Spectrum-RG saw an event of tidal destruction of a star by a black hole

In early November 2020, the eROSITA telescope, installed on board the Russian orbiting X-ray observatory “ Spektr-RG “, registered a new source in the sky, which attracted attention of domestic astrophysicists to the softness of its X-ray spectrum. Observations at the world’s largest 10-meter Keck telescope (Hawaii, USA) confirmed that the radiation of an accretion disk with a luminosity ten billion times higher than the luminosity of our Sun in all spectral ranges was recorded. Such sources with a lifetime of about six months should appear during the tidal destruction of a star that has flown too close to a supermassive black hole.

By mid-December 2020, the telescopes of the Spektr-RG X-ray observatory completed the second sky survey. Thus, in the year that has passed since the beginning of scanning in December 2019, the entire sky was “scanned” by the observatory twice. Comparison of two sky maps obtained by the eROSITA telescope makes it possible to study the variability of X-ray sources and, in particular, to look for X-ray transients – objects from which radiation was not detected in the first survey, but which became bright in the second (or vice versa). Such sources, which have increased their brightness more than 10 times in six months, are found by eROSITA on average about once a day.

Among the extragalactic transients detected by eROSITA, astrophysicists are especially interested in events associated with tidal destruction of stars in the gravitational field of a supermassive black hole. One of these events was discovered by the staff of the High Energy Astrophysics Department of the Space Research Institute of the Russian Academy of Sciences on November 9, 2020.

“The extragalactic X-ray transient SRGeJ213527.3-181634 attracted our attention by the softness of its spectrum, which had a temperature of only 70 electron volts (eV), and by the fact that it was located in an unremarkable, relatively small galaxy in which it had not previously been recorded activity of the nucleus – a supermassive black hole at its center. These are classic signs of a tidal star destruction event, ”says RAS Corresponding Member Marat Gilfanov.

X-ray images of a 5 × 5 arc-minute area of the sky in the 0.3-2.2 keV range, obtained by the eROSITA telescope in the first (left) and second (right) sky survey. Each light dot represents one (or more) X-ray photons. In the first survey, not a single photon was detected from the vicinity of the source, in the second survey, more than one hundred X-ray photons

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