The Spectrum-RG orbital X-ray observatory, launched from the Baikonur kosmodrom on July 13, 2019, begins the survey of the entire sky. On December 8, a spacecraft moving along the orbit around the L2 libration point at a distance of one and a half million kilometers from the Earth, started rotation around the axis directed towards the Earth. Thus, the SRG observatory began scanning the sky along the big circle on the celestial sphere, marking the start of the 4-years long all-sky survey. Following the Earth movement around the Sun, the ART-XC and eRosita telescopes will obtain every six months the map of the whole sky more sensitive than any all-sky data obtained by X-ray astronomers so far. The sum of eight independent sky maps that are expected after four years of the survey will make it possible to achieve record-high sensitivity and find about three million active nuclear galaxies and quasars, hundreds of thousands of clusters and group galaxies, and about half a million active stars, white dwarfs, pulsars and remnants of supernova explosions, neutron stars and black holes in our Galaxy. Comparison of individual sky maps will allow astrophysicists to monitor variability of millions of X-ray sources over the entire sky.
The main scientific goal of sky survey is the study of the large-scale structure of the Universe and the study of the nature of Dark Matter and Dark Energy. At the same time, the unprecedented sensitivity of the survey and numerous selection of X-ray sources of different types that will be discovered during the survey have enormous potential for new discoveries and will gain research in all branches of modern high-energy astrophysics.
The beginning of the sky survey was preceded by the painstaking work of scientists and engineers at the Space Research Institute (IKI) of the Russian Academy of Sciences in Moscow and at the Institute of Extraterrestrial Physics (MPE) of the Max Planck Society in Germany to set up and calibrate two unique X-ray telescopes. This work ended with extensive verification observations, during which the telescopes were tested during real observations of astrophysical objects. The figures below demonstrate the capabilities of the SRG / eRosita telescope for conducting deep sky fields with the size of tens of square degrees (see also the image of the eFEDS mini-survey on the MPE website).
Figure 1 shows an X-ray map of the Galactic disk region (the so-called “Ridge of the Galaxy”) obtained by the eRosita telescope in October 2019. As seen from the image map with a size of 25 squared degrees, numerous X-ray sources were detected, both located in our Galaxy, and quasars located at large distances from the Earth and observed through the Galaxy. Of great interest are Galactic objects: clusters of young stars emitting in X-rays, stars even less massive than our Sun, but having corona emitting in X-rays thousands of times brighter than the corona of our Sun. The map also shows X-ray pulsars: rapidly rotating magnetized neutron stars, the remnants of supernova explosions in which shock waves emit X-ray photons due to collisions of gas detached from a dead star with the surrounding interstellar matter. The map exhibits zones of diffuse X-ray emission. Blue and green colors correspond to high photon energies emitted by a gas with a temperature of tens of millions of degrees, and red color corresponds to a colder gas with a temperature of hundreds of thousands to a million degrees.
The Lockman Hole is a unique area in the sky where the absorption of X-rays by the interstellar medium of our Galaxy reaches its minimum value, which allows us to study distant quasars and clusters of galaxies with record sensitivity. In the sky region with the size of 20 square degrees, the eRosita telescope detected about 6,000 X-ray sources (Figure 2). The vast majority of these sources are the active galactic nuclei and quasars, the radiation of which is associated with the accretion of matter into a supermassive black hole. According to photometric redshift estimate, the most distant of them are located at redshifts up to z ~ 4-5. Also found more than 100 clusters of galaxies and several hundred active stars located in our galaxy.
The images shown above were obtained within the Russian quota of observational time of the eRosita telescope, and were analyzed by the scientists of the high-energy astrophysics department at IKI.