The fusion of two neutron stars that generated gravitational waves detected last year could have led to the birth of the lowest mass black hole found, say scientists who analyzed data from NASA’s Chandra X-ray Observatory. The data were taken in the days, weeks and months after the detection of gravitational waves by the Observatory of Gravitational Waves of the Laser Interferometer (LIGO) and the gamma rays by the Fermi mission of NASA on August 17, 2017.
While almost all telescopes observed this source, officially known as GW170817, Chandra’s X-rays are critical to understanding what happened after the two neutron stars collided. From the LIGO data, astronomers have a good estimate that the mass of the object resulting from the fusion of the neutron star is about 2.7 times the mass of the Sun.
This places it on a loose identity string, which implies that it is the most massive neutron star ever found or the lowest mass black hole found. The owners of the previous record for the latter are not less than about four or five times the mass of the Sun.
“While neutron stars and black holes are mysterious, we have studied many of them throughout the universe using telescopes like Chandra,” said Dave Pooley of Trinity University in the United States, who led the study. “That means we have data and theories about how we expect those objects to behave on X-rays,” Pooley said. If the neutron stars merged and formed a heavier neutron star, 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 a bright X-ray emission.
In contrast, Chandra’s data show X-ray levels that are a factor of a few to several hundred times lower than expected for a rapidly fused neutron star and the associated bubble of high-energy particles, which implies a probably formed black hole. 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 an orbit sufficiently adjusted for the gravitational wave radiation to bind the neutron stars.
“Astronomers have long suspected that fusions of neutron stars would form a black hole and produce bursts of radiation, but until now we lacked solid arguments,” said Pawan Kumar of the University of Texas at Austin, in the United States. A Chandra observation two or three days after the event did not detect a source, but subsequent observations 9, 15 and 16 days after the event resulted in detections. The source was behind the Sun shortly thereafter, but an increase in brightness was observed in Chandra’s observations around 110 days after the event, followed by a comparable intensity of X-rays after approximately 160 days.
The researchers said that the observed X-ray emission is entirely due to the shock wave, similar to a sonic boom from a supersonic aircraft, from the merger that crashed into the surrounding gas. There are no signs of X-rays as a result of a neutron star. The affirmations of Pooley’s team can be tested by radiographs and future radio observations. If the remnant turns out to be a neutron star with a strong magnetic field, the source will become much brighter in X-rays and radio wavelengths in about a couple of years when the high-energy particle bubble reaches the deceleration of the shock wave .
If it is indeed a black hole, astronomers expect it to remain weaker than recently observed as the shockwave weakens.