An artistic rendering of two neutron stars merging. Credit: NSF/LIGO/Sonoma State/A. Simonnet
In 2017, LIGO (Laser-Interferometer Gravitational Wave Observatory) and Virgo detected gravitational waves coming from the merger of two neutron stars. They named that signal GW170817. Two seconds after detecting it, NASA’s Fermi satellite detected a gamma ray burst (GRB) that was named GRB170817A. Within minutes, telescopes and observatories around the world honed in on the event.
The Hubble Space Telescope played a role in this historic detection of two neutron stars merging. Starting in December 2017, Hubble detected the visible light from this merger, and in the next year and a half it turned its powerful mirror on the same location over 10 times. The result?
The deepest image of the afterglow of this event, and one chock-full of scientific detail.
In new study, UChicago astronomers find no evidence for extra spatial dimensions to the universe based on gravitational wave data. Credit: NASA’s Goddard Space Flight Center CI Lab
In August of 2017, astronomers made another major breakthrough when the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves that were believed to be caused by the merger of two neutron stars. Since that time, scientists at multiple facilities around the world have conducted follow-up observations to determine the aftermath this merger, as even to test various cosmological theories.
For instance, in the past, some scientists have suggested that the inconsistencies between Einstein’s Theory of General Relativity and the nature of the Universe over large-scales could be explained by the presence of extra dimensions. However, according to a new study by a team of American astrophysicists, last year’s kilonova event effectively rules out this hypothesis.
As the jet from the neutron-star merger event emerged into space, simulated radio images in this artist's conception illustrate its extremely fast motion. Credit: D. Berry, O. Gottlieb, K. Mooley, G. Hallinan, NRAO/AUI/NSF
In August of 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected waves that were believed to be caused by a neutron star merger. This “kilonova” event, known as GW170817, was the first astronomical event to be detected in both gravitational and electromagnetic waves – including visible light, gamma rays, X-rays, and radio waves.
In the months that followed the merger, orbiting and ground-based telescopes around the world have observed GW170817 to see what has resulted from it. According to a new study by an international team of astronomers, the merger produced a narrow jet of material that made its way into interstellar space at velocities approaching the speed of light.
Artist's illustration of two merging neutron stars. The narrow beams represent the gamma-ray burst while the rippling spacetime grid indicates the isotropic gravitational waves that characterize the merger. Swirling clouds of material ejected from the merging stars are a possible source of the light that was seen at lower energies. Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet
This source, known as GW170817/GRB, has been the target of many follow-up surveys since it was believed that the merge could have led to the formation of a black hole. According to a new study by a team that analyzed data from NASA’s Chandra X-ray Observatory since the event, scientists can now say with greater confidence that the merger created a new black hole in our galaxy.
The study, titled “GW170817 Most Likely Made a Black Hole“, recently appeared in The Astrophysical Journal Letters. The study was led by David Pooley, an assistant professor in physics and astronomy at Trinity University, San Antonio, and included members from the University of Texas at Austin, the University of California, Berkeley, and Nazarbayev University’s Energetic Cosmos Laboratory in Kazakhstan.
Illustration of the kilonova merger (top), and the resulting object (left and right) over time. Credit: NASA/CXC/Trinity University/D. Pooley et al. Illustration: NASA/CXC/M.Weiss
For the sake of their study, the team analyzed X-ray data from Chandra taken in the days, weeks, and months after the detection of gravitational waves by LIGO and gamma rays by NASA’s Fermi mission. While nearly every telescope in the world had observed the source, X-ray data was critical to understanding what happened after the two neutron stars collided.
While a Chandra observation two to three days after the event failed to detect an X-ray source, subsequent observations taken 9, 15, and 16 days after the event resulted in detections. The source disappeared for a time as GW170817 passed behind the Sun, but additional observations were made about 110 and 160 days after the event, both of which showed significant brightening.
While the LIGO data provided astronomers with a good estimate of the resulting object’s mass after the neutron stars merged (2.7 Solar Masses), this was not enough to determine what it had become. Essentially, this amount of mass meant that it was either the most massive neutron star ever found or the lowest-mass black hole ever found (the previous record holders being four or five Solar Masses). As Dave Pooley explained in a NASA/Chandra press release:
“While neutron stars and black holes are mysterious, we have studied many of them throughout the Universe using telescopes like Chandra. That means we have both data and theories on how we expect such objects to behave in X-rays.”
Illustration of the resulting black hole caused by GW170817. Credit: NASA/CXC/M.Weiss
If the neutron stars merged to form a heavier neutron star, then astronomers would expect it to spin rapidly and generate and very strong magnetic field. This would have also created an expanded bubble of high-energy particles that would result in bright X-ray emissions. However, the Chandra data revealed X-ray emissions that were several hundred times lower than expected from a massive, rapidly-spinning neutron star.
By comparing the Chandra observations with those by the NSF’s Karl G. Jansky Very Large Array (VLA), Pooley and his team were also able to deduce that the X-ray emission were due entirely to the shock wave caused by the merger smashing into surrounding gas. In short, there was no sign of X-rays resulting from a neutron star.
This strongly implies that the resulting object was in fact a black hole. If confirmed, these results would indicate that the formation process of a blackhole can sometimes be complicated. Essentially, GW170817 would have been the result of two stars undergoing a supernova explosion that left behind two neutron stars in a sufficiently tight orbit that they eventually came together. As Pawan Kumar explained:
“We may have answered one of the most basic questions about this dazzling event: what did it make? Astronomers have long suspected that neutron star mergers would form a black hole and produce bursts of radiation, but we lacked a strong case for it until now.”
Simulated view of a black hole. Credit: Bronzwaer/Davelaar/Moscibrodzka/Falcke, Radboud University
Looking ahead, the claims put forward by Pooley and his colleagues could be tested by future X-ray and radio observations. Next-generation instruments – like the Square Kilometer Array (SKA) currently under construction in South Africa and Australia, and the ESA’s Advanced Telescope for High-ENergy Astrophysics (Athena+) – would be especially helpful in this regard.
If the remnant turns out to be a massive neutron star with a strong magnetic field after all, then the source should get much brighter in the X-ray and radio wavelengths in the coming years as the high-energy bubble catches up with the decelerating shock wave. As the shock wave weakens, astronomers expect that it will continue to become fainter than it was when recently observed.
Regardless, future observations of GW170817 are bound to provide a wealth of information, according to J. Craig Wheeler, a co-author on the study also from the University of Texas. “GW170817 is the astronomical event that keeps on giving,” he said. “We are learning so much about the astrophysics of the densest known objects from this one event.”
If these follow-up observations find that a heavy neutron star is what resulted from the merger, this discovery would challenge theories about the structure of neutron stars and how massive they can get. On the other hand, if they find that it formed a tiny black hole, then it will challenge astronomers notions about the lower mass limits of black holes. For astrophysicists, it’s basically a win-win scenario.
As co-author Bruce Grossan of the University of California at Berkeley added:
“At the beginning of my career, astronomers could only observe neutron stars and black holes in our own galaxy, and now we are observing these exotic stars across the cosmos. What an exciting time to be alive, to see instruments like LIGO and Chandra showing us so many thrilling things nature has to offer.”
Indeed, looking farther out into the cosmos and deeper back in time has revealed much about the Universe that was previously unknown. And with improved instruments being developed for the sole purpose of studying astronomical phenomena in greater detail and at even greater distances, there seems to be no limit to what we might learn. And be sure to check out this video of the GW170817 merger, courtesy of the Chandra X-ray Observatory:
Artist's illustration of two merging neutron stars. The narrow beams represent the gamma-ray burst while the rippling spacetime grid indicates the isotropic gravitational waves that characterize the merger. Swirling clouds of material ejected from the merging stars are a possible source of the light that was seen at lower energies. Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet
In August of 2017, a major breakthrough occurred when scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves that were believed to be caused by the collision of two neutron stars. This source, known as GW170817/GRB, was the first gravitational wave (GW) event that was not caused by the merger of two black holes, and was even believed to have led to the formation of one.
As such, scientists from all over the world have been studying this event ever since to learn what they can from it. For example, according to a new study led by the McGill Space Institute and Department of Physics, GW170817/GRB has shown some rather strange behavior since the two neutron stars colliding last August. Instead of dimming, as was expected, it has been gradually growing brighter.
Chandra images showing the X-ray afterglow of the GW170817/GRB event. Credit: NASA/CXC/McGill University/J. Ruan et al.
For the sake of their study, the team relied on data obtained by NASA’s Chandra X-ray Observatory, which showed that the remnant has been brightening in the X-ray and radio wavelengths in the months since the collision took place. As Daryl Haggard, an astrophysicist with McGill University whose research group led the new study, said in a recent Chandra press release:
“Usually when we see a short gamma-ray burst, the jet emission generated gets bright for a short time as it smashes into the surrounding medium – then fades as the system stops injecting energy into the outflow. This one is different; it’s definitely not a simple, plain-Jane narrow jet.”
What’s more, these X-ray observations are consistent with radiowave data reported last month by another team of scientists, who also indicated that it was continuing to brighten during the three months since the collision. During this same period, X-ray and optical observatories were unable to monitor GW170817/GRB because it was too close to the Sun at the time.
However, once this period ended, Chandra was able to gather data again, which was consistent with these other observations. As John Ruan explained:
“When the source emerged from that blind spot in the sky in early December, our Chandra team jumped at the chance to see what was going on. Sure enough, the afterglow turned out to be brighter in the X-ray wavelengths, just as it was in the radio.”
Collisions of neutron stars produce powerful gamma-ray bursts – and heavy elements like gold (Credit: Dana Berry, SkyWorks Digital, Inc.)
This unexpected behavior has led to a serious buzz in the scientific community, with astronomers trying to come up with explanations as to what type of physics could be driving these emissions. One theory is a complex model for neutron star mergers known as “cocoon theory”. In accordance with this theory, the merger of two neutron stars could trigger the release of a jet that shock-heats the surrounding gaseous debris.
This hot “cocoon” around the jet would glow brightly, which would explain the increase in X-ray and radiowave emissions. In the coming months, additional observations are sure to be made for the sake of confirming or denying this explanation. Regardless of whether or not the “cocoon theory” holds up, any and all future studies are sure to reveal a great deal more about this mysterious remnant and its strange behavior.
As Melania Nynka, another McGill postdoctoral researcher and a co-author on the paper indicated, GW170817/GRB presents some truly unique opportunities for astrophysical research. “This neutron-star merger is unlike anything we’ve seen before,” she said. “For astrophysicists, it’s a gift that seems to keep on giving.”
It is no exaggeration to say that the first-ever detection of gravitational waves, which took place in February of 2016, has led to a new era in astronomy. But the detection of two neutron stars colliding was also a revolutionary accomplishment. For the first time, astronomers were able to observe such an event in both light waves and gravitational waves.
In the end, the combination of improved technology, improved methodology, and closer cooperation between institutions and observatories is allowing scientists to study cosmic phenomena that was once merely theoretical. Looking ahead, the possibilities seem almost limitless!
Artist's illustration of two merging neutron stars. The narrow beams represent the gamma-ray burst while the rippling spacetime grid indicates the isotropic gravitational waves that characterize the merger. Swirling clouds of material ejected from the merging stars are a possible source of the light that was seen at lower energies. Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet
In February of 2016, scientists working at the Laser Interferometer Gravitational-Wave Observatory (LIGO) made history when they announced the first-ever detection of gravitational waves. Since that time, the study of gravitational waves has advanced considerably and opened new possibilities into the study of the Universe and the laws which govern it.
For example, a team from the University of Frankurt am Main recently showed how gravitational waves could be used to determine how massive neutron stars can get before collapsing into black holes. This has remained a mystery since neutron stars were first discovered in the 1960s. And with an upper mass limit now established, scientists will be able to develop a better understanding of how matter behaves under extreme conditions.
The study which describes their findings recently appeared in the scientific journal The Astrophysical Journal Letters under the title “Using Gravitational-wave Observations and Quasi-universal Relations to Constrain the Maximum Mass of Neutron Stars“. The study was led by Luciano Rezzolla, the Chair of Theoretical Astrophysics and the Director of the Institute for Theoretical Physics at the University of Frankfurt, with assistance provided by his students, Elias Most and Lukas Wei.
Collisions of neutron stars produce powerful gamma-ray bursts – and heavy elements like gold. Credit: Dana Berry, SkyWorks Digital, Inc.
For the sake of their study, the team considered recent observations made of the gravitational wave event known as GW170817. This event, which took place on August 17th, 2017, was the sixth gravitational wave to be discovered by the Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo Observatory. Unlike previous events, this one was unique in that it appeared to be caused by the collision and explosion of two neutron stars.
And whereas other events occurred at distances of about a billion light years, GW170817 took place only 130 million light years from Earth, which allowed for rapid detection and research. In addition, based on modeling that was conducted months after the event (and using data obtained by the Chandra X-ray Observatory) the collision appeared to have left behind a black hole as a remnant.
The team also adopted a “universal relations” approach for their study, which was developed by researchers at Frankfurt University a few years ago. This approach implies that all neutron stars have similar properties which can be expressed in terms of dimensionless quantities. Combined with the GW data, they concluded that the maximum mass of non-rotating neutron stars cannot exceed 2.16 solar masses.
Artist’s impression of gravitational-wave emissions from a collapsing star. Credit: aktuelles.uni-frankfurt.de
As Professor Rezzolla explained in a University of Frankfurt press release:
“The beauty of theoretical research is that it can make predictions. Theory, however, desperately needs experiments to narrow down some of its uncertainties. It’s therefore quite remarkable that the observation of a single binary neutron star merger that occurred millions of light years away combined with the universal relations discovered through our theoretical work have allowed us to solve a riddle that has seen so much speculation in the past.”
This study is a good example of how theoretical and experimental research can coincide to produce better models ad predictions. A few days after the publication of their study, research groups from the USA and Japan independently confirmed the findings. Just as significantly, these research teams confirmed the studies findings using different approaches and techniques.
In the future, gravitational-wave astronomy is expected to observe many more events. And with improved methods and more accurate models at their disposal, astronomers are likely to learn even more about the most mysterious and powerful forces at work in our Universe.
Artist's illustration of two merging neutron stars. The narrow beams represent the gamma-ray burst while the rippling spacetime grid indicates the isotropic gravitational waves that characterize the merger. Swirling clouds of material ejected from the merging stars are a possible source of the light that was seen at lower energies. Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet
Shortly thereafter, scientists at LIGO, Advanced Virgo, and the Fermi Gamma-ray Space Telescope were able to determine where in the sky the neutron star merger occurred. While many studies have focused on the by-products of this merger, a new study by researchers from Trinity University, the University of Texas at Austin and Eureka Scientific, has chosen to focus on the remnant, which they claim is likely a black hole.
For the sake of their study, which recently appeared online under the title “GW170817 Most Likely Made a Black Hole“, the team consulted data from the Chandra X-ray Observatory to examine what resulted of the supernova merger. This data was obtained during Director’s Discretionary Time observations that were made on December 3rd and 6th, 2017, some 108 days after the merger.
This data showed a light-curve increase in the X-ray band which was compatible to the radio flux increase that was reported by a previous study conducted by the same team. These combined results suggest that radio and X-ray emissions were being produced at the same source, and that the rising light-curve that followed the merger was likely due to an increase in accelerated charged particles in the external shock – the region where an outflow of gas interacts with the interstellar medium.
As they indicate in their study, this could either be explained as the result of a more massive neutron star being formed from the merger, or a black hole:
“The merger of two neutron stars with mass 1.48 ± 0.12 M and 1.26 ± 0.1 M — where the merged object has a mass of 2.74 +0.04-0.01 M… could result in either a neutron star or a black hole. There might also be a debris disk that gets accreted onto the central object over a period of time, and which could be source of keV X-rays.”
The team also ruled out various possibilities of what could account for this rise in X-ray luminosity. Basically, they concluded that the X-ray photons were not coming from a debris disk, which would have been left over from the merger of the two neutron stars. They also deduced that they would not be produced by a relativistic jet spewing from the remnant, since the flux would be much lower after 102 days.
Collisions of neutron stars produce powerful gamma-ray bursts – and heavy elements like gold. Credit: Dana Berry, SkyWorks Digital, Inc.
All of this indicated that the remnant was more likely to be a black hole than a hyper-massive neutron star. As they explained:
“We show next that if the merged object were a hyper-massive neutron star endowed with a strong magnetic field, then the X-ray luminosity associated with the dipole radiation would be larger than the observed luminosity 10 days after the event, but much smaller than the observed flux at t ~ 100 days. This argues against the formation of a hyper-massive neutron star in this merger.”
Last, but not least, they considered the X-ray and radio emissions that were present roughly 100 days after the merger. These, they claim, are best explained by continued emissions coming from the merger-induced shock (and the not remnant itself) since these emissions would continue to propagate in the interstellar medium around the remnant. Combined with early X-ray data, this all points towards GW170817 now being a black hole.
The first-ever detection of gravitational waves signaled the dawn of a new era in astronomical research. Since that time, observatories like LIGO, Advanced Virgo, and GEO 600 have also benefited from information-sharing and new studies that have indicated that mergers are more common than previously thought, and that gravity waves could be used to probe the interior of supernovae.
With this latest study, scientists have learned that they are not only able to detect the waves caused by black hole mergers, but even the creation thereof. At the same time, it shows how the study of the Universe is growing. Not only is astronomy advancing to the point where we are able to study more and more of the visible Universe, but the invisible Universe as well.
