Astronomers were surprised by the continued X-rays years after a neutron star collided

Art show of merging two neutron stars. Credit: NSF / LIGO / Sonoma State / A. Simonet

The new, more complete show from start to finish for Neutron star Merger is rewriting the way scholars understand these events.

It has been three years since the historic discovery of the merging of a neutron star from Gravitational waves. Since that day, an international team of researchers led by University of Maryland astronomer Eleonora Troja has been constantly monitoring the subsequent radiation emissions to provide the most complete picture of such an event.

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

The researchers constantly monitored the radiation emitted from the first (and so far only) cosmic event that was detected in both the gravitational waves and the entire spectrum of light. The collision of a neutron star detected on August 17, 2017 is shown in this image emanating from the galaxy NGC 4993. A new analysis provides possible explanations for the X-rays that continued to emit from the collision long after the other radiation had faded away and exceeded the model’s expectations. Credit: E. Troja

“We are entering a new phase in our understanding of neutron stars,” said Troja, an associate research scientist in UMD’s astronomy division and lead author of the paper. “We don’t really know what to expect from this point on, because all of our models did not anticipate any x-rays and were surprised to see them 1,000 days after the crash was discovered. It might take years to figure out the answer to what’s going on, but our research opens the door to many possibilities.

The merger of neutron stars that Troja’s team – GW170817 – was studying – was first recognized by the gravitational waves detected by the LIWO and its counterpart Virgo on August 17, 2017. Within hours, telescopes around the world began monitoring electromagnetic radiation, including Gamma rays and light emitted from the explosion. It was the first and only time that astronomers had been able to observe radiation associated with gravitational waves, even though they had long known that this radiation was occurring. All other gravitational waves observed to date originate from events so weak and far from the rays that cannot be detected from Earth.

Seconds after discovering GW170817, the scientists recorded the initial flux of energy, known as a gamma-ray burst, then the slower kilonova, a cloud of gas that exploded behind the initial plane. The light from Kilonova lasted about three weeks and then faded away. Meanwhile, nine days after detecting the gravitational wave for the first time, telescopes noticed something they hadn’t seen before: X-rays. Scientific models based on known astrophysics predict that as the initial jet moves from a neutron star collision through interstellar space, it creates its own shock wave, which emits X-rays, radio waves and light. This is known as aurora. But such a subsequent glow was not noticed before. In this case, subsequent twilight peaked about 160 days after the gravitational waves were detected and then rapidly faded. But the x-rays remained. It was last spotted by the Chandra X-ray Observatory two and a half years after GW170817 was first detected.

The new paper proposes some possible explanations for the long-lived X-ray emissions. One possibility is that these X-rays represent a completely new feature of post-collision glare, and the dynamics of the gamma-ray burst are somewhat different than expected.

“A collision so close to us that it’s visible opens a window into the whole process that we’re rarely able to access,” said Troja, who is also a research scientist at Troja. NASAGoddard Space Flight Center. “There may be physical processes that we haven’t included in our models because they are irrelevant in the earlier stages that we know best, when planes are forming.”

Another possibility is that the Kilonova and the expanding gas cloud behind the primary jet of radiation may have created its own shock wave that took longer to reach Earth.

“We’ve seen Kilonova, so we know that this gas cloud is there, and the X-rays from the shock wave might just be reaching us,” said Geoffrey Ryan, a postdoctoral fellow in the UMD Department of Astronomy and one of the authors of the book. studying. “But we need more data to understand whether this is what we see. If so, it might give us a new tool, which is a signature of these events that we did not recognize before. This could help us find neutron star collisions in previous records of radiation radiation.” X. “

The third possibility is that something was left behind after the collision, possibly the remnants of a neutron star emitting X-rays.

More analysis is needed before researchers can confirm the exact source of the trapped X-ray. Some answers may come in December 2020, when telescopes are once again pointed at the source of GW170817. (The last note was February 2020).

“This may be the last breath of a historical source or the beginning of a new story, where the sign shines again in the future and may remain visible for decades or even centuries,” Troja said. “Whatever happens, this event changes what we know about neutron star fusion and rewrites of our models.”

The reference: “A Thousand Days After Merger: Continued X-ray Emission from GW170817” By E. Troja, H. van Eerten, B. Zhang, G. Ryan, L. Piro, R. Ricci, B. O’Connor, MH Wieringa And SB Cenko and T. Sakamoto, October 12 12 2020, Monthly Notices of the Royal Astronomical Society.
DOI: 10.1093 / mnras / staa2626

Additional authors of the research paper from the UMD Department of Astronomy are College Assistant Brendan O’Connor and Associate Associate Professor Stephen Cinco.

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