Madrid, 1 (European press)
For the first time, astronomers led by Northwestern University may have detected Kilonova’s afterglow.
A kilonova occurs when two neutron stars, some of the densest objects in the universe, merge to produce an explosion 1,000 times brighter than a classical supernova. In this case, a narrow off-axis jet of high-energy particles accompanied the fusion event, designated GW170817. Three and a half years after the merger, the jet has faded away, revealing a new source of X-rays.
As the main explanation for the new X-ray source, astrophysicists believe the expanding debris from the merger generated an effect similar to the sonic boom of a supersonic jet. This shock then heated the surrounding material, generating X-ray emissions, known as kilonova afterglow. An alternative explanation is that material falling toward the black hole, formed as a result of merging neutron stars, caused the X-rays to appear.
Either scenario would be scientific first. The study was published in The Astrophysical Journal Letters.
“We are entering uncharted territory here in studying the consequences of a neutron star merger,” said Abrajita Hajela of Northwestern, who led the new study. “We are witnessing something new and unusual for the first time. This gives us the opportunity to study and understand new physical processes that have not been observed before.”
Hajela is a graduate student at the Northwestern Center for Interdisciplinary Research and Exploration in Astrophysics (CIERA) and in the Department of Physics and Astronomy at the Weinberg College of Arts and Sciences.
On August 17, 2017, GW170817 went down in history as the first neutron star merger detected by both gravitational waves and electromagnetic radiation (or light). Since then, astronomers have used telescopes around the world and in space to study the event across the electromagnetic spectrum.
Using NASA’s Chandra X-ray Observatory, astronomers have observed X-ray emissions from an aircraft moving very close to the speed of light produced by merging neutron stars. Beginning in early 2018, the aircraft’s X-ray emission steadily faded as the aircraft continued to slow and expand. Then Hajila and his team observed that from March 2020 until the end of 2020, the decrease in brightness stopped and the X-ray emission was nearly constant in brightness. This was an important clue.
“The fact that the X-rays stopped fading quickly was the best evidence we had yet that something beside the plane was being detected in the X-rays from this source,” Raffaella Margotti, an astrophysics student, said in a university statement. California at Berkeley and study author. “It appears that a completely different source of X-rays is needed to explain what we are seeing.”
The researchers believe that it is possible that there is a kilonova glow or a black hole behind the X-rays, and neither scenario has been observed before.
Study co-author Joe Bright, also of the University of California at Los Angeles, Berkeley, said. “Any result would be very exciting.”
To distinguish between the two interpretations, astronomers will continue to observe GW170817 in X-rays and radio waves. If the kilonova afterglow, X-ray and radio emissions are expected to be brighter in the coming months or years. If the explanation included matter falling into a newly formed black hole, the X-ray output should either remain constant or decline rapidly, and no radio emission would be detected over time.
“Further study of GW170817 may have far-reaching implications,” said study co-author Kate Alexander, a CIERA postdoctoral fellow at Northwestern University. The discovery of the kilonova glow means that the merger did not immediately produce a black hole. Alternatively, this object may provide astronomers with the opportunity to study how matter falls into a black hole a few years after its birth.”
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