When two neutron stars collide, it creates a kilonova. The event causes both gravitational waves and emissions of electromagnetic energy. In 2017 the LIGO-Virgo gravitational-wave observatories detected a merger of two neutron stars about 130 million light-years away in the galaxy NGC 4993. The merger is called GW170817, and it remains the only cosmic event observed in both gravitational waves and electromagnetic radiation.
Astronomers have watched the expanding debris cloud from the kilonova for years. A clearer picture of what happens in the aftermath is emerging.
Neutron stars are the remnants of massive stars that explode as supernovae at the end of their fusion lives. They’re super-dense cores where all of the protons and electrons are crushed into neutrons by the overpowering gravity of the dead star. They’re the smallest and densest stellar objects, except for black holes, and possibly other arcane, hypothetical objects like quark stars.
When two neutron stars merge, we can detect the resulting gravitational waves. But some aspects of these mergers are poorly-understood. One question surrounds short-lived gamma-ray bursts from these mergers. Previous studies have shown that these bursts may come from the decay of heavy elements produced in a neutron star merger.
A new study strengthens our understanding of these complex mergers and introduces a model that explains the gamma rays.
Astronomers have spotted Strontium in the aftermath of a collision between two neutron stars. This is the first time a heavy element has ever been identified in a kilonova, the explosive aftermath of these types of collisions. The discovery plugs a hole in our understanding of how heavy elements form.
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.
For almost a century, astronomers have been studying supernovae with great interest. These miraculous events are what take place when a star enters the final phase of its lifespan and collapses, or is stripped by a companion star of its outer layers to the point where it undergoes core collapse. In both cases, this event usually leads to a massive release of material a few times the mass of our Sun.
However, an international team of scientists recently witnessed a supernova that was a surprisingly faint and brief. Their observations indicate that the supernova was caused by an unseen companion, likely a neutron star that stripped its companion of material, causing it to collapse and go supernova. This is therefore the first time that scientists have witnessed the birth of a compact neutron star binary system.
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.
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.
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.”
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.”
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:
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.
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.
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.
As astute readers of Universe Today, you likely know what a supernova is: a stellar explosion that signals the end game for certain kinds of stars. Above, however, is a picture of a kilonova, which happens when two really dense objects come together.
This fireball arose after a short-term (1/10 of a second) gamma-ray burst came into view of the Swift space telescope on June 3. Nine days later, the Hubble Space Telescope looked at the same area to see if there were any remnants, and spotted a faint red object that was confirmed in independent observations.
It’s the first time astronomers have been able to see a connection between gamma-ray bursts and kilonovas, although it was predicted before. They’re saying this is the first evidence that short-duration gamma ray bursts arise as two super-dense stellar objects come together.
So what’s the connection? Astronomers suspect it’s this sequence of events:
Two binary neutron stars (really dense stars) start to move closer to each other;
The system sends out gravitational radiation that make ripples in space-time;
These waves make the stars move even closer together;
In the milliseconds before the explosion, the two stars “merge into a death spiral that kicks out highly radioactive material,” as NASA states, with material that gets warmer, gets bigger and sends out light;
The kilonova occurs with the detonation of a white dwarf. While it’s bright, 1,000 times brighter than a nova, it’s only 1/10th to 1/100th the brightness of an average supernova.
“This observation finally solves the mystery of the origin of short gamma ray bursts,” stated Nial Tanvir of the University of Leicester in the United Kingdom, who is also the lead author.
“Many astronomers, including our group, have already provided a great deal of evidence that long-duration gamma ray bursts (those lasting more than two seconds) are produced by the collapse of extremely massive stars. But we only had weak circumstantial evidence that short bursts were produced by the merger of compact objects. This result now appears to provide definitive proof supporting that scenario.”