelerated over time, its kinetic energy was converted into heat by the shocks.
“It would just fall in. Done.”
“If the merged neutron stars were to collapse directly to a black hole with no intermediate stage, it would be very hard to explain this X-ray excess that we see right now, because there would be no hard surface for stuff to bounce off and fly out at high velocities to create this afterglow,” said co-author Raffaella Margutti of the UC Berkeley. “It would just fall in. Done. The true reason why I’m excited scientifically is the possibility that we are seeing something more than the jet. We might finally get some information about the new compact object.”
Brian Metzger of Columbia University proposed an alternative scenario: the X-ray emission could be powered by matter falling into the back hole that was formed during the merger. This is also a scientific first, according to Hajela, since this kind of long-term accretion has never been observed before.
There are more observations planned going forward, and that data will help resolve the issue. If the X-rays and radio emissions brighten over the next few months or years, this would confirm the kilonova afterglow scenario. If the X-ray emissions decline steeply or remain steady, with no accompanying radio emission, that would confirm the accreting black hole scenario.
Regardless, “This would either be the first time we’ve seen a kilonova afterglow or the first time we’ve seen material falling onto a black hole after a neutron star merger,” said co-author Joe Bright, postdoc at UC Berkeley. “Either outcome would be extremely exciting.”
DOI: Astrophysical Journal Letters, 2022. 10.48550/arXiv.2104.02070 (About DOIs).
Mysterious Kilonova Explosion Afterglow Potentially Spotted for First Time
An artist’s conception illustrates the aftermath of a “kilonova,” a powerful event that happens when two neutron stars merge. Credit: X-ray: NASA/CXC/Northwestern Univ./A. Hajela et al.; Illustration: NASA/CXC/M.Weiss
Strange ‘sonic boom’ accompanied unprecedented event.
- Mysterious X-rays observed 3.5 years after the merger of two neutron stars
- Astrophysicists believe a kilonova afterglow or materials falling into a
Either scenario would be a first for the field. The study was published on February 28, 2022, in The Astrophysical Journal Letters.
“We have entered uncharted territory here in studying the aftermath of a neutron star merger,” said Northwestern’s Aprajita Hajela, who led the new study. “We are looking at something new and extraordinary for the very first time. This gives us an opportunity to study and understand new physical processes, which have not before been observed.”
Hajela is a graduate student at Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and in the Department of Physics and Astronomy in the Weinberg College of Arts and Sciences.
On August 17, 2017, GW170817 made history as the first neutron-star merger detected by both (function(d, s, id){ var js, fjs = d.getElementsByTagName(s)[0]; if (d.getElementById(id)) return; js = d.createElement(s); js.id = id; js.src = "https://connect.facebook.net/en_US/sdk.js#xfbml=1&version=v2.6"; fjs.parentNode.insertBefore(js, fjs); }(document, 'script', 'facebook-jssdk'));
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Possible ‘kilonova’ explosion leaves an epic afterglow
Astronomers may have spotted the afterglow from an epic cosmic event known as a “kilonova.”
Kilonovas occur after the collision of two hyper-dense neutron stars, which are the remnants of stars that have died in supernova explosions. Astronomers think they have spotted an afterglow in X-rays from the event, which is dubbed GW170817.
The discovery team suggests that as the debris expanded out from the collision, the resulting sonic-boom-like shock heated up surrounding materials. The heating generated X-rays.
Related: Monstrous ‘Kilonova’ Explosions May Be Showering a Nearby Galaxy in Gold
Alternatively, however, a similar effect may be produced due to materials falling towards a black hole caused by the neutron star merger, so astronomers caution the finding is tentative. Either type of find, however, would be the first known to science.
“We have entered uncharted territory here in studying the aftermath of a neutron star merger,” lead researcher Aprajita Hajela, an astrophysics graduate student at Northwestern University, said in a statement. “We are looking at something new and extraordinary for the very first time. This gives us an opportunity to study and understand new physical processes, which have not before been observed.”
The neutron star event was already known to science, following an Aug. 17, 2017 merger that produced the first-ever detection of such an event by gravitational waves, or ripples of space-time. Astronomers continue to study the region to find out how the area is evolving.
Astronomers spotted X-ray emissions soon after the event, using NASA’s Chandra X-ray Observatory, but the emission began to fade in early 2018. Hajela’s team, however, showed the decline in brightness stopped in 2020, with the X-ray emission remaining nearly constant.
The consistency in X-ray brightness was what pointed to this being an unusual event, team members said. “A completely different source of X-rays appears to be needed to explain what we’re seeing,” Raffaella Margutti, an astrophysicist at the University of California at Berkeley and a senior author of the study, said in the same statement.
Figuring out what the ultimate cause was, however, will require more follow-up study. If it is indeed a kilonova, the researchers expect to see the X-ray and radio emissions get brighter as the shock continues to plow through the nearby environment. But if it is a black hole, the output should decline or remain steady.
A study based on the research was published Monday (Feb. 28) in The Astrophysical Journal Letters.
Follow Elizabeth Howell on Twitter @howellspace. Follow us on Twitter @Spacedotcom or Facebook.
Kilonova afterglow potentially spotted for first time
For the first time, Northwestern University-led astronomers may have detected an afterglow from a kilonova.
A kilonova occurs when two neutron stars—some of the densest objects in the universe—merge to create a blast 1,000 times brighter than a classical nova. In this case, a narrow, off-axis jet of high-energy particles accompanied the merger event, dubbed GW170817. Three-and-a-half years after the merger, the jet faded away, revealing a new source of mysterious X-rays.
As the leading explanation for the new X-ray source, astrophysicists believe expanding debris from the merger generated a shock—similar to the sonic boom from a supersonic plane. This shock then heated surrounding materials, which generated X-ray emissions, known as a kilonova afterglow. An alternative explanation is materials falling toward a black hole—formed as a result of the neutron star merger—caused the X-rays.
Either scenario would be a first for the field. The study was published today (Feb. 28), in The Astrophysical Journal Letters.
“We have entered uncharted territory here in studying the aftermath of a neutron star merger,” said Northwestern’s Aprajita Hajela, who led the new study. “We are looking at something new and extraordinary for the very first time. This gives us an opportunity to study and understand new physical processes, which have not before been observed.”
Hajela is a graduate student at Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and in the Department of Physics and Astronomy in the Weinberg College of Arts and Sciences.
On Aug. 17, 2017, GW170817 made history as the first neutron-star merger detected by both gravitational waves and electromagnetic radiation (or light). Since then, astronomers have been using telescopes around the world and in space to study the event across the electromagnetic spectrum.
Using NASA’s Chandra X-ray Observatory, astronomers observed X-ray emissions from a jet moving very close to the speed of light produced by the neutron star merger. Starting in early 2018, the jet’s X-ray emission steadily faded as the jet continued to slow and expand. Hajela and her team then noticed from March 2020 until the end of 2020, the decline in brightness stopped, and the X-ray emission was approximately constant in brightness.
This was a significant clue.
“The fact that the X-rays stopped fading quickly was our best evidence yet that something in addition to a jet is being detected in X-rays in this source,” said Raffaella Margutti, astrophysicist at the University of California at Berkeley and a senior author of the study. “A completely different source of X-rays appears to be needed to explain what we’re seeing.”
The researchers believe a kilonova afterglow or black hole are likely behind the X-rays. Neither scenario has ever before been observed.
“This would either be the first time we’ve seen a kilonova afterglow or the first time we’ve seen material falling onto a black hole after a neutron star merger,” said study co-author Joe Bright, also from the University of California at Berkeley. “Either outcome would be extremely exciting.”
To distinguish between the two explanations, astronomers will keep monitoring GW170817 in X-rays and radio waves. If it is a kilonova afterglow, the X-ray and radio emissions are expected to get brighter over the next few months or years. If the explanation involves matter falling onto a newly formed black hole, then the X-ray output should stay steady or decline rapidly, and no radio emission will be detected over time.
“Further study of GW170817 could have far-reaching implications,” said study co-author Kate Alexander, a CIERA postdoctoral fellow at Northwestern. “The detection of a kilonova afterglow would imply that the merger did not immediately produce a black hole. Alternatively, this object may offer astronomers a chance to study how matter falls onto a black hole a few years after its birth.”
Astronomers find x-rays lingering years after landmark neutron star collision
More information:
The emergence of a new source of X-rays from the binary neutron star merger GW170817, arXiv:2104.02070 [astro-ph.HE] arxiv.org/abs/2104.02070
The emergence of a new source of X-rays from the binary neutron star merger GW170817, arXiv:2104.02070 [astro-ph.HE] arxiv.org/abs/2104.02070
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