The first X-ray sky map created thanks to the eROSITA instrument

The sky seen at X-rays by eROSITA
The Max Planck Institute for Extraterrestrial Physics (MPE) has published an X-ray universe map obtained thanks to the eROSITA instrument, which the institute built for the Spektr-RG space telescope. This map includes about a million objects related to the hot and energetic part of the universe. It’s about 4 times deeper than the previous map of this type and contains about 10 times the number of energy sources, the equivalent of those discovered by all X-ray telescopes combined in over half a century of observations. It took about six months for eROSITA to complete this survey, which is only the first of eight expected in the course of about four years that will offer information that is likely to be used for decades.

The Spektr-RG space telescope was launched on July 13, 2019. Its main instrument, eROSITA, is the successor to the ROSAT X-ray telescope, which between 1990 and 1991 created an X-ray universe map. The experience of that mission contributed to the significant progress that made it possible to build the eROSITA instrument, which now shows its first great results with an extraordinary new map.

The image (Courtesy Jeremy Sanders, Hermann Brunner and the eSASS team (MPE); Eugene Churazov, Marat Gilfanov (on behalf of IKI). All rights reserved) shows the energetic universe seen during this survey in which Spektr-RG rotated continuously providing a uniform exposure of approximately 150-200 second over the whole sky. The entire sky is projected on an ellipse, in the so-called Aitoff projection, with the center of the Milky Way in the center of the image and the galactic plane running horizontally. The collected photons have been colored according to their energy (red for energies 0.3-0.6keV, green for 0.6-1keV, blue for 1-2.3keV). The diffuse red glow that can be seen far from the galactic plane originates from the hot gas in the vicinity of the solar system, the so-called Local Bubble. Along the galaxy’s plane, dust and gas absorb low-energy photons, so only the sources that emit the highest energy photons remain visible and have the characteristic blue color. The hotter gas near the galactic center, shown in green and yellow, contains imprinted information on the history of the most energetic processes in the history of the Milky Way, such as supernova explosions, which drive gas fountains outside the galactic plane, and possibly outbursts occurred in the past from our supermassive black hole, now dormant.

The hot sources of the Milky Way are those shown in greater detail including stars, even dead but more active than ever such as neutron stars and black holes, and structures such as the so-called circum-galactic medium formed by hot gas that surrounds the galaxy. Many sources from the satellite galaxies, such as Magellanic Clouds, are also included in this new map in large amounts and with much greater detail than previous observations.

Mara Salvatore of the MPE, who leads the effort to combine the observations of eROSITA with those of other telescopes that span the entire electromagnetic spectrum, spoke of the expectation of having the new complete map of the sky. The scientist explained that large areas of the sky have already been covered in many other wavelengths, and now we have the matching X-ray data. These surveys are used to identify X-ray sources and understand their nature. In this sense, the survey also offers a treasure trove of rare and exotic phenomena since eROSITA often sees X-ray outbursts coming from the sky and it’s necessary to alert ground-based telescopes immediately to understand what produces them.

This type of survey can lead to the discovery of new objects, and in the case of the map generated by the eROSITA instrument there was the discovery of a number of energy sources equivalent to those discovered by all previous X-ray telescopes. This represents only the start of studies because astronomers around the world will be able to use this new map for specific research. The combination of multiple instruments is increasingly important in the field of astronomy, even more with the development of multimessenger astronomy, which combines observations of different emissions such as electromagnetic and gravitational waves.

Rashid Sunyaev, who leads the Russian Spektr-RG team, explained that the space telescope started its second survey. Eight of them are planned in total, and their combination will offer further depth in the energetic universe mapping. For this reason, the scientist predicts that the final maps will be used for decades in astrophysics and cosmology studies. In short, we can expect these investigations to enable further steps forward in our understanding of the universe.

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