The binary neutron-star merger GW170817 was discovered in August 2017. Many telescopes saw different kinds of light after the discovery, but only NASA’s Chandra X-ray Observatory is still making a detection. Early Chandra data revealed the presence of a narrow jet that has slowed down and expanded over time. The new study presents X-ray evidence for a shock — similar to a sonic boom — in the aftermath of the neutron-star merger.
An artist’s conception illustrates the aftermath of two neutron stars merging. Image credit: NASA / CXC / M. Weiss.
“We have entered uncharted territory here in studying the aftermath of a neutron star merger,” said Dr. Aprajita Hajela, an astronomer at Northwestern University.
“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.”
GW170817 was the first — and thus far the only — cosmic event where both gravitational waves and electromagnetic radiation were detected.
This combination provides astronomers with critical information about the physics of neutron star mergers and related phenomena, using observations at many different parts of the electromagnetic spectrum.
Chandra is the only observatory still able to detect light from this extraordinary cosmic collision more than four years after the original event.
Astronomers think that after neutron stars merge, the debris generates visible and infrared light from the decay of radioactive elements like platinum and gold formed in the debris from the merger. This burst of light is called a kilonova.
Indeed, visible light and infrared emission were detected from GW170817 several hours after the gravitational waves.
Initially, the neutron star merger likely produced a jet of high-energy particles that was not pointed directly at Earth, explaining an initial lack of X-rays seen by Chandra. The jet then slowed down and widened upon impact with surrounding gas and dust.
These changes caused an increase in X-rays observed by Chandra followed by a decline in early 2018.
However, since the end of 2020, X-rays detected by Chandra have remained at a nearly constant level.
The Chandra image from data taken in December 2020 and January 2021 shows X-ray emission from GW170817 and from the center of its host galaxy, NGC 4993.
Dr. Hajela and colleagues think this steadying of the X-ray emission comes from a shock as the merger debris responsible for the kilonova strikes gas around GW170817.
Material heated by such a shock would glow steadily in X-rays giving a kilonova afterglow, like Chandra has observed.
There is also an alternative explanation suggesting that the X-rays come from material falling towards a black hole that formed after the neutron stars merged.
“Further study of GW170817 could have far-reaching implications,” said Dr. Kate Alexander, also from Northwestern University.
“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.”
The team’s paper will be published in the Astrophysical Journal Letters.
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A. Hajela et al. 2022. The emergence of a new source of X-rays from the binary neutron star merger GW170817. ApJL, in press; arXiv: 2104.02070
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