ROSCOSMOS & DLR - Spectrum-RG Space Telescope patch.
August 24, 2021
The Russian orbital observatory Spektr-RG scans the Universe during the fourth survey of the entire sky in X-rays. The record sensitivity of the eROSITA telescope (one of the two telescopes on board the Spectra-RG) makes it possible to find very rare and unusual sources of X-ray radiation in the celestial sphere.
One of these sources is a "round" object, the angular size of which is 8 times the apparent diameter of the Moon. Russian astrophysicists, who discovered this object and named it G116.6-26.1 in accordance with the coordinates in the sky, believe that this is the remnant of a thermonuclear supernova explosion that exploded 40,000 years ago. Its main difference from several hundred similar objects is the properties of the gas in which the exploding star was located.
The discovery of an old thermonuclear supernova remnant in our galaxy is a rather rare event. Moreover, G116.6-26.1 is not in the plane of the Galaxy (a "disk" about 1 thousand light-years thick, where the stellar population is mainly concentrated; it is surrounded by a more rarefied stellar and gas halo, stretching tens and hundreds of thousands of light-years from us ), where it would be expected, but at an impressive distance of 4 thousand light years above it and 10 thousand light years from the Sun.
And although this is still our Milky Way galaxy, supernova remnants have not yet been observed so high above its plane. A short-lived (only millions of years) massive star could not explode there, they simply do not exist at such a "height". It was a thermonuclear explosion of a white dwarf that occurred about 40 thousand years ago. All matter from a star with a mass of 1.4 solar masses was ejected by an explosion with a tremendous speed of about 3000 km / s, and now the remnant has a gigantic physical size with a diameter of about 600-700 light years.
Image above: X-ray spectrum from a circle with a radius of 1.95 degrees around the supernova remnant (red dots). For comparison, the black dots show the halo emission spectrum outside the supernova remnant. The difference between these spectra (blue dots) is the emission of the remnant itself, in which the lines of hydrogen and helium-like oxygen ions dominate (the positions of the corresponding lines are marked at the top) X-ray spectrum from a circle with a radius of 1.95 degrees around the supernova remnant (red dots). For comparison, the black dots show the halo emission spectrum outside the supernova remnant. The difference between these spectra (blue dots) is the emission of the remnant itself, in which the lines of hydrogen- and helium-like oxygen ions dominate (the positions of the corresponding lines are indicated above).
In the course of thermonuclear fusion reactions with a gigantic energy release, which caused the explosion, and radioactive decay, more than half of the star's mass turned into iron. The resulting shock wave during its propagation "raked" hot gas in front of itself into the galactic halo with a total mass of about 100 solar masses. The eROSITA X-ray telescope "saw" the emission of this gas in the lines of hydrogen-like (O VIII) and helium-like (O VII) oxygen ions, consisting of an oxygen nucleus with a charge of Z = 8 and only one or two electrons, respectively.
“Such radiation is typical for equilibrium astrophysical plasma with a temperature of about 1–2 million degrees. The main characteristics of the spectrum of the object we found suggest that the ratio of the number of different ions in the swept gas should have changed not much compared to the surrounding unperturbed gas. Generally speaking, this is surprising, since it could be expected that the passage of a shock wave, heating and an increase in the gas density by several times should have changed these relations.
Our explanation is that the gas density even after compression was very low, and the time it takes to establish ionization equilibrium is longer than the age of the supernova. As a result, we observe an example of an “overheated” plasma that “remembers” the initial relationship between the number of different ions. In this case, the efficiency of collisional excitation of the most important transitions changes noticeably, and the emission in oxygen lines increases by more than 10 times in comparison with the equilibrium situation at the same temperature. It is this circumstance, in our opinion, that makes the found supernova remnant a source of bright X-ray radiation in the lines of oxygen ions, as well as a unique “living” laboratory of processes in a nonequilibrium astrophysical plasma, ”says one of the authors of the discovery, Ph.D. Ildar Khabibullin.
Image above: X-ray image (8x8 degrees) of the vicinity of the supernova remnant SRGe J0023 + 3625 = G116.6-26.1, obtained by the SRG / eROSITA telescope for the first three all-sky scans. The circle seen in the figure is a shock wave propagating through the hot gas in the halo of our Galaxy. Bright white dots correspond to compact sources of radiation that are far beyond the Galaxy (mainly distant quasars and active galactic nuclei) X-ray image (8x8 degrees) of the vicinity of the supernova remnant SRGe J0023 + 3625 = G116.6-26.1, obtained by the SRG / telescope eROSITA for the first three full sky scans. The circle seen in the figure is a shock wave propagating through the hot gas in the halo of our Galaxy. Bright white dots correspond to compact radiation sources that are far beyond the Galaxy (mainly distant quasars and active galactic nuclei).
“The study of the properties of gas in the halo of our galaxy is the most important task for understanding the formation and evolution of galaxies,” says the lead author of the article, Academician Yevgeny Churazov. - The gigantic size of the halo and the negligible density of matter make this task very difficult. It is remarkable that we now have the opportunity to use the remnants of supernova explosions for direct measurements of gas temperature and density at distances of tens of thousands of light years from us, high above the plane of the Milky Way."
It is believed that thermonuclear supernovae occur in our Galaxy less often than explosions of massive stars, accompanied by gravitational collapse and the formation of neutron stars or black holes. Until today, it is reliably known about five such relatively young (age from one hundred to a thousand years) remnants of thermonuclear explosions.
“It is hoped that a detailed study of the gas in the central part of the remnant will reveal iron ions with a total mass of almost the mass of the Sun, which were synthesized in the course of a thermonuclear explosion and the death of a white dwarf. Perhaps it will be possible to understand how and for what time the mixing of this “iron” plasma with the environment and the enrichment of the gas in the halo with iron occurs, ”says the co-author of the article, Academician Rashid Sunyaev, scientific leader of the Spectrum-Roentgen-Gamma project. - It is also striking that the discovered remnant of a supernova explosion is not visible in radio beams. This means that the shock wave in the hot plasma of the galactic halo accelerates cosmic rays extremely ineffectively. After all, most of the old supernova remnants in the plane of our Galaxy were discovered by their radio emission”.
Spectrum-RG Space Telescope
Russian astrophysicists hope in the coming months and years to report on other previously unknown supernova remnants detected by the eROSITA telescope in an all-sky X-ray survey. But they still need to be found among millions of X-ray sources of a different nature and rarefied clouds of hot diffuse gas from our Galaxy on the X-ray sky maps received by the Spectrum-Roentgen-Gamma observatory.
The eROSITA Discovery of a Large and Circular Supernova Remnant G116.6-26.1: A Type Ia Supernova Explosion as an Indicator of the Properties of Gas in the Halo of Our Galaxy is published in the Monthly Notices of the Royal Astronomical Society and posted in the archive of electronic preprints arxiv.org. The authors of the work are members of the Russian scientific group eROSITA on diffuse X-ray sources, employees of the IKI RAS E.M. Churazov, I.I. Khabibullin, R.A. Sunyaev, Institute of Astronomy of the Russian Academy of Sciences N.N. Chugai, A.F. Ioffe A.M. Bykov and the Institute of Applied Physics of the Russian Academy of Sciences I.I. Zinchenko.
Source: IKI RAN.
Related links:
IKI RAN: https://www.roscosmos.ru/tag/iki-ran/
Spectrum-RG: https://www.roscosmos.ru/tag/spektr-rg/
ROSCOSMOS Press Release: https://www.roscosmos.ru/32299/
Images, Text, Credits: ROSCOSMOS/Orbiter.ch Aerospace/Roland Berga.
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