NICER / ISS Science Nugget
for October 27, 2022
Formation of an X-ray "corona" around a supermassive black hole 3 years after it destroyed a star
A low-density, high-temperature plasma is thought to exist around actively accreting black holes. Dubbed a "corona" because it may adorn the black hole and its accretion disk like a crown, the hot electrons in this plasma reveal themselves via scattering of low-energy X-rays (typically produced in the accretion disk), redirecting and further energizing them in a process known as Compton up-scattering. The result is a boost in the high-energy tail of an otherwise thermal X-ray emission spectrum. Despite this characteristic observational signature, the origin of ubiquitous X-ray coronae remains a mystery. Stellar tidal disruption events (TDEs), in which a dormant black hole shreds an unfortunate passing star, provide an ideal opportunity to study the formation of X-ray coronae, but they require dedicated monitoring over months to years to capture the right moment.
In October 2019, the Zwicky Transient Facility, a ground-based optical sky survey at Palomar Observatory, California, detected brightening of the nucleus of a galaxy at a redshift of 0.0878 (luminosity distance of roughly 1.3 billion light-years). Based on its optical spectrum and lack of prior activity from the galaxy, the event was classified as the tidal disruption of a star. NASA's Swift and ESA's XMM-Newton telescopes observed this optical transient, AT2019teq, in December 2019 and September 2022, respectively. Both of the resulting X-ray spectra were consistent with thermal emission. NICER observed the target on 20 and 21 October 2022, some six weeks after XMM-Newton. Surprisingly, the NICER spectrum shows an excess in hard (> 1 keV photon energy) X-rays, which can be interpreted as originating in a newly formed, or still forming, X-ray corona. Y. Ajay (IISER, India) and collaborators reported these initial NICER results in Astronomer's Telegram #15724 to alert the transient community about this rare and fortuitously detected astrophysical transformation.
NICER and XMM X-ray spectra of the tidal disruption event AT2019teq. NICER data were acquired 3 years after the stellar disruption triggered a visible-light outburst, while the XMM spectrum was obtained less than 2 months prior to the NICER observation. The NICER data show the emergence of an excess at high energies. (The peak in the NICER spectrum near 0.5 keV photon energy is a feature that originates in ionized oxygen atoms in the Earth's upper atmosphere.)