Explosive neutron star collisions still emit X-rays and confuse astronomers

When two neutron stars hit each otherAbout 130 million light years from Earth, the universe glowed. The collision between some of the densest objects in the cosmos created gravitational waves and a splash of fireworks on August 17, 2017. Dozens of telescopes on Earth captured the rare merger over different wavelengths of the electromagnetic spectrum. First there was an outbreak of high energy gamma rays, followed by light and UV, radio and infrared signals.

About nine days after the collision Chandra-Observatory der NASA recorded an X-ray signal. From our understanding of neutron stars, it should be gone by now.

In a new study published Monday in the Royal Astronomical Society’s Monthly Notices, researchers examined the influence of neutron stars on neutron star labeled GW170817 and found that 1000 days later the X-ray signal was still detectable.

“We really don’t know what to expect from this point on, as all of our models didn’t predict X-rays,” said Eleonora Troja, astrophysicist at NASA’s Goddard Space Flight Center and lead author of the study, in a press release.

GW170817 is the first neutron star fusion discovered by the three gravitational wave observatories stationed on Earth. The three observatories were able to triangulate the position of the fusion as soon as it entered, allowing the researchers to turn their telescopes into space and get a good look at the event. And it’s violent.

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Since we haven’t seen many neutron star collisions (only two have been recorded and confirmed so far), scientists had to rely on models to predict the consequences. The models were largely consistent with what was detected with GW170817. When two neutron stars collide, they release a beam of gamma rays and a huge beam of bright gas known as the “kilonova”. These events are temporary – we see them for a few days or weeks and then they go away. That was the case with GW170817.

But Chandra, NASA’s X-ray observatory, was still spotting X-rays in the location when it focused on the fusion in February, two and a half years after it was brought to life. The latest measurements show that the signal has faded, but the ghost of an X-ray is still visible and slightly brighter than the predicted models. Why are these x-rays still visible? This is a puzzle the researchers are trying to solve.

There may be an additional component of the neutron star fusion models that have not previously been considered. Or maybe the dynamics of the energy released after the collision is a little different from what we expect. One exciting possibility is that the remnants of the fusion represent an x-ray-emitting neutron star – although a lot more analysis is required to determine where the signal is coming from. Astronomers will switch their telescopes to GW170817 in December, offering another opportunity to solve the puzzle of fusion.

“Whatever happens, this event changes what we know about neutron star fusions and rewrites our models,” Troy said.