ROSCOSMOS & DLR - Spectrum-RG Mission patch.
Jan. 5, 2021
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 the attention of Russian astrophysicists by 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 the order of 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 has been "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 search 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.
"Spectrum-RG" or Spektr-RG X-ray observatory
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 such events was discovered by employees 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 the 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.
Image above: X-ray images of a 5 × 5 arc-minute area of the sky in the range 0.3-2.2 keV, 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.
“Analysis of archived data showed that a few months earlier, the Zwicky Transient Facility at the California Institute of Technology registered an optical flare ZTF20abgbdpr from this galaxy, which continues to this day and was originally classified as a probable supernova candidate. Our results showed that it was not a supernova, ”continues RAS professor Sergei Sazonov.
Less than two weeks after the discovery of the object with the eROSITA telescope, American astronomers at the 10-meter telescope of the Keck Observatory in Hawaii obtained the spectrum of this object, which recorded the emission lines of hydrogen and helium and oxygen ions.
“These spectral features and the appearance of a bright object in less than six months confirmed our assumption that we are dealing with an event of tidal destruction of a star. The redshift of the host galaxy was also measured at z = 0.0942, says Academician Rashid Sunyaev, scientific director of the Spectr-RG observatory. - We have all heard from childhood about the tides in the oceans and seas. And these tides are the result of the presence of the Moon 300,000 km from Earth. One can easily imagine how tidal gravitational forces tear apart a star passing by a supermassive black hole even in a hundred gravitational radii. Much of the torn star's material forms an accretion disk around the black hole and slowly falls into the black hole, sending us a signal in the form of powerful X-rays. Over the past 25 years, astronomers have observed two passes of a star near (but beyond the tidal radius) from a black hole with a mass of 4 million solar masses in the center of our Galaxy. So the tidal destruction of stars by black holes is not so exotic. "
Black Holes; Neutron Stars; White Dwarfs; Beyond Space and Time
The Russian orbiting X-ray observatory Spektr-RG continues to scan the sky - two weeks ago, the third (out of eight planned) sky survey began. The enterprises of the State Corporation "Roscosmos" control the satellite, antennas for long-range space communications receive scientific data every day and send commands to the satellite and its scientific instruments, which are located at a distance of 1.5 million kilometers from the Earth (four times farther than the Moon). Scientists from IKI RAS are processing scientific data on computers in the project data center.
ROSCOSMOS Press Release: https://www.roscosmos.ru/29780/
Spectrum-RG: http://roscosmos.ru/srg/
Images, Text, Credits: ROSCOSMOS/Orbiter.ch Aerospace/Roland Berga/Video: NASA/ESA/Music: Château - Rob Dougan.
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