ROSCOSMOS / DLR - Spectrum-RG Mission patch.
Oct. 5, 2021
Based on the data of the X-ray sky survey of the eROSITA telescope of the Russian astrophysical observatory “Spektr-RG”, the first catalog of events of tidal destruction of stars near supermassive black holes in the cores of distant galaxies has been compiled.
In the 70s and 80s of the last century, astrophysical researchers, studying galaxies and supermassive black holes in their cores, suggested that very violent events could occur around the latter. If an ordinary star flies near such a supermassive black hole, then it can be torn apart by tidal forces, and the subsequent accretion of matter from the destroyed star onto the black hole will lead to a powerful burst of X-ray radiation.
Image above: Image from an article by Sazonov et al. 2021.
This scenario seemed very plausible, but at that time such "tidal disruption events" (TDE) could not be observed - there was not enough sensitivity of telescopes.
For the first time, several events of this type were detected in the early 1990s using an X-ray telescope aboard the German orbiting ROSAT observatory. Then the events of tidal destruction began to be found also with the help of optical and ultraviolet telescopes. But these discoveries were still very rare. The small number of registered tidal destruction events did not allow a sufficiently good study of their statistical and physical properties.
The Russian Astrophysical Observatory Spektr-RG, which began work in space in 2019, surveys the entire sky using two X-ray telescopes: the German eROSITA and the Russian ART-XC named after M.N. Pavlinsky. The high sensitivity of the detectors and the wide field of view of the eROSITA telescope, for the first time in the history of X-ray astronomy, made it possible to begin a massive search for tidal destruction events in the Universe.
The first results of this search were published in a paper accepted for publication in the Monthly Notices of the Royal Astronomical Society and published on the arXiv.org preprint site. It was attended by 25 researchers from the Institute for Space Research and the Special Astrophysical Observatory of the Russian Academy of Sciences, the Institute of Solar-Terrestrial Physics of the Siberian Branch of the Russian Academy of Sciences, the State Astronomical Institute named after V.I. PC. Sternberg Moscow State University. M.V. Lomonosov, Kazan Federal University, California Institute of Technology (USA) and Leiden University (Netherlands).
At the first stage of the study, we compiled a catalog of X-ray sources that were detected with the eROSITA telescope during the second half-year (June 10 - December 14, 2020) sky survey, but were not registered during the first survey. We also set the condition that the brightness of the source in the second scan should be at least ten times higher than its brightness in the first scan. Thus, a sample of about a hundred bright variable sources (transients) of X-ray radiation, located in half of the sky, was compiled. Pavel Medvedev, Researcher, High Energy Astrophysics Department, IKI RAS.
Image above: The position in the sky (against the background of optical images according to the Pan-STARRS survey) of the tidal destruction of stars discovered by the SRH / eROSITA telescope. The circles show the regions of localization of X-ray flares, and the arrows show the "parent" galaxies in which the star was destroyed.
Comparison with the Gaia Astrometric Catalog (ESA) showed that many of these objects are stars in our Galaxy. To study the rest, presumably extragalactic, transients, observations were organized on almost all large optical telescopes in our country: the 6-meter telescope BTA SAO RAS, the 2.5-meter telescope of the Caucasian Mountain Observatory GAISH MSU, the 1.6-meter telescope AZT-33IK ISTP SB RAS, 1.5 -meter Russian-Turkish telescope, as well as one of the largest telescopes in the world - the 10-meter Keck telescope in Hawaii (USA).
Based on the obtained optical spectra, it was possible to classify these objects, measure the distances to them, and distinguish among them sixteen events of tidal destruction, thirteen of which are discussed in the article.
“The fact that we are dealing with the destruction of a star near a massive black hole is evidenced by the softness of the obtained X-ray spectra, the presence of an extended object - a galaxy - in the region of the localization of the X-ray source and the absence of powerful emission lines in the optical spectrum of the galaxy. According to their properties, these objects are clearly different from active galactic nuclei, in which there is a prolonged (thousands and millions of years) accretion of interstellar gas onto a supermassive black hole, ”explains Ph.D. Georgy Khorunzhev, Researcher, High Energy Astrophysics Department, IKI RAS.
Tidal destruction of stars is most easily found in the "passive" cores of galaxies, in which the black hole usually "sleeps" and can erupt only for a short time after the star burst. Such a "dormant" black hole exists in the center of our Milky Way Galaxy, but in our Galaxy we have never seen TDE and are unlikely to see it in the next thousand years.
The closest of the tidal destruction events discovered by the Spektr-RG observatory took place in a galaxy at a distance of about 500 million light years from us, and the farthest - in a galaxy at a cosmological redshift of 0.581, i.e. happened about 6 billion years ago. And their X-ray luminosities reached 1038 watts in some cases.
Spectrum-RG (Spektr-RG)
“To provide such a colossal energy release, a black hole must absorb matter at a rate of about a hundred Earth masses per day,” says RAS professor Sergei Sazonov, head of the laboratory of experimental astrophysics at IKI RAS and the first author of the article. - The X-ray flare lasts at least several months, as evidenced by the repeated detection of a number of transients six months later (in the third sky scan of the eROSITA telescope) after their discovery. During this time, a black hole with its own mass of the order of ten thousand - one hundred million solar masses manages to absorb about half of the substance of the destroyed star.
Using the obtained unique sample, scientists have constructed for the first time a function of the X-ray luminosity of tidal destruction events. It turned out that the frequency of occurrence of such events in the Universe decreases with increasing luminosity (the more X-ray photons in a flare, the less frequent), and on average, the destruction of stars occurs approximately once every hundred thousand years per galaxy. Therefore, we do not register such events in the core of our Galaxy or in nearby galaxies, and in order to search for them, we have to examine a large volume of the Universe, containing millions of galaxies.
Most of the tidal destruction events that the eROSITA telescope discovers manifest themselves only in X-rays. This is how they differ from the events that are detected in optical surveys, in particular with the help of the Installation for the search for transients to them. Zwicky (Zwicky Transient Facility, USA). But some of the events that SRH / eROSITA detects in X-rays still manifest themselves as optical transients.
One of these - SRGe J131014.2 + 444315 - was recently reported in the ATel astronomical telegram No. 14800 (this was reported in a press release by the SRG / eROSITA saw the beginning of the tidal destruction of a star by a supermassive black hole). Scientists continue to argue why the events of tidal destruction of stars can manifest themselves in different ways.
“There is still no complete picture of how exactly the destruction of the star by tidal forces occurs and how radiation is formed at different wavelengths. Some theoretical models predict that when a star collapses, a thick accretion disk appears around a black hole, with X-rays being generated in the inner region of this disk. If such an object is observed along the axis of the accretion disk, we will see X-rays from the vicinity of the black hole, and when viewed from the side, along the plane of the disk, X-rays are shaded by the thickness of the disk and we observe only optical radiation, ”explains RAS Corresponding Member Marat Gilfanov. Leading researcher of the Astrophysics Department of the IKI RAS.
“The Spectr-RG observatory continues to search for events of tidal destruction of stars by black holes. To date, optical observations from Earth have already confirmed more than forty such events discovered by Russian scientists using eROSITA. This is more than the number of events of this type known before the launch of the observatory. All in all, in four years of survey across the entire sky, there is a chance to detect about seven hundred such events. In every galaxy, such events are extremely rare, but, as we can see, eROSITA is able to record the rarest TDE cases from billions of galaxies, where supermassive black holes lurk, which do not manifest themselves as bright X-ray sources until a star flies too close. from her, "concludes Academician Rashid Sunyaev, scientific director of the Spektr-RG orbital observatory.
Source: IKI RAN.
Related links:
ROSCOSMOS Press Release: https://www.roscosmos.ru/32817/
IKI RAN: https://www.roscosmos.ru/tag/iki-ran/
Spectrum-RG: https://www.roscosmos.ru/tag/spektr-rg/
Images, Text, Credits: ROSCOSMOS/IKI RAN/Orbiter.ch Aerospace/Roland Berga.
Greetings, Orbiter.ch
