Astronomers Surprised by Lingering X-rays Years After Landmark Neutron Star Collision

New, most complete view from start to finish of Neutron star The merger will rewrite the way scientists understand these events.

It’s been three years since the groundbreaking discovery of a neutron star fusion Gravitational waves. And since that day, an international team of researchers led by the astronomer Eleonora Troja from the University of Maryland has been continuously monitoring the subsequent radiation emissions in order to obtain the most complete possible picture of such an event.

Their analysis provides possible explanations for x-rays emanating from the collision long after the models were predicted to stop. The study also shows that current models of neutron stars and compact body collisions lack important information. The study was published in the journal on October 12, 2020 Monthly announcements from the Royal Astronomical Society.

The researchers continuously monitored the radiation emanating from the first (and so far only) cosmic event, which was detected both in gravitational waves and in the entire spectrum of light. The neutron star collision detected on August 17, 2017 can be seen in this image from the galaxy NGC 4993. A new analysis provides possible explanations for X-rays that emitted from the collision long after other radiation faded and far behind the model predictions. Photo credit: E. Troy

“We are entering a new phase in our understanding of neutron stars,” said Troy, associate research scientist at the UMD Department of Astronomy and lead author of the paper. “We really don’t know what to expect from this point on as all of our models didn’t predict X-rays and we were surprised to see them 1,000 days after the collision event was detected. It may take years to figure out the answer to what is happening, but our research opens the door to many possibilities.

The neutron star fusion studied by Troy’s team – GW170817 – was first identified from gravitational waves captured by the laser interferometer gravitational wave observatory and its counterpart Virgo on August 17, 2017. Within hours, telescopes around the world began observing electromagnetic radiation, including gamma rays, and light from the explosion. It was the first and only time that astronomers could observe the radiation associated with gravitational waves, although they long knew that such radiation was occurring. All other gravitational waves observed so far originate from events that are too weak and too far away for the radiation to be detected by the earth.

Seconds after GW170817 was discovered, the scientists recorded the initial beam of energy known as the gamma-ray burst, then the slower kilonova, a cloud of gas that erupts behind the initial beam. The light from the Kilonova lasted about three weeks and then faded. Meanwhile, nine days after the gravitational wave was first detected, the telescopes were observing something they hadn’t seen before: X-rays. Scientific models based on well-known astrophysics predicted that the initial beam of a neutron star collision travels through interstellar space, creating its own shock wave that emits X-rays, radio waves, and light. This is known as afterglow. However, such afterglow had never been observed. In this case, the afterglow peaked about 160 days after the gravitational waves were detected and then quickly disappeared. But the X-rays stayed. They were last observed by the Chandra X-ray Observatory two and a half years after GW170817 was first detected.

The new research paper suggests some possible explanations for the long-lived X-ray emissions. One possibility is that these x-rays are an entirely new feature of the afterglow of a collision, and the dynamics of a gamma-ray burst is somewhat different than expected.

“A collision that is so close to us that it is visible opens a window into the whole process that we rarely have access to,” said Troy, who is also a scientist at NASAGoddard Space Flight Center. “It may be that there are physical processes that we have not included in our models because they are not relevant in the earlier phases that we are more familiar with when the jets form.”

Another possibility is that the kilonova and the expanding gas cloud behind the initial radiation beam 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 of its shock wave may just be reaching us,” said Geoffrey Ryan, postdoctoral fellow in the UMD department of astronomy and co-author of the study. “But we need more data to understand whether we’re seeing this. If it does, we may get a new tool, a signature of these events that we haven’t seen before. This could help us find neutron star collisions in previous records of X-rays. ”

A third possibility is that something may have been left behind after the collision, possibly the remainder of an x-ray emitting neutron star.

Much more analysis is needed before researchers can confirm exactly where the remaining x-rays are coming from. Some answers could come in December 2020 when the telescopes are again aimed at the source of GW170817. (The last observation was in February 2020.)

“This can be the last breath of a historical source or the beginning of a new story in which the signal will lighten up again in the future and remain visible for decades or even centuries,” Troy said. “Whatever happens, this event changes what we know about neutron star fusions and rewrites our models.”

Reference: “A thousand days after the merger: Continuation of the X-ray emission from GW170817” by E. Troja, H. van Eerten, B. Zhang, G. Ryan, L. Piro, R. Ricci, B. O’Connor, MH Wieringa, SB Cenko and T. Sakamoto, October 12, 2020, Monthly announcements from the Royal Astronomical Society.
DOI: 10.1093 / mnras / staa2626

Other authors of the UMD Department of Astronomy Paper include Faculty Assistant Brendan O’Connor and Associate Associate Professor Stephen Cenko.

This work was supported in part by NASA (Chandra Award No. G0920071A, NNX16AB66G, NNX17AB18G, and 80NSSC20K0389), the Postdoctoral Fellowship of the Joint Space-Science Institute Prize, and the European Union’s Horizon 2020 Program (Award No. 871158). The contents of this article do not necessarily reflect the views of these organizations.