Spectrum-RG begins all-sky survey

December 18, 2019

The Spectrum-RG observatory, launched from Baikonur on July 13, 2019, now begins with scanning the entire sky. On December 8, the spacecraft moving on a wide orbit around the L2 liberation point at a distance of 1.5 million kilometres started rotating around the axis directed towards the Earth. Both, the ART-XC and eROSITA telescopes began scanning the sky along the big circle on the celestial sphere, thus marking the start of the 4-years long all-sky survey.

X-ray map of a 25 squared degrees region in the disk of our Milky Way (the so-called Galactic Ridge) obtained by the eROSITA telescope in October 2019. In this field, hundreds of galactic X-ray sources are detected, as well as a number of quasars located at much larger distances and observed through the Galactic disk. In addition to many individual sources, the map also shows unresolved X-ray emission from hot gas and from a multitude of faint unresolved sources. Blue and green colors correspond to high photon energies emitted by a gas with a temperature of tens of millions of degrees, while red colored regions reveal colder gas of lower temperature.

Following the Earth’s motion around the Sun, every six months, ART-XC and eROSITA will obtain a map of the whole sky which will be more sensitive than any all-sky data obtained by X-ray astronomers so far. At the end of the four year survey, the sum of all eight independent sky maps will achieve record-high sensitivity. The scientists expect to discover about three million active galactic nuclei and quasars, hundreds of thousands of clusters and groups of galaxies, as well as about half a million active stars, white dwarfs, pulsars and remnants of supernova explosions, neutron stars and black holes in our own Galaxy. Comparison of the individual sky maps will allow astrophysicists to monitor the variability of millions of X-ray sources over the entire sky.

The main scientific goal of the all-sky survey is to study the large-scale structure of the Universe and deduce information on the nature of Dark Matter and Dark Energy. On the other hand, the unprecedented sensitivity and the sheer number of X-ray sources of different types that will be discovered harbours enormous discovery potential, fostering research in all areas of modern high-energy astrophysics.

In the Lockman Hole, a sky region with the size of 20 square degrees, the eROSITA telescope detected about 6,000 X-ray sources. The vast majority of these sources are active galactic nuclei and quasars, the radiation from which is associated with the accretion of matter into a supermassive black hole. There are also more than 100 clusters of galaxies and several hundred active stars located in our Galaxy.

The all-sky survey was preceded with the careful work of scientists and engineers at the Space Research Institute (IKI) of the Russian Academy of Sciences in Moscow and at the Institute for Extraterrestrial Physics (MPE) of the Max Planck Society in Germany to commission and calibrate the two unique X-ray telescopes aboard the SRG spacecraft. The calibration phase was concluded with the performance verification phase, during which the telescopes were tested in deep observations of various astrophysical objects and fields. The figures shown here are based on data obtained during the PV phase of the mission and impressively demonstrate the capabilities of SRG/eROSITA for studying sky areas of tens of square degrees in size (see also the MPE press release and the map obtained in the course of the eFEDS mini-survey).

In the Galactic Ridge, eROSITA detected stars with active X-ray emitting coronae, which are thousands times brighter than the corona of our Sun, star-forming regions and clusters of young stars, X-ray pulsars (rapidly rotating magnetized neutron stars), and supernova remnants. In these, X-ray photons are emitted by gas compressed in shocks where the material of the exploded star collides with the surrounding interstellar matter. While the Milky Way image also includes a number of extragalactic sources, the long range of eROSITA was demonstrated with an observation of the Lockman Hole. In this unique area in the sky absorption of X-rays by the interstellar medium of the Galaxy reaches its minimum value, which allows astronomers to study distant quasars and clusters of galaxies with unprecedented detail. According to photometric redshift estimates, the most distant of the sources detected by eROSITA in the Lockman Hole are located at redshifts up to z ~ 4-5.

The images shown here are based on the data from the Russian share of observing time of the SRG/eROSITA telescope.


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