A new study using X-ray data from NASA’s Chandra X-ray Observatory indicates that the neutron star merger that became the gravitational wave source, GW170817, likely created the lowest mass black hole known.
The GW170817 gravitational signal was first detected on August 17, 2017.
The detection was made by the two Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors, located in Hanford, Washington, and Livingston, Louisiana. The information provided by the third detector, Virgo, situated near Pisa, Italy, enabled an improvement in localizing the event.
In the night following the initial discovery of GW170817, professional astronomers started their hunt to locate the source of the event.
They found it in NGC 4993, a lenticular galaxy located about 130 million light-years from Earth in the constellation Hydra.
While nearly every telescope at astronomers’ disposal observed GW170817, X-rays from NASA’s Chandra X-ray Observatory are critical for understanding what happened after the spectacular merger of two neutron stars.
From the LIGO data astronomers have a good estimate that the mass of the object resulting from the neutron star merger is about 2.7 times the mass of the Sun.
This puts it on a tightrope of identity, implying it is either the most massive neutron star ever found or the lowest mass black hole ever found. The previous record holders for the latter are no less than about 4-5 times the Sun’s mass.
“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,” said lead author Dr. Dave Pooley, from Trinity University in San Antonio, Texas.
The Chandra observations are telling, not only for what they revealed, but also for what they did not.
If the neutron stars merged and formed a heavier neutron star, then astronomers would expect it to spin rapidly and generate a very strong magnetic field. This, in turn, would have created an expanding bubble of high-energy particles that would result in bright X-ray emission.
Instead, the Chandra data show levels of X-rays that are a factor of a few to several hundred times lower than expected for a rapidly spinning, merged neutron star and the associated bubble of high-energy particles, implying a black hole likely formed instead.
If confirmed, this result shows that a recipe for making a black hole can sometimes be complicated.
In the case of GW170817, it would have required two supernova explosions that left behind two neutron stars in a sufficiently tight orbit for gravitational wave radiation to bring the neutron stars together.
“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, said co-author Dr. Pawan Kumar, from the University of Texas at Austin.
By comparing the Chandra observations with those by NSF’s Karl G. Jansky Very Large Array (VLA), the team explains the observed X-ray emission as being due entirely to the shock wave from the merger smashing into surrounding gas. There is no sign of X-rays resulting from a neutron star.
The claims by the researchers can be tested by future X-ray and radio observations.
If the remnant turns out to be a neutron star with a strong magnetic field, then the source should get much brighter at X-ray and radio wavelengths in about a couple of years when the bubble of high energy particles catches up with the decelerating shock wave.
If it is indeed a black hole, they expect it to continue to become fainter that has recently been observed as the shock wave weakens.
“GW170817 is the astronomical event that keeps on giving. We are learning so much about the astrophysics of the densest known objects from this one event,” said co-author Dr. J. Craig Wheeler, also from the University of Texas.
If follow-up observations find that a heavy neutron star has survived, such a discovery would challenge theories for the structure of neutron stars and how massive they can get.
“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,” said co-author Dr. Bruce Gossan, from the University of California, Berkeley.
The study will be published in the Astrophysical Journal Letters (arXiv.org preprint).
David Pooley et al. 2018. GW170817 Most Likely Made a Black Hole. ApJL, in press; arXiv: 1712.03240