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The explosive collision of neutron stars still emits X-rays, baffling astronomers

to destroy

Two neutron stars collide, generating gravitational waves and a huge jet of light.

Caltech / LIGO

when two neutron stars collided with each other, about 130 million light years from Earth, the universe was illuminated. The collision, between some of the densest objects in the cosmos, produced gravitational waves and splashes of fireworks on August 17, 2017. Dozens of telescopes on Earth have captured the rare merger between different wavelengths of the electromagnetic spectrum. First, there was an explosion of highly energetic gamma rays, followed by bursts of light and UV, radio and infrared signals.

About nine days after the collision, NASA’s Chandra Observatory detected an X-ray signal. According to our understanding of neutron stars, it should be gone by now.

But in a new study, published Monday in the Royal Astronomical Society’s Monthly Notice, researchers studied the neutron star’s impact on a neutron star, designated GW170817, and found that 1,000 days later, the X-ray signal it was still detectable.

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

GW170817 is the first neutron star merger detected by the three gravitational wave observatories stationed on Earth. The triad of observers were able to triangulate the location of the merger moments after it happened, allowing the researchers to spin their telescopes in space and get a good look at the event. And it’s violent.

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Because we haven’t seen many neutron star collisions (only two have been recorded and confirmed so far), scientists have had to rely on models to predict the consequences. For the most part, the models aligned with what we found with GW170817. When two neutron stars collide, they release a jet of gamma rays and a huge explosion of bright gas, known as a “kilonova”. These events are transient: we see them for a few days or weeks and then they disappear. This was the case with GW170817.

But Chandra, NASA’s X-ray observatory, was still detecting X-rays at the location when it focused on the merger in February, two and a half years after it turned on. The latest measurements show that the signal has faded, but the spectrum of an X-ray burst is still visible and is a bit brighter than the models predicted. Why are these X-rays still visible? This is a puzzle that researchers are trying to solve.

There may be an add-on to neutron star fusion patterns that were never taken into account. Or perhaps the dynamics of the energy released in the aftermath of the collision are slightly different from what we expect. An interesting possibility is that the remnants of the fusion represent an X-ray emitting neutron star, although much more analysis is needed to determine where the signal is coming from. Astronomers will turn their telescopes into GW170817 in December, offering another opportunity to unravel the mystery of the merger.

“Whatever happens, this event is changing what we know about neutron star mergers and rewriting our models,” Troja said.

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