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Astronomers examined in detail the gap of a star with a black hole

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The TESS space telescope, designed to search for exoplanets, allowed scientists to obtain the most detailed information about the absorption of a star by a black hole. The high sensitivity of the telescope provided data on the growth of the brightness of the object during the week before it became noticeable to automatic telescopes on Earth.

If a star passes close enough to a large black hole, then it is torn apart by tidal forces: in this case, part of the material of the star flies away, and part is absorbed by an extreme body. The matter attracted by the hole forms an accretion disk around it and is absorbed in a relatively short time. In this case, a distant observer sees the flash of radiation associated with the heating of the material in the disk – this is called an event tidal disruption (tidal disruption event, TDE).

Early theoretical work predicted the heating of the disk to about 10 thousand Kelvin and, accordingly, the maximum spectral density of radiation in the soft x-ray range. It was also believed that the total luminosity and rate of absorption of a substance should follow a single power-law time dependence with an indicator of −5/3.

However, in the last decade, many TDEs have been discovered that deviate significantly from initial estimates. In particular, the radiation maxima in some cases are in the ultraviolet region, the temperatures turn out to be an order of magnitude lower, and the nature of the decrease in luminosity can vary over a considerable range.

Because of this, it becomes necessary to build new, more detailed models, but their verification is complicated by the fact that the dependence of brightness on time (light curve) is known at the initial stages for a small amount of TDE: as a rule, there are qualitative measurements only at the stage after the maximum. As a result, the details of the formation of a disk from the remnants of a star are not well known.

A team of scientists from the United States, Denmark, Chile, China, Brazil, Australia, and Canada led by Thomas Holoien from the Carnegie Institution Observatory described a successful set of circumstances that made it possible to obtain the most comprehensive information about the tidal destruction event. Astronomers studied an event called ASASSN-19bt, detected by an automatic ground-based telescope on January 29, but later it turned out that a piece of the sky with this object also observed the TESS space telescope created to search for exoplanets, and he saw an increase in luminosity as early as January 21.

Launched in 2018, TESS automatically sequentially observes various regions of the sky for 27 days. It is not designed to quickly transfer to new objects and transmits information to the Earth once every two weeks, after which it must be processed. Therefore, despite the data obtained on the beginning of the growth of brightness, they became available to scientists only on March 13.

In addition to the TESS data, scientists also studied the object using the Swift and XMM-Newton space telescopes, which measured ultraviolet and X-ray emission. Together with these data, ASASSN-19bt is the most fully studied TDE. The total luminosity, temperature, size and evolution of the spectrum make it similar to other similar events, but other features, on the contrary, are unique, although this may be due to the lack of such complete data obtained for previously studied objects.

In particular, ASASSN-19bt was not very bright in the X-ray range, but at the same time, during the luminosity growth phase relative to other TDEs, there were many hard-range X-rays in its radiation. Then the spectrum became softer, which testifies to the formation of a jet at an early stage of star destruction. TESS data allows to determine with high accuracy the time to reach the maximum brightness – 41 days in the frame of reference relative to the object. Also, these data accurately confirm the quadratic increase in luminosity with time.

The main task of TESS is the search for exoplanets by registering small variations in the brightness of stars. In this regard, astronomers speculated that it could help investigate short-term flares, such as supernova explosions. However, the ASASSN-19bt was the first tidal disruption event observed by this instrument.

“The early TESS data allowed us to see light from the immediate vicinity of the black hole, much closer than we saw before (in the case of TDE – approx. N + 1 ),” said co-author Patrick Vallely of Ohio State University. “They also showed us that the brightness increase in the case of ASASSN-19bt was very smooth – this confirms the classification of the event as tidal destruction, and not another type of outburst, such as supernovae or occur in the centers of galaxies.”

Previously, astrophysicists using observations of the destruction of a star were able to calculate the rotation speed of a supermassive black hole. The initial light curve of growth stages of tidal disruption events in the radio for the first time managed to obtain in 2015.

Via | arXiv | The article is also accepted for publication in The Astrophysical Journal.

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