Astronomers have potentially spotted kilonova afterglow for the first time

And it’s much like a sonic boom from a supersonic aircraft.
Ameya Paleja
An artist’s perspective on the aftermath of a kilonovaNASA/ CXC/ M.Weiss

Researchers at the Northwestern University and Weinberg College of Arts and Sciences may have potentially come across a kilonova afterglow, the first of its kind ever to be observed, according to a university press release

A kilonova is the merger of two neutron stars that creates a blast 1,000 times brighter than a classical nova. On August 17, 2017, astronomers observed the first-ever neutron star merger, GW170817, using light as well as gravitational waves. Ever since researchers across the globe have been pointing ground and space telescopes towards this event to study it across the electromagnetic spectrum. 

What the astronomers observed

Aprajita Hajela, a graduate student at Northwestern University, was also one of the many astronomers who was looking at GW170817. Using NASA's Chandra X-ray observatory, Hajela and her team noticed that the merger event had created a jet emitting X-rays at speeds very close to those of light. Starting early 2018, the jet's X-ray emissions began to steadily fade as they slowed down and expanded. However, from March 2020, the dimming of emissions stopped and the jet's brightness stayed constant. 
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"The fact that the X-rays stopped fading quickly was our best evidence yet that something in addition to a jet is being detected in X-rays in this source," said Raffaella Margutti, an astrophysicist as well Hajela's advisor. 

What could be behind the X-ray emissions?

The researchers believe that the expanding debris from the merger has generated a shock-much like a sonic boom from a supersonic plane. The shock is heating up the surrounding materials which are now emitting X-rays. This is the kilonova afterglow that hasn't been observed before. 

An alternate explanation could be the merger of the neutron stars has created a black hole into which the debris is now falling and is emitting the X-rays prior to its fall. Even if either of the explanations is true, it would still be a first for the field of astronomy. 

"We have entered uncharted territory here in studying the aftermath of a neutron star merger," said Hajela in the press release. "We are looking at something new and extraordinary for the very first time. This gives us an opportunity to study and understand new physical processes, which have not before been observed."

To find out which of these theories can explain the kilonova afterglow, the researchers will continue to observe GW170817 using X-rays and radiowaves. If it is the afterglow, the X-ray and radio emissions will get brighter over the next few months or years. However, if a black hole is causing this, the X-ray output will decline over time until no more emissions can be observed. 

Either way, astronomers will either learn that a neutron star does not immediately form a black hole or be able to observe how debris falls into a black hole over a period of time. 

The research is due to be published in The Journal of Astrophysical Letters. It is also available as a non-peer-reviewed publication at arxiv.org

Study Abstract:
The binary neutron-star (BNS) merger GW170817 is the first celestial object from which both gravitational waves (GWs) and light have been detected enabling critical insight on the pre-merger (GWs) and post-merger (light) physical properties of these phenomena. For the first ∼3 years after the merger the detected radio and X-ray radiation has been dominated by emission from a structured relativistic jet initially pointing ∼15−25 degrees away from our line of sight and propagating into a low-density medium. Here we report on observational evidence for the emergence of a new X-ray emission component at δt>900 days after the merger. The new component has luminosity Lx≈5×1038ergs−1 at 1234 days, and represents a ∼3.5σ - 4.3σ excess compared to the expectations from the off-axis jet model that best fits the multi-wavelength afterglow of GW170817 at earlier times. A lack of detectable radio emission at 3 GHz around the same time suggests a harder broadband spectrum than the jet afterglow. These properties are consistent with synchrotron emission from a mildly relativistic shock generated by the expanding merger ejecta, i.e. a kilonova afterglow. In this context our simulations show that the X-ray excess supports the presence of a high-velocity tail in the merger ejecta, and argues against the prompt collapse of the merger remnant into a black hole. However, radiation from accretion processes on the compact-object remnant represents a viable alternative to the kilonova afterglow. Neither a kilonova afterglow nor accretion-powered emission have been observed before.

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