The eROSITA space telescope entered outer space on July 13, 2019 from Baikonur attached to a Proton-M launch vehicle. Because high-energy X-rays are fully absorbed by the Earth’s atmosphere, this is the only way to carry out these observations. The data that has now been published represents the first of many all-sky surveys using the eROSITA telescope. Within the most sensitive energy range of the eROSITA detectors, the telescope detected 170 million X-ray photons during this time, whose incoming energy and arrival time can be measured precisely using the cameras.
The eRASS1 catalogue covers half the X-ray sky and represents the data share attributable to the German eROSITA consortium. It consists of more than 900,000 sources, including some 710,000 supermassive black holes in distant galaxies (active galactic nuclei), 180,000 X-ray-emitting stars in our own Milky Way and 12,000 clusters of galaxies, plus a small number of other exotic classes of sources such as X-ray-emitting binary stars, supernova remnants, pulsars, and other objects.
“These are mind-blowing numbers for X-ray astronomy,” says Andrea Merloni, eROSITA principal investigator and first author of the eROSITA catalogue paper. “We’ve detected more sources in 6 months than the big flagship missions XMM-Newton and Chandra have done in nearly 25 years of operation.”
To coincide with the release, the German eROSITA Consortium has submitted more than 40 new scientific publications to peer-reviewed journals, adding to the more than 200 that the team had already published before the data release. Most of the new papers appear today with selected discoveries including the giant filament of pristine warm-hot gas extending between two galaxies. Further studies of how X-ray irradiation from a star may affect the atmosphere and water retention of orbiting planets.
“The scientific breadth and impact of the survey is quite overwhelming; it’s hard to put into a few words,” says Mara Salvato, who as spokesperson for the German eROSITA consortium co-ordinates the efforts of about 250 scientists organized into 12 working groups. “But the papers published by the team will speak for themselves.”
The first authors also include astrophysicists from the University of Bonn led by Prof. Dr. Thomas Reiprich from the Argelander Institute for Astronomy. “The hot gas inside clusters of galaxies (>10 million degrees Celsius) can be observed especially well within the X-ray range,” says Jakob Dietl, a master’s student at the Argelander Institute. “According to our theoretical models of the cosmos, these clusters should be connected by filaments of hot gas (1–10 million degrees Celsius). eROSITA has now enabled us to discover and describe a particularly long filament between two clusters for the first time.” But eROSITA is also ideal for investigating the clusters themselves: “The instrument’s large field of view has allowed us to study not only details of our nearest cluster, the Virgo Cluster, but also the area around it,” explains Hannah McCall, who is now studying for a doctorate at the University of Chicago having completed her master’s thesis at the University of Bonn. She adds: “If we could see Virgo in the night sky with the naked eye, it’d have a diameter equivalent to 15 full moons, so eROSITA is just made for analyzing objects like these. It’s the first time we’ve been able to study the outer regions of the Virgo Cluster on a grand scale.” Another researcher, Dr. Konstantinos Migkas, who obtained his doctorate from the University of Bonn and is now Oort Postdoctoral Fellow at Leiden Observatory, carried out important cross-calibration work with other X-ray satellites and is first author of a further study.
One of the research aims being pursued by the telescope is to use clusters of galaxies to test cosmological models. The cosmological findings based on a detailed analysis of the eRASS1 clusters will be published in around two weeks.
eRASS1 Facts & Figures:
- Observation period: 12 December 2019 – 11 June 2020
- Days of observations: 184
- Observing efficiency (avg. fraction of time spent by the telescope collecting data): 96.5%
- Total number of individual photons detected in the 0.2-2 keV energy range: 170 million [half-sky]
- Total number of detected X-ray sources: ~900k [half-sky]
- Total number of detected AGN (accreting supermassive black holes): ~710k [half-sky]
- Total number of detected stars in the Milky Way: ~180k [half-sky]
- Total number of detected clusters of galaxies: ~12k [half-sky]
- Total volume of scientific data transmitted down to earth by the instrument: 75 GB [all-sky]
- German eROSITA Consortium: ~250 members (incl. 80 early career researchers)
Animation regarding the filament between galaxy clusters: https://www.youtube.com/watch?v=Uqye0DfOWII
