In 2017, astronomers detected a gravitational-wave signal, designated GW170817, from the merger of two neutron stars. And since the detection, they have been continuously monitoring the subsequent emissions to provide the most complete picture of such an event. Their analysis provides possible explanations for X-rays that continued to radiate from the GW170817 collision long after models predicted they would stop.
This X-ray image, taken by NASA’s Chandra X-ray Observatory, shows GW170817. The central panel shows the stacked image of the field, with total exposure of 783 ks. The position of GW170817 is marked. In addition, several X-ray point sources as well as extended diffuse X-ray emissions are visible. The image stamps are centered on the location of GW170817, showing the main phases of its evolution. Image credit: Troja et al., doi: 10.1093/mnras/staa2626.
The GW170817 event was first spotted by the Laser Interferometer Gravitational-wave Observatory and its counterpart Virgo on August 17, 2017.
It occurred in the lenticular galaxy NGC 4993, which is located about 130 million light-years from Earth in the constellation Hydra.
Within hours after the detection, telescopes around the world began observing electromagnetic radiation, including gamma rays and light emitted from the merger.
It was the first and only time astronomers were able to observe the radiation associated with gravity waves, although they long knew such radiation occurs.
Seconds after GW170817 was detected, the telescopes recorded the initial jet of energy, known as a gamma-ray burst, then the slower kilonova, a cloud of gas which burst forth behind the initial jet.
Light from the kilonova lasted about three weeks and then faded. Meanwhile, nine days after the gravity wave was first detected, the telescopes observed something they’d not seen before: X-rays.
Astrophysical models predicted that as the initial jet from a neutron star collision moves through interstellar space, it creates its own shockwave, which emits X-rays, radio waves and light. This is known as the afterglow.
But such an afterglow had never been observed before. In this case, the afterglow peaked around 160 days after the gravity waves were detected and then rapidly faded away.
But the X-rays remained. They were last observed by NASA’s Chandra X-ray Observatory about 2.5 years after GW170817 was first detected.
A research team led by Dr. Eleonora Troja from the University of Maryland and NASA’s Goddard Space Flight Center propose a few possible explanations for the long-lived X-ray emissions.
One possibility is that these X-rays represent a completely new feature of a collision’s afterglow, and the dynamics of a gamma-ray burst are somehow different than expected.
“Having a collision so close to us that it’s visible opens a window into the whole process that we rarely have access to,” Dr. Troja said.
“It may be there are physical processes we have not included in our models because they’re not relevant in the earlier stages that we are more familiar with, when the jets form.”
Another possibility is that the kilonova and the expanding gas cloud behind the initial jet of radiation may have created their own shock wave that took longer to reach Earth.
“We saw the kilonova, so we know this gas cloud is there, and the X-rays from its shock wave may just be reaching us,” said Dr. Geoffrey Ryan, a postdoctoral researcher at the University of Maryland.
“But we need more data to understand if that’s what we’re seeing. If it is, it may give us a new tool, a signature of these events that we haven’t recognized before. That may help us find neutron star collisions in previous records of X-ray radiation.”
A third possibility is that something may have been left behind after the collision, perhaps the remnant of an X-ray emitting neutron star.
“Long-term monitoring of this source will be essential to test these different models,” the researchers said.
Their paper was published in the Monthly Notices of the Royal Astronomical Society.
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E. Troja et al. 2020. A thousand days after the merger: Continued X-ray emission from GW170817. MNRAS 498 (4): 5643-5651; doi: 10.1093/mnras/staa2626
