Years after we detected two neutron stars crashing into each other, we're still picking up X-rays. We don't know why

'Discovering a new type of celestial source is very exciting' lead astroboffin tells us

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After a thousand days of observations, the continuing X-ray radiation from two neutron stars smashing into one another has left astronomers puzzled.

The collision, code-named GW170817, was picked up by our planet's LIGO and Virgo gravitational wave detectors in mid-2017. The incredible crash, some 130 million light-years away, spilled a heady mix of electromagnetic signals into space as well as the gravitational wave we detected here on Earth.

One remarkable feature of the merger was its kilonova, which immediately followed an initial gamma-ray burst. Light from this kilonova faded about three weeks after GW170817 was observed, as expected. Curiously, though, X-rays were picked up nine days after the merger's gravitational wave was detected, and they continue to linger well past what’s typically considered normal. The X-ray afterglow is still visible three years after the detection of GW170817.

“The X-rays persist for much longer, and we do not understand why,” Eleonora Troja, an associate research scientist at the University of Maryland in the US, who helped detect GW170817, told The Register.

It's thought the X-rays are the result of a physical process, related to the neutron star merger, not yet observed or considered by scientists, or that the kilonova created its own shock wave through space that resulted in the delayed X-ray emissions.

“It may be that our models are somehow incomplete and do not fully capture the complexity of these extreme phenomena, or we may be seeing something else," Troja said. "Maybe it’s the remnant of the kilonova explosion, or signs from the stellar object that survived the gravitational wave merger.”

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Indeed, a paper, of which Troja is lead author, was published in the Monthly Notices of the Royal Astronomical Society on Monday, and suggests the neutron star collision created another neutron star that’s been emitting x-rays for all these years.

“This third possibility is intriguing, because it will place an important constraint on the poorly known equation of state of nuclear matter,” added Bing Zhang, a co-author of the paper and an astrophysics professor at the Uni of Maryland.

The X-rays are incredibly energetic and bright: Troja said the emissions are 100 billion times brighter than those from the Sun, judging from the latest readings.

“We think that the jet has traveled for over 10,000 billion kilometres, that is larger than the size of our Solar System,” she told us. "If this jet was launched by our Sun, it would be way further than Pluto, making its way through the Oort cloud.

“In the same way we see the remnants of supernova explosions, like the Crab Nebula or the Jellyfish nebula, we may discover the remnant of a kilonova explosion. We have never seen such a thing before, it only exists in our theories and models. This could really change the way we search for gravitational wave mergers, and the idea of discovering a new type of celestial source is very exciting." ®

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