Gravitational wave event likely signaled birth of black hole

The merger of two neutron stars that generated gravitational waves detected last year may have led to the birth of the lowest mass black hole ever found, say scientists who analysed data from NASA's Chandra X-ray Observatory. The data was taken in the days, weeks, and months after the detection of gravitational waves by the Laser Interferometer Gravitational Wave Observatory (LIGO) and gamma rays by NASA's Fermi mission on August 17, 2017. While nearly every telescope observed this source, known officially as GW170817, X-rays from Chandra are critical for 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 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 four or five 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," said Dave Pooley of Trinity University in the US, who led the study. "That means we have both data and theories on how we expect such objects to behave in X-rays," said Pooley. 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. "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 Pawan Kumar of the University of Texas at Austin in the US.