The merger of two neutron stars that generated gravitational waves detected in 2017 may have led to the creation of a lowest-mass black hole ever found, say scientists who analyzed data from NASA’s Chandra X-ray Observatory.

The data was taken in the months after the detection of gravitational waves by the Laser Interferometer Gravitational Wave Observatory (LIGO) and gamma rays by NASA’s Fermi mission in August 2017.

X-rays from Chandra are critical for understanding what happened after the two neutron stars collided, especially with every telescope observing this source, known officially as GW170817.

Astronomers have a solid estimate that the mass of the object resulting from the neutron star merger is about 2.7 times the mass of the Sun, based on the LIGO data.

“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.

By contrast, the Chandra data shows 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, indicating a black hole may have formed.

“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.

A Chandra observation two to three days after the event failed to detect a source, but subsequent observations 9, 15 and 16 days after the event, resulted in detections. The source went behind the Sun soon after, but further brightening was seen in Chandra observations about 110 days after the event, followed by comparable X-ray intensity after about 160 days.

Researchers said that the observed X-ray emission as being due entirely to the shock wave – akin to a sonic boom from a supersonic plane – from the merger smashing into surrounding gas.

Claims by Pooley’s research team can be tested by future radio and X-ray observations. According to the researchers, 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 within a few of years when the bubble of high energy particles catches up with the decelerating shock wave.