Consider kilonova GW170817. Back in 2017, the light and gravitational waves from two merging neutron stars reached our world, but not everything arrived at exactly the same time. On August 17, gravitational waves were detected by LIGO & Virgo, and high-energy gamma-ray light was detected by Fermi. Researchers quickly turned pretty much any telescope they thought might be useful at this event, and on the list was the Chandra X-ray Observatory. Initially, this telescope saw nothing, but undeterred, Chandra kept returning to that spot in the sky, and on August 26, nine days later, X-ray emission was detected.
And then things got weird. According to a release from Chandra: This non-detection of X-rays quickly followed by a detection provides evidence for a narrow jet of high-energy particles produced by the neutron star merger. The jet is “off-axis” – that is, not pointing directly towards Earth. Researchers think that Chandra originally viewed the narrow jet from its side, and therefore saw no X-rays immediately after the gravitational waves were detected. However, as time passed, the material in the jet slowed down and widened as it slammed into the surrounding material. This caused the cone of the jet to begin to expand more into Chandra’s direct line of sight, and X-ray emission was detected. Since early 2018, the X-ray emission caused by the jet had steadily been getting fainter as the jet further slowed down and expanded. Hajela and her team then noticed that from March 2020 until the end of 2020 the decline stopped and the X-ray emission was approximately constant in brightness.
That last step – the X-Ray emission appearing constant in brightness – is weird and seems to indicate there is something more than just a jet generating this light.
Option 1, which has been turned into some truly amazing artwork that we’ll link to on our website, DailySpace.org, has a shockwave from the merger hitting the surrounding material and superheating it so that it glows in X-rays. This kind of afterglow is something we see with other kinds of gamma-ray bursts and would be the first time we’ve seen an afterglow with a neutron star merger.
Alternatively, we could be seeing light from material falling into the black hole that formed when those neutron stars merged. This would be the final photon screams of material about to fall into the event horizon.
Further observations should be able to sort out which of these options matches reality. For now, these results are published in The Astrophysical Journal Letters in a paper led by Aprajita Hajela, and while the X-ray observations themselves are just a blob, go check out the artist’s rendering. It is truly amazing.
More Information
CXO press release
UC Berkeley press release
“The emergence of a new source of X-rays from the binary neutron star merger GW170817,” A. Hajela et al., to be published in The Astrophysical Journal Letters (preprint on arxiv.org)
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