Mysterious X-rays could be kilonova ‘afterglow’ from 2017 neutron star mergers

Zoom / Artistic representation of the merging of two neutron stars to form a black hole (hidden within a bright bulge in the center of the image). The merger generates opposing, high-energy jets of matter (blue) that heat the material around the stars, causing them to emit X-rays (reddish clouds).

NASA/CXC/M. Weiss

In 2017, astronomers discovered a phenomenon known askilonova“: A merger of two neutron stars accompanied by powerful bursts of gamma rays. Three and a half years later, astrophysicists have discovered a mysterious X-ray that they believe could be the first detection of the ‘after-kilonova glow’,” according to a new research paper published. Astrophysicists could be the first observation of matter falling into the black hole that formed after the merger.

as inform us Previously, Discover LIGO through gravitational waves laser interferometry. This method uses high-powered lasers to measure small changes in the distance between two objects located kilometers apart. (LIGO has detectors in Hanford, Washington, and in Livingston, Louisiana. A third detector in Italy, known as Advanced VIRGO, was commissioned in 2016.) Having three detectors means scientists can pinpoint where any night-sky chirps are coming from.

In addition to seven binary black hole mergers, discover LIGO’s second run, from November 30, 2016 to August 25, 2017, Binary fusion between neutron stars with one time gamma ray burst and signals in the rest of the electromagnetic spectrum. The event is now known as GW170817. These signals included telltale signs of heavy elements – particularly gold, platinum and uranium – created by the collision. Most of the lighter elements are formed in the stifling explosions of massive stars known as supernovae, but astronomers have long assumed that heavier elements may originate in the kilonova produced when two neutron stars collide.

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Kilonova’s discovery in 2017 provided evidence that these astronomers were right. The recording of this kind of celestial event was unprecedented, and it officially marked the dawning of a new era in the so-called “Astronomy multi-message. “

Since then, astronomers have been searching for a matching optical signature when LIGO/VIRGO picks up a gravitational wave signal of neutron star mergers or potential mergers between neutron stars and a black hole. The assumption was that black hole and black hole mergers wouldn’t produce any optical signature, so there was no point in looking for one — until 2020. That’s when astronomers found first guide for such a phenomenon. Astronomers made the discovery by combining gravitational wave data with data collected during an automated sky survey.

But Kilonova 2017 remains unique, according to Abrajita Hajela, lead author of the new paper and a graduate student at Northwestern University. Hajela Calls Kilonova “The only event of its kind” and “a treasure chest of several initial observations in our field”. Along with other astronomers at Northwestern and the University of California, Berkeley, it has been monitoring the evolution of GW170817 since it was first discovered by LIGO/Virgo using space-based spacecraft. Chandra X-ray Observatory.

Illustration of the Chandra X-ray Observatory in space, the most sensitive X-ray telescope ever.
Zoom / Illustration of the Chandra X-ray Observatory in space, the most sensitive X-ray telescope ever.

NASA/CXC/NGST (Public Domain)

Chandra first detected X-ray and radio emissions from GW170817 two weeks after the merger, which lasted 900 days. But those initial X-rays, powered by a jet from near-light-speed fusion, began fading out in early 2018. However, from March 2020 through the end of that year, the sharp drop in brightness stopped, and X-ray emission became constant. Somewhat in terms of brightness.

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To help solve the mystery, Hajela and her team collected additional observational data with both Chandra and Very Large Array (VLA) in December 2020, 3.5 years after the merger. It was Hajela who woke up at 4 a.m. on the notice of a surprisingly strong and bright X-ray emission – four times higher than would be expected at this point if the emissions were powered only by the jet. (The VLA did not detect any radio emissions.) These new emissions remained at a constant level for 700 days.

This means that a completely different source of X-rays must be the source of energy for them. One possible explanation is that the expanding debris from the merger generated a shock wave, similar to a sonic boom, as well as jets. In this case, the merging neutron stars cannot instantly collapse into a black hole. Instead, the stars rotate rapidly for a second. This rapid spin would have countered the gravitational collapse briefly enough to produce a rapid tail of Kilonova’s heavy projectiles, which were the impetus for the shock wave. As those heavy projectiles slowed down over time, their kinetic energy was converted into heat by the shocks.

“You’ll fall into it. Done.”

“If the merging neutron stars were to collapse directly into a black hole without an intermediate phase, it would be very difficult to explain the excess X-rays we see now, because there would be no solid surface for things to bounce back flying at high speeds to create these auroras.” Co-author Raffaella Margutti said from the University of California at Berkeley. “You’ll fall in. Done. The real reason I’m scientifically excited is because we might see something more from the plane. We might finally get some information about the new compact object.”

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Brian Metzger of Columbia University proposed an alternative scenario: the X-ray emission could be triggered by material falling into the backslit formed during fusion. This is also a scientific first, Hagel said, since this kind of long-term buildup hasn’t been observed before.

There are more observations planned from now on, and this data will help solve the problem. If X-rays and radio emissions brighten over the next few months or years, this will confirm the kilonova aurora scenario. If the X-ray emissions declined sharply or remained constant, with no accompanying radio emissions, that would confirm the growing black hole scenario.

Regardless, “this will be the first time we see a kilonova auroras or the first time we see matter falling into a black hole after a neutron star merger,” Co-author Joe Bright said:Postdoc at the University of California, Berkeley. “Neither outcome would be very exciting.”

DOI: The Astrophysical Journal Letters, 2022. 10.48550 / arXiv.2104.02070 (About DOIs).

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