ROSCOSMOS & DLR - Spectrum-RG (or Spektr-RG) Mission patch.
Dec. 10, 2020
Image above: Map of diffuse X-ray radiation in the range 0.6–1.0 keV, obtained by the SRG / eROSITA telescope. The contribution of point sources was removed. Map of diffuse X-rays in the range 0.6–1.0 keV, obtained by the SRG / eROSITA telescope.
The structures of hot gas on both sides of the Galactic disk, clearly visible in an X-ray survey of the entire sky, arose, most likely, due to shock waves caused by a powerful burst of activity in the center of our Galaxy tens of millions of years ago. In the first all-sky map produced by the eROSITA X-ray telescope, one of two instruments aboard Russia's Spektr-RG orbital observatory, astronomers have discovered an astonishing new detail: a huge circular structure below the plane of the Milky Way that occupies a significant portion of the Southern Sky.
A similar structure in the Northern Sky, the so-called North Polar Spur, has been known since the days of radio astronomy and X-ray astronomy. For many years it was believed that it arose as a result of a supernova explosion close to the Sun tens or hundreds of thousands of years ago. However, taken together, the northern and southern structures on the map resemble an hourglass-shaped halo, quite symmetrical about the center of the Galaxy, which is 25 thousand light years from the Sun (1 light year is about 9.46 trillion kilometers).
Image above: Overlay of maps of our Galaxy, obtained by the telescopes EROSITA and "Fermi". Diffuse X-ray radiation recorded by the erosite telescope (0.6–1 keV, indicated in shades of blue) surrounds a region of harder radiation (gigaelectron-volts, indicated in red), which is called Fermi bubbles. Comparison of these maps indicates a close relationship between Fermi and erosite bubbles. Overlay of maps of our Galaxy, obtained by the telescopes EROSITA and "Fermi". Diffuse X-ray radiation recorded by the erosite telescope (0.6–1 keV, indicated in shades of blue) surrounds a region of harder radiation (gigaelectron-volts, indicated in red), which is called Fermi bubbles. Comparison of these maps indicates a close relationship between Fermi and erosite bubbles.
“With its high sensitivity, good spectral and angular resolution and low background, the eROSITA telescope, which scans the entire sky every six months, has become a unique tool for detecting and studying objects that are much larger than the telescope's field of view and make up a significant portion of the entire sky,” explains Michael Freiberg, a scientist working with data from the eROSITA telescope at the Institute for Extraterrestrial Physics of the Society. Max Planck (MPE, Germany).
Large-scale X-rays observed in the 0.6–1.0 keV range show the manifestation of these giant bubbles with slowly varying brightness over a large part of the sky. Their angular dimensions are comparable to the dimensions of our entire Milky Way Galaxy, which corresponds to linear dimensions of ten kiloparsecs, that is, up to 30,000 light years across.
"Bubbles eROSITA" have a striking morphological similarity with the well-known "Fermi bubbles", but do not coincide with them geometrically, and the size of the latter is noticeably smaller. Fermi Bubbles were discovered years ago by the Fermi Gamma Observatory at much higher photon energies (gamma rays), a million times more energetic than the X-ray photons recorded by the Russian Spektr-RG Observatory.
Image above: An illustration of the possible position of eROSITA bubbles (EB, eROSITA bubbles, yellow) and Fermi bubbles (FB, Fermi bubbles, pink) relative to the Galaxy and the Solar System. The approximate dimensions of the structures obtained in this study are indicated next to the arrows of the corresponding color. An illustration of the possible position of eROSITA bubbles (EB, eROSITA bubbles, yellow) and Fermi bubbles (FB, Fermi bubbles, pink) relative to the Galaxy and the Solar System. The approximate dimensions of the structures obtained in this study are indicated next to the arrows of the corresponding color.
“The eROSITA telescope is now completing the second scan of the entire sky, doubling the number of X-ray photons recorded, in particular, from the bubbles it discovered,” says academician Rashid Sunyaev, scientific director of the Spektr-RG orbital observatory. “We have a tremendous amount of work ahead of us, because the eROSITA data allow us to isolate many X-ray spectral lines emitted by highly ionized gas ions in various regions of the“ bubbles ”. We were able to investigate the abundance of chemical elements, the degree of their ionization, the density and temperature of the emitting gas in many zones of the bubbles, to investigate the position of shock waves and to estimate the characteristic times that have passed since the time of the giant flash that generated these bubbles. It is striking that the eROSITA and Fermi bubbles are separated in space and the eROSITA bubbles are much larger. Most likely, magnetic fields play an important role at their boundary, which impede the escape of cosmic rays outside the Fermi bubbles.
This discovery helps to understand the circulation of matter in and around the Milky Way, as well as in other galaxies that we cannot observe with such a degree of detail due to the enormous distance to them. Most of the normal matter in the universe is invisible to our eyes. All the stars and galaxies that we observe with optical telescopes make it possible to see less than 10% of the total number of baryons. Huge amounts of unobservable baryonic matter are expected to be found in the rarefied halos that surround galaxies like cocoons, as well as in "filaments" that connect galaxy clusters like a cosmic web. These halos are hot, their temperature is millions of degrees, and therefore they are available for observation in the X-ray wavelength range.
Image above: An X-ray map of the entire sky obtained by the SRG / EOSITA telescope in galactic coordinates (the Galactic plane passes horizontally through the center of the map). An X-ray map of the entire sky obtained by the SRG / EOSITA telescope in galactic coordinates (the Galactic plane passes horizontally through the center of the map).
The bubbles seen by the eROSITA telescope are “reflections” of disturbances in this envelope of hot gas. They were caused by the ejection of matter due to the activity of a supermassive black hole in the center of our Galaxy, or by a giant burst of star formation in the gas of the central part of the Galaxy.
“The size of the bubbles and the temperature of the gas in them allow us to judge only the total released energy and approximately the time scale,” says Yevgeny Churazov, academician of the Space Research Institute of the Russian Academy of Sciences, one of the authors of the article. "But to unambiguously choose one of the hypotheses, this is not enough."
Now "our" black hole manifests itself as a very weak X-ray and radio source, from time to time faintly flashing in X-rays and infrared rays. However, she may well have been quite active in the past. We know examples of such activity from observations of supermassive black holes in other galaxies. In any case, the energy required to form these huge bubbles must have been very high - 1056 erg. This corresponds to the release of energy from 100,000 supernovae, which is similar to the estimates of other explosions in active nuclei of distant galaxies.
“The sharp edges of these bubbles are most likely the traces of shock waves caused by the powerful release of energy from the center of our Galaxy into its halo,” notes Peter Predel, one of the two leading authors of the article. "Such an explanation was previously proposed for the Fermi bubbles, and now, according to the eROSITA telescope, the full volume and morphology of these structures have become apparent." “The scars left by such flashes take a long time to heal in such halos,” adds Andrea Merloni, scientific director of the eROSITA telescope.
Spectrum-RG (or Spektr-RG) observatory
The eROSITA “multicolor” X-ray map of the sky contains a colossal amount of information about the interstellar medium of the Milky Way as a whole, says Corresponding Member of the Russian Academy of Sciences, one of the creators of the X-ray sky map and co-author of the article Marat Gilfanov (Space Research Institute, RAS). - It can be seen that the radiation of hot and warm gas comes to us from all directions, but its brightness in the direction of the Galactic plane is greatly reduced due to absorption by cold matter located in the spiral arms and in the disk of our Galaxy. The observed picture is complicated by the contribution of radiation from the so-called. “Local Bubble”, the nature of which is still not fully understood, and we expect eROSITA to contribute to solving this mystery as well.
The discovery was published in the journal Nature on December 9, 2020. Half of the co-authors of the article are employees of Russian research institutes, members of scientific groups of the eROSITA telescope.
The Russian spacecraft "Spektr-RG", developed at the Scientific and Production Association named after S.A. Lavochkin (part of the Roscosmos State Corporation), was launched on July 13, 2019 from the Baikonur cosmodrome. It was created with the participation of Germany within the framework of the Federal Space Program of Russia by order of the Russian Academy of Sciences. The observatory is equipped with two unique X-ray mirror telescopes: ART-XC (IKI RAS, Russia) and eROSITA (MPE, Germany), operating on the principle of oblique incidence X-ray optics. The telescopes are installed on the Navigator space platform (NPO Lavochkina, Russia), adapted to the project's objectives. The main goal of the mission is to build a map of the entire sky in the soft (0.3–8 keV) and hard (4–20 keV) ranges of the X-ray spectrum with unprecedented sensitivity. The observatory must operate in space for at least 6.5 years.
ROSCOSMOS Press Release: https://www.roscosmos.ru/29661/
Spektr-RG (SRG): http://www.iki.rssi.ru/eng/srg.htm
Images, Text, Credits: ROSCOSMOS/DLR/Orbiter.ch Aerospace/Roland Berga.
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