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It's been three years since the landmark detection of a neutron star merger from gravitational waves. And since that day, an international team of researchers, including ���ӽ紫ý astrophysicist Bing Zhang, has been continuously monitoring the subsequent radiation emissions to provide the most complete picture of such an event. 

Their analysis offers possible explanations for X-rays that continued to radiate from the collision long after models predicted they would stop. The study also reveals that current models of neutron stars are missing important information. The paper, “A thousand days after the merger: continued X-ray emission from GW170817,” for which Zhang was a collaborating theorist and co-author, was published Oct. 12 in the journal . 

“We are entering a new phase in our understanding of neutron stars,” said Eleonora Troja, associate research scientist at the University of Maryland and lead author of the paper. “We really don’t know what to expect from this point forward, because all our models were predicting no X-rays and we were surprised to see them 1,000 days after the collision event was detected. It may take years to find out the answer to what is going on, but our research opens the door to many possibilities.”

The neutron star merger that the team studied—GW170817—was first identified from gravitational waves detected on August 17, 2017. Within hours, telescopes around the world began observing electromagnetic radiation, including gamma rays and light emitted from the explosion. It was the first and only time astronomers were able to observe the radiation associated with gravitational waves, although they long knew such radiation occurs. All other gravitational waves observed to date have originated from events that are either too far away or do not emit bright electromagnetic radiation to be detected from Earth. 

Seconds after GW170817 was detected, scientists 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. 

Scientific models based on known astrophysics 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. This afterglow was observed to rise early on, peaked around 160 days after the gravitational waves were detected and then faded rapidly. After three years, the radio and light faded away, but the X-rays remained. They were last observed by the Chandra X-ray Observatory two and a half years after GW170817 was first detected.

The research paper suggests 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. 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. A third possibility is that something may have been left behind after the collision, perhaps the remnant of a massive X-ray emitting neutron star. 

“This third possibility is intriguing, because it will place an important constraint on the poorly known equation of state of nuclear matter,” said Zhang. “Long-term monitoring of electromagnetic radiation from this and other future binary neutron star mergers will help to address this fundamental problem in physics.”

Much more analysis is needed before researchers can confirm exactly where the lingering X-rays came from. Some answers may come in December 2020, when the Chandra X-ray telescope will once again be aimed at the source of GW170817. 

“This may be the last breath of an historical source or the beginning of a new story, in which the signal brightens up again in the future and may remain visible for decades or even centuries,” Troja said. “Whatever happens, this event is changing what we know about neutron star mergers and rewriting our models.”