NICER / ISS Science Nugget
for February 18, 2021
A Changing Look for NICER
Supermassive black holes (SMBH) are ubiquitous inhabitants of the centers of galaxies. When a star orbiting nearby comes too close to a SMBH it can be torn apart, and the resulting debris may light up as it falls toward the black hole and is heated. Typically, these "tidal disruption events" (TDEs) are first spotted by optical telescopes dedicated to scanning the sky for sources of light that suddenly appear. Material closer to the SMBH that is heated to hundreds of thousands of degrees will emit X-rays. With its fast response, flexible scheduling, and sensitivity to X-rays emitted at these temperatures, NICER is a superb instrument to investigate the heated stellar debris from many TDEs through follow-up monitoring campaigns.
In early 2018, the galaxy 1ES 1927+654 was observed to go through a remarkable "changing-look" event, probably related to a stellar disruption event. Even among this spectacular class of phenomena, this event was extreme, since it was detected in a galactic nucleus which was already actively accreting and emitting X-rays. NICER monitored the source for more than a year, from the X-ray counterpart of the initial transient event, through a strange, extended period of X-ray quiescence (almost as if the TDE disrupted the previous accretion flow), followed by a recovery in X-ray luminosity to levels far exceeding historic values.
A comprehensive analysis of the NICER observations, led by Claudio Ricci (Universidad Diego Portales, Chile), was recently accepted for publication in the Astrophysical Journal Supplement Series. This investigation takes a deep look at the evolution of 1ES 1927 in X-rays, combining data from NICER and other X-ray observatories, and considers a wide range of spectral models commonly applied to accreting systems. Both the luminosity and the X-ray spectrum show large-scale, rapid variations, with brighter emission associated with a larger proportion of higher energy X-rays, but none at very high X-ray energies. In addition, the changes in X-rays are very different from observed ultraviolet variations, something not seen before. Ricci et al. also discovered two prominent bumps in the spectrum that so far have defied explanation, as well as evidence that a very fast wind of hot gas is being driven from the nucleus of the galaxy. The unique characteristics of 1ES 1927 in the X-ray band suggest that it belongs to a new type of changing-look AGN, one that underwent a catastrophic event that restructured the flow of material onto the central supermassive black hole.
Figure: NICER measures a very tight correlation between the luminosity and temperature of material falling into the supermassive black hole at the heart of galaxy 1ES 1927+654, across 450 days of evolution following a transient event. The temperature increases as the source gets brighter, indicating that the flow of X-ray emitting gas is following a remarkably coherent track, even as the brightness varies by a factor of more than 100.