Merger Of Two Neutron Stars That Generated Gravitational Waves May Have Spawned A Black Hole




The merger of two neutron stars that generated gravitational waves and was observed by LIGO and Virgo in 2017 is 2.7 times more massive than the sun. Here’s why researchers think this remnant is a black hole. 
( NASA/CXC/M.Weiss )

Last year, the U.S.-based Laser Interferometer Gravitational-Wave Observatory(LIGO) and the Europe-based Virgo detector detected the collision of two neutron stars  that generated gravitational waves.

Remnant Of The Gravitational Wave Event

The event marks the first time that scientists have witnessed the merger of two neutron stars.

Now, a new study suggests that the merger also birthed a black hole. Researchers said that the remnant of the event could be the lowest mass black hole ever found.

Dave Pooley of Trinity University in San Antonio, Texas, and colleagues analyzed data from NASA’s Chandra X-ray Observatory before the August 2017 detection of gravitational waves. Chandra’s X-rays are crucial to a better understanding of what occurred after the collision of the two neutron stars.

Using data from LIGO, the researchers were able to estimate that the mass of the object produced by the neutron star merger is about 2.7 times the solar mass, which suggests this could either be the most massive neutron star ever found or the lowest mass black hole ever found.

Black Hole Or Neutron Star?

If a heavier neutron star formed as a result of the merger, astronomers said it would spin rapidly and produce a very strong magnetic field. This would create an expanding bubble of high-energy particles that would result in bright X-ray emission.

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Chandra’s data, however, showed levels of X-rays that are lower than what is expected for the merged neutron star. This suggests that what formed is a black hole.

“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 study coauthor Pawan Kumar of the University of Texas at Austin.

The idea that the remnant is a neutron star still has to be confirmed.

The theory can be tested with the help of future radio and X-ray observations. If the object turns out to be a neutron star, it is expected to get brighter at radio wavelengths and X-rays in a few years. If it is a black hole, it would continue to become fainter as the shock wave weakens.

“If the remnant is a rapidly rotating magnetized neutron star, the total energy in the external shock should rise by a factor ~102 (to ~1052 erg) after a few years; therefore, Chandra observations over the next year or two that do not show substantial brightening will rule out such a remnant,” the researchers wrote in their study, which was published in the Astrophysical Journal Letters on May 31.

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