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Astronomers surprised by prolonged X-rays years after a collision of a remarkable neutron star

Fusion of neutron stars

Artistic transmission of two merging neutron stars. Credit: NSF / LIGO / Sonoma State / A. Simonnet

New, most complete view from start to finish neutron star the merger rewrites the way scientists understand these events.

Three years have passed since the remarkable discovery of a neutron star fusion from gravitational waves. And since that day, an international team of researchers led by Eleanor Troy, an astronomer at the University of Maryland, has been constantly monitoring subsequent radiation emissions to provide the most complete picture of such an event.

Their analysis provides possible explanations for the X-rays that continue to emit from the collision long after the models predicted they would stop. The study also reveals that current models of neutron stars and compact body collisions lack important information. The study was published on October 12, 2020 in the journal Monthly notices of the Royal Astronomical Society.

GW170817 Broadcast

Researchers are constantly observing the radiation emitted by the first (and so far only) cosmic event found in both gravitational waves and the entire spectrum of light. The collision of neutron stars, discovered on August 17, 2017, is seen in this image emitted by the galaxy NGC 4993. The new analysis provides possible explanations for the X-rays that continue to emit from the collision long after the disappearance of other radiation and bypass the predictions of model. Credit: E. Troy

“We are entering a new phase in our understanding of neutron stars,” said Troy, an associate scientist in the Department of Astronomy at UMD and lead author of the article. “We really don’t know what to expect from now on, because all our models predicted no X-rays and we were surprised to see them 1,000 days after the collision was discovered. It may take years to understand the answer to what is happening, but our research opens the door to many possibilities.

The fusion of neutron stars, which the Troy team is studying – GW170817 – was first identified by gravitational waves discovered by the Gravitational Wave Observatory of the laser interferometer and its counterpart Virgo on August 17, 2017. Within hours, telescopes around the world began to observe electromagnetic radiation, including gamma rays and light emitted by the explosion. This was the first and only time that astronomers have been able to observe the radiation associated with gravitational waves, although they have long known that such radiation occurs. All other gravitational waves observed so far originate from events that are too weak and too far away to detect radiation from Earth.

Seconds after the discovery of GW170817, scientists recorded the initial jet of energy known as a gamma-ray burst, then the slower kilonova, a cloud of gas that exploded behind the original jet. The light from the kilogram lasted for about three weeks and then faded. Meanwhile, nine days after the first discovery of the gravitational wave, telescopes are observing something they have never seen before: X-rays. Scientific models based on known astrophysics have predicted that while the initial jet from a neutron star collision travels through interstellar space, it creates its own shock wave that emits X-rays, radio waves, and light. This is known as afterglow. But such radiation has never been observed before. In this case, the subsequent glow peaked about 160 days after the gravitational waves were detected and then quickly disappeared. But the X-rays remained. They were last observed by the Chandra X-ray Observatory two and a half years after the first discovery of GW170817.

The new research paper offers several possible explanations for long-term X-ray emissions. One possibility is that these X-rays represent a completely new characteristic of the subsequent glow of a collision, and the dynamics of the gamma-ray burst is somewhat different than expected.

“The presence of a collision so close to us opens a window to the whole process, to which we rarely have access,” said Troy, who is also a researcher at NASAGoddard Space Flight Center. “There may be physical processes that we have not included in our models because they are not relevant in the earlier stages that we are more familiar with when jets form.”

Another possibility is that the kilonian and expanding cloud of gases behind the initial stream of radiation may have created their own shock wave, which takes longer to reach Earth.

“We’ve seen the kilogram, so we know that this cloud of gas is there, and the X-rays from its shock wave may just be reaching us,” said Jeffrey Ryan, a postdoctoral fellow in UMD’s astronomy department and co-author of the study. “But we need more data to see if this is what we see. If it is, it can give us a new tool, a signature of those events that we have not recognized before. This can help us detect neutron star collisions in previous X-ray recordings. “

A third possibility is that there may be something left after the collision, perhaps the remnant of an X-ray neutron star.

Much more analysis is needed before researchers can confirm exactly where the continuous X-rays come from. Some answers may come in December 2020, when the telescopes will again be directed to the source of GW170817. (The last observation was in February 2020)

“This could be the last breath of a historical source or the beginning of a new history, in which the signal is re-illuminated in the future and may remain visible for decades or even centuries,” Troy said. “Whatever happens, this event changes what we know about neutron star fusions and rewrites our patterns.”

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

Additional authors of the article from the Department of Astronomy at UMD are Assistant Professor Brendan O’Connor and Associate Professor Stephen Chenko.

This work was partially supported by NASA (Chandra Award number G0920071A, NNX16AB66G, NNX17AB18G and 80NSSC20K0389.), Postdoctoral Fellowship of the Joint Space Science Award and the European Union Horizon 2020 Award (award 58 8711). The content of this article does not necessarily reflect the views of these organizations.

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