Astronomers detected an afterglow from a kilonova

Strange ‘sonic boom’ accompanied the unprecedented event.


When neutron stars collide, they produce an explosion called a kilonova. The event produces short gamma-ray bursts and intense electromagnetic radiation.

They are 1⁄10 to 1⁄100 the brightness of a typical supernova, the self-detonation of a massive star.

Northwestern University-led astronomers may have detected an afterglow from a kilonova for the first time. They observed this event in GW170817, a merger of two neutron stars.

On Aug. 17, 2017, astronomers discovered gravitational waves from such a merger using the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo, coinciding with a burst of gamma rays. Since then, astronomers have been using telescopes worldwide to study GW170817 across the electromagnetic spectrum.

According to astronomers, the debris- generated after the collision of neutron stars- generates a shock. This shock then generated X-ray emissions when heated by surrounding materials, known as a kilonova afterglow. An alternative explanation is that materials falling toward a black hole—formed due to the neutron star merger—caused the X-rays.

Northwestern’s Aprajita Hajela, who led the new study, said, “We have entered uncharted territory here in studying the aftermath of a neutron star merger. 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.”

Using NASA’s Chandra X-ray Observatory, Astronomers observed X-ray emissions from a jet. The jet was moving very close to the speed of light produced by the neutron star merger. In 2018, the emissions from the jet were noticed to get dimmer as the jet continued to slow and expand.

Until the end of 2020, the dimming stopped, and the X-ray emission was found to have constant brightness. This was a significant sign.

Raffaella Margutti, an astrophysicist at the University of California at Berkeley and a senior author of the study, said, “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.”

“A completely different source of X-rays appears to be needed to explain what we’re seeing.”

According to astronomers, this is either a kilonova afterglow or a black hole likely behind the X-rays. And surprisingly, such a scenario was never observed before.

Study co-author Joe Bright, also from the University of California at Berkeley, said, “This would either be the first time we’ve seen a kilonova afterglow or the first time we’ve seen material falling onto a black hole after a neutron star merger. Either outcome would be fascinating.”

Astronomers will continue monitoring GW170817 in X-rays and radio waves to distinguish between the two explanations.

Study authors noted“If it is a kilonova afterglow, the X-ray and radio emissions are expected to get brighter over the next few months or years. Suppose the explanation involves matter falling onto a newly formed black hole. In that case, the X-ray output should stay steady or decline rapidly, and no radio emission will be detected over time.”

Study co-author Kate Alexander, a CIERA postdoctoral fellow at Northwestern, said“Further study of GW170817 could have far-reaching implications. The detection of a kilonova afterglow would imply that the merger did not immediately produce a black hole. Alternatively, this object may offer astronomers a chance to study how matter falls onto a black hole a few years after its birth.”

Journal Reference:

  1. The emergence of a new source of X-rays from the binary neutron star merger GW170817, arXiv:2104.02070 [astro-ph.HE]
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