With the start of the SRG all-sky survey, eROSITA promises most accurate maps of the X-ray sky ever

Launched from Baikonur on July 13th 2019 to the second Sun-Earth Lagrange point (L2), the Russian-German SRG mission has now started its main task. On December 8th, after an extensive program of commissioning, calibration and performance verification of its two X-ray telescopes (ART-XC and eROSITA), the satellite has begun observing the sky in continuous scanning mode. As SRG follows the revolution of Earth, and hence also of the L2 point, around the Sun, it will perform eight complete surveys of the whole sky, one every six months, for the next 4 years. Pre-launch predictions suggest that, over that time, the eROSITA instrument, conceived, designed and built at MPE, should discover approximately 100,000 clusters of galaxies, around 3 million accreting supermassive black holes and half a million active stars.

To confirm those predictions for the all-sky survey, eROSITA scientists have performed a number of test observations over the past few weeks. Among those, a mini-survey called eFEDS (eROSITA Final Equatorial Depth Survey) was devised in order to image a small patch of the sky to the same depth expected at the end of the 4-years all-sky survey. The eFEDS data show the same stunning image quality demonstrated by the eROSITA first light (see also IKI press-release featuring more eROSITA images of the Lockman Hole and the Galactic Plane). More importantly, they allowed the scientists to confirm with great accuracy the sensitivity of the X-ray telescope to its main target classes.

Over an area of just 1/300 of the full sky, eROSITA revealed more than 18,000 point-like X-ray sources, around 85% of them being distant Active Galactic Nuclei (AGN) harboring growing supermassive black holes, and most of the remainder X-ray stars. The mini-survey also discovered more then 400 clusters of galaxies (including a few at a redshift around 1), easily recognizable from their extended, diffuse morphology in the sharp X-ray images (see image).

It took around 4 days of eROSITA observations to generate these maps, and it will take about 1500 more to make such exquisite maps for the whole sky. The main goal of the SRG/eROSITA all-sky survey is to use these maps to study the large-scale structure of the Universe, and to measure its expansion rate by counting clusters of galaxies and the distribution of AGN in space and time. At the same time, eROSITA will reveal a large population of hot stars, neutron stars and stellar mass black holes scattered around the body of our Milky Way.