X-rays detected years after neutron stars collided

The neutron star merger the Maryland team studied - GW170817 - was first identified by

gravitational waves, which werediscovered by LIGO in the constellation Virgo on August 17, 2017. Within hours, telescopes around the world began observing electromagnetic radiation, including gamma rays, rays and light coming from the explosion. This was the first and only time that astronomers were able to observe radiation associated with gravitational waves, although they had long known that such an event was possible in principle. All other gravitational waves observed to date have resulted from events too weak and too distant for the radiation to be detected from Earth.

A few seconds after GW170817was discovered, scientists detected an initial blast of energy known as a gamma-ray burst, followed by a slower kilonova. The light from it lasted about three weeks before fading. Meanwhile, nine days after the gravitational wave was first detected, scientists saw telescopes observe something they had never seen before: X-rays.

Scientific models predicted that when the originalthe jet from the collision of a neutron star moves through interstellar space, it creates its own shock wave, which emits X-rays, radio waves and light. This phenomenon is known as afterglow. But such an afterglow has not been observed before.

Researchers constantly tracked the radiation,emanating from the first (and so far the only) cosmic event detected both in gravitational waves and in the entire spectrum of light. This image shows a neutron star collision detected on August 17, 2017, emanating from the galaxy NGC 4993. The new analysis provides possible explanations for the X-rays that continued to be emitted from the collision long after other radiation disappeared and far exceeded model predictions. Credit: E. Troy.

In the case of GW170817, the afterglow peaked about 160 days after the gravitational waves were detected and then quickly disappeared. But the X-rays remained.

The new study suggests severalpossible explanations for long-lived X-rays. One possibility is that these X-rays represent a completely new feature of the collision afterglow, and the dynamics of the gamma-ray burst is somehow different from what is expected.

“The collision is so close to us that it can be seen, andopens up a window into the whole process that we rarely have access to, ”explains astronomer Eleanor Troy, who is also a research assistant at NASA's Goddard Space Flight Center. "There may be physical processes that we have not included in our models."

Another possibility is thatThe kilonova and expanding gas cloud behind the original radiation jet could create their own shock wave, which took longer to reach Earth.

A third possibility is that something might have been left over from the collision, possibly the remnants of an X-ray emitting neutron star.

Much more analysis is needed beforeresearchers will be able to confirm exactly where the X-rays came from. Some answers may appear in December 2020, when the telescopes will again target the source GW170817. The last observation was in February 2020.

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