But what that exactly is remains shrouded in mystery.

In 2017, GW170817 was world news for a while. For the first time, researchers had detected gravitational waves caused by the merger of two neutron stars. And for the first time, it was possible to investigate its aftermath using electromagnetic radiation (or light). Much has already been published about the event. But a new study, published in the magazine The Astrophysical Journal Letters, reveals that nearly five years later, there is still more to be said about GW170817. In fact, scientists believe they have spotted something completely new in the wake of the violent merger: a mysterious source of X-rays. It may be the afterglow of a kilonova (something scientists have never seen) or material falling into a black hole created by the merging neutron stars (and that too has never been observed). “We’re looking at something new and extraordinary here that we’re seeing for the first time,” said researcher Aprajita Hajela. “And that gives us the opportunity to observe and understand new physical processes that have never been observed before.”

Kilonova

As mentioned, the study deals with the much-discussed GW170817. Two neutron stars fused together, creating a kilonova (a very powerful explosion, about 1000 times more powerful than a classic nova). At the same time, however, a jet emerged: a focused, bundled stream of high-energy particles. Those particles sped away at a speed approaching the speed of light, generating X-rays as well. Initially, that X-ray radiation from Earth was not visible; Chandra’s X-ray telescope searched for it shortly after the gravitational waves were observed, but unsuccessfully. But that changed a few days later. It is therefore suspected that the merging neutron stars spawned a very narrow jet that was not aimed at Earth, but as time progressed — hitting surrounding gas and dust, then slowing down and expanding — nevertheless entered Chandra’s field of view. .

New source

In the years that followed, Chandra continued to monitor GW170817. And from early 2018, the observatory saw X-ray emissions begin to decline. It can be traced back to the fact that the jet is doomed to continue to slow down and expand, the researchers say. And with that, the X-ray emission will only decrease further. But then something strange happens: from March 2020, the decline in X-ray emissions will stop and X-ray emissions will remain stable for months in a row. And that’s when the astronomers realized that something special was going on. “The fact that the X-rays had stopped rapidly decreasing was the best evidence that there had to be another source of X-rays in addition to the jet,” said researcher Raffaella Margutti. “A completely different source of X-rays is needed to explain what we see here.”

Black hole or afterglow?

But what kind of source is that? It may be the afterglow of the kilonova, the researchers say. That glow is said to be caused by a shock wave created by the expanding debris cloud created during the quite violent merger of the neutron stars. This shock wave heats up surrounding material, creating the emission that the researchers refer to as the afterglow of a kilonova. Another possibility is that the additional X-rays are created because a black hole – created after the neutron stars merged – attracts and consumes material. “It would be the first time we see the afterglow of a kilonova, and it would also be the first time we see material fall into a black hole after a merger of neutron stars,” said study researcher Joe Bright. “Both outcomes would be extremely exciting.”

Future research

But which outcome is the right one, remains shrouded in mystery for now. For this, GW170817 must first be studied in more detail. Astronomers certainly intend to. In addition, GW170817 is monitored not only in X-rays, but also in radio waves. If the additional X-rays are the result of the afterglow of the kilonova, you can expect that radio radiation will also be generated in the long term. However, if the additional X-rays are the work of a newly formed black hole, no radio emissions will be generated and X-ray emissions will remain stable or decrease rapidly.

Astronomers can’t wait to see what happens in the coming years. “Further investigation into GW170817 could have far-reaching implications,” said researcher Kate Alexander. “The detection of the afterglow of a kilonova would mean that the merger did not immediately produce a new black hole.” But it is also interesting if it turns out that a black hole has formed immediately that is busy consuming material and is thus responsible for the additional X-rays. “The object will then offer astronomers the opportunity to study how matter falls into a newborn black hole.”