NICER / ISS Science Nugget
for February 24, 2022
A Relativistic Jet from a Shredded Star 7 Billion Light Years Away
When a star passes close to a massive black hole, it can be ripped apart by strong tidal forces. The resulting debris falls back onto the black hole, and this sudden influx of matter can launch a powerful jet in which particles are accelerated to nearly the speed of light. More than 100 such tidal disruption events (TDEs) have been identified to date, with most of them within a few hundred million light years from us. However, if by chance a TDE's relativistic jet is aligned along our line of sight, the overall emission is "Doppler boosted" by several orders of magnitude. This bright, beamed emission in the X-ray band enables us to witness TDEs occurring much further back in time - several billion light years away.
On February 11, the Zwicky Transient Facility and other ground-based optical sky surveys detected a fast-fading optical explosion, dubbed AT2022cmc, later measured to be at a distance of roughly 7 billion light years. Rapidly decaying visible-light emission is common for gamma-ray burst (GRB) afterglows, where a massive star has collapsed under its own gravity and forms a black hole, also launching a jet along our line of sight. Astronomers have observed thousands of such GRB afterglows.
However, when NICER observed AT2022cmc, it found surprisingly bright X-ray emission, at least 5 times brighter than the brightest known GRB afterglow at a similar phase of evolution. Moreover, NICER monitoring over the last week has revealed that the X-ray brightness varies on timescales of hours, while the energy spectrum can change shape over a day. These dramatic properties have never been seen in a GRB afterglow. The current best hypothesis is that this event is likely a TDE with a newly launched relativistic jet fortuitously pointing directly at us. This would make AT2022cmc the most distant TDE yet seen, and coincidentally also the farthest and brightest object ever observed by NICER. Dr. Dheeraj Pasham (MIT) and collaborators reported these initial NICER results in Astronomer's Telegram #15232 to alert the transient community to the novel properties of this exceptional astrophysical transient.
Using data from NASA's NuSTAR high-energy X-ray telescope in combination with NICER data, Yuhan Yao (Caltech) and collaborators reported in Astronomer's Telegram #15230 the detection of a break in the X-ray spectrum of AT2022cmc near 11 keV photon energy. Such a break is a crucial measurement for understanding the energetics and the size of the region within the jet where particle acceleration is occurring. NICER plans to continue monitoring AT2022cmc along with additional simultaneous and complementary observations with other facilities. These data will provide astronomers a unique opportunity to probe the underlying physics of jets, a major puzzle in high-energy astrophysics for decades.
Figure: NICER X-ray brightness, in photon counts per second, as a function of time for the early evolution of the unusual transient AT2022cmc.
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