eROSITA

eROSITA (extended Roentgen Survey with an Imaging Telescope Array) is the first mission to scan the entire sky in the medium X-ray energy range. eROSITA is located as one of two instruments on the satellite Spektrum-Röntgen-Gamma. The launch of the satellite is scheduled for 2019. The project is supported by the German Aerospace Center (German: Deutsches Zentrum für Luft- und Raumfahrt e.V., abbrv. DLR) and coordinated by the Max Planck Society for the Advancement of Science (German: Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.V., abbrv. MPG), in corporation with the Russian space agency Roskosmos. eROSITA consists of seven identical cameras and mirrors. The plan is a scan the whole sky for about four years in an energy range of 0.5 to 10 keV. Another three years of eROSITA’s lifetime is planned for the observation of selected sources and regions. By scanning the entire sky, eROSITA is expected to detect many new sources and clusters of galaxies. Hence, it would be possible to learn more about the distribution of dark matter and dark energy in the universe and to test new cosmological models.

Launch: scheduled for 2019 with a Sojus-2 rocket

Energy range: 0.5 – 10 keV

Scientific goals: Observation of clusters of galaxies to get new information about the distribution of dark matter and energy, as well as detection of active galactic nuclei (AGN), gamma-ray bursts (GRBs), the diffuse emission of interstellar dust and the analysis of dust scattering halos.

The size of the pixels is 75 μm x 75 μm. Taking the total number of 384 x 384 pixels, the total area of the detector is 28.8 mm x 28.8 mm. This region is called the image area. The eROSITA detector is similar to the one of XMM-Newton, but has in addition also a framstore area, which consists of another 384 x 384 pixels. In contrast to the image area, the framestore area is shielded against X-ray radiation. After an exposure time of 50 ms, the electrons , which are collected in the pixels, are transported line by line in a time of 200 μs to the framestore area. The readout electronics processes the data in less than 10 ms, so that there are no misinterpreted photon counts during the readout process. Then, the framestore area is readout line by line by three CAMEX chips (CMOS Amplifier and MultiplEXer). Afterwards, the signals are amplified and digitized for further processing on the board computer.

The image area is shielded against UV-radiation with a 170 nm thick filter on the CCDs made of silicon dioxide, silicon nitride and a layer of aluminium. The CCD and CAMEX layer has an additional proton shield out of copper with a graded-Z-shield. The temperature is constantly kept at about -80°C to minimize thermal noise. For calibration, a Fe⁵⁵ source is placed in front of the CCD.

One of the main advantages of eROSITA is that the instrument can measure fluxes down to very small values. Therefore, it is possible to detect weak sources with a large field of view. We hope to detect many new sources with eROSITA, which is also described in the next section.

The main goal of eROSITA is to get new information about the distribution of dark matter and dark energy. To achieve this goal, about 100 000 clusters of galaxies will be detected during the four year scan of the entire sky. Clusters of galaxies contain about 1000 galaxies. These galaxies make up about 2% of the total mass of the cluster. The rest is about 11% intracluster medium and 87% dark matter. Clusters are good objects to analyze the structure and evolution of large-scale structures in the universe because they are bound by gravity. They are observed in the X-ray range because the hot intracluster gas emits mainly thermal X-ray radiation due to the Planck’s law.

By studying large-scale structures in the universe, it is possible to draw conclusions about the origin, geometry and evolution of the universe. The evolutionary process is related to the gravitational force and therefore to the distribution of the dark matter, while the dark energy mainly influences the evolution of the spatial distribution.

Another scientific goal is the detection of new AGN in the local universe. By making population studies of AGN, we can draw new conclusions about the structure of the early universe. Furthermore, a lot of unexplored black holes are thought to be located in surrounding galaxies and could be detected by eROSITA.

By observing dust scattering halos, new results on the chemical composition of interstellar dust will be obtainted. Those halos evolve out of flat angle scattering processes of X-ray radiation in the interstellar medium.