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An appetizer to the all-sky banquet

First eROSITA X-ray data now released to the public

June 28, 2021

As announced during the 2021 meeting of the European Astronomical Society, the German eROSITA collaboration has released the first set of data taken with the eROSITA X-ray telescope onboard the SRG observatory. For the first time, astronomers throughout the world will have the chance to download and analyse data from this new powerful telescope. The Early Data Release will be accompanied by the publication of xxx eROSITA science papers by the German eROSITA Consortium on the arXiv preprint server, with these and more to be published in a forthcoming special issue of the journal Astronomy and Astrophysics.

“This is the first public release of SRG/eROSITA data,” points out Andrea Merloni, eROSITA principal investigator. “Since the start of observations with the X-ray telescope at the end of 2019, we have been impressed with the high-quality data, which have already led to numerous astronomical discoveries and breakthroughs. Now is the time to give astronomers worldwide a first taste of what is to come over the next few years. This is going to open up a whole new Universe of possibilities.”

The “eFEDS” (the eROSITA Final Equatorial Depth Survey) was designed as a preview to the final eROSITA all-sky survey. The top panel shows all sources, color-coded by the energy of the photons. The inset is a zoom onto a "supercluster", i.e. a conglomeration of clusters of galaxies about to merge. In bottom panel out all stars and AGN (i.e. all the point sources) have been filtered out. What is left is the diffuse emission from clusters and groups of galaxies, the matter in between, and the halo of the MilkyWay in front. As such, it gives a visual illustration of the large-scale structure that is almost impossible to gain otherwise.

The Early Data Release (EDR) observations were taken during the so-called ‘Calibration and Performance Verification Phase’, which lasted approximately from mid-September to mid-December 2019. Since then, the eROSITA X-ray telescope on-board the SRG spacecraft has been scanning the whole sky and producing sensitive X-ray all-sky maps, and it will continue to do so until the end of 2023. eROSITA is the first large X-ray focusing telescope optimized for surveys, thanks to its large field of view, high quality mirrors and sensitive CCD cameras.

The EDR contains almost 100 individual observations of 29 distinct fields taken before the start of the all-sky scans. They cover a wide range of different astronomical objects, from galactic neutron stars to clusters of galaxies and showcase the potential and versatility of the eROSITA telescope for imaging, spectroscopy and time domain analysis.

“Organizing these first eROSITA datasets in a comprehensive manner was a huge challenge”, says Miriam Ramos-Ceja at the Max Planck Institute for Extraterrestrial Physics (MPE), who is the main EDR coordinator. “First we had to gather and process the data in a uniform way, and then check and validate them to make sure they were of high quality.” In addition to the data itself, the MPE-led team will also make available dedicated software developed to reduce and analyze the eROSITA data. “We put a lot of emphasis on documenting all relevant steps taken to make the data and software easy to use by scientists all over the world,” adds Ramos-Ceja.

Among the datasets released to the public, there is one in particular that holds a special place: a mini-survey called “eFEDS” (the eROSITA Final Equatorial Depth Survey). Designed as a preview to the final all-sky survey, eFEDS covers uniformly a patch of about 140 square degrees of the sky (about 1/300th of the all-sky survey), providing a glimpse of what the whole extra-galactic sky will look like in X-rays after eROSITA completes its all-sky survey program in 2023. In just four days of eFEDS observations, eROSITA has carried out the largest contiguous X-ray survey to date. The total number of sources detected was almost 30,000, more than any other single, dedicated X-ray survey. The release includes not only the processed data, but several catalogues of eROSITA sources describing their X-ray and and multi-wavelength properties.

The cluster of galaxies called Abell 3266 is one of the brightest in the sky; it is in the process of merging with a smaller group of galaxies. The eROSITA image, color-coded by the energy of the photons, shows the disturbed cluster gas (white diffuse emission) as well as many background AGN and foreground stars. The inset shows the entropy distribution of the intra-cluster gas, based on the pressure and temperature of the gas and measured entirely from eROSITA data. As part of the merging process, the cold and dense low-entropy gas in the core (dark-blue) is stirred and mixed with the hotter gas in the outskirts (yellow-white).

“We didn’t stop just with the X-rays – we have now combined the eROSITA X-ray data with UV, optical and infrared data from many different instruments both on the ground and in space,” explains Mara Salvato, eROSITA spokesperson and chair of the eROSITA follow-up working group. “The location of the eFEDS field was partly chosen because a large set of other observations are available from powerful telescopes across most of the electromagnetic spectrum. This step is crucial to classify the X-ray sources discovered by eROSITA and work out their physical properties. It’s exciting to see this all come together in eFEDS. It’s proof-of concept that we can do this for all the sources that that the all-sky survey will bring – though we still have a huge task ahead of us.”

The associated suite of xxx papers led by the German eROSITA Consortium mostly focus on the EDR observations, but also include a few exciting highlights from the ongoing all-sky survey. The objects under study range from stars and diffuse emission in our own Milky Way or the neighboring Large Magellanic Cloud to Active Galactic Nuclei hosting supermassive black holes, and huge clusters of galaxies. “Apart from the groundbreaking science, another thing that makes me really proud is the contribution of female scientists to the effort” adds Salvato “Almost half the papers that accompany the data release are led by women. The eROSITA collaboration will keep working towards making scientific opportunities available to all.” 

Along with everyone else, of course, COVID-19 has complicated matters for the eROSITA team. “Only six months after the start of eROSITA science observations, the global pandemic forced us to modify almost all aspects of our work,” says Andrea Merloni. “Even operating the telescope 1.5 million kilometers away had to be done from home. I’d like to think that the unique opportunity of working with a brand new ‘discovery machine’ has helped many of us to keep some sort of focus or balance – at least it did for me. eROSITA has given us many reasons to celebrate, and we are all looking forward to having a real party soon!”

Notes to editors

eROSITA is the primary instrument aboard Spektrum-RG (SRG), a joint Russian-German science mission supported by the Russian Space Agency (Roskosmos), in the interests of the Russian Academy of Sciences represented by its Space Research Institute (IKI), and the Deutsches Zentrum für Luft- und Raumfahrt (DLR). The SRG spacecraft was built by Lavochkin Association (NPOL) and its subcontractors, and is operated by NPOL with support from the Max-Planck Institute for Extraterrestrial Physics (MPE).

The telescope was launched into space onboard the RG mission on July 13, 2019. Its large collecting area and wide field of view are designed to perform to a deep all-sky survey in the X-ray band. Over the course of six months (December 2019 to June 2020), SRG/eROSITA completed the first survey of the whole sky at energies 0.2-8 keV, which is significantly deeper than the only existing all-sky survey with an X-ray imaging telescope, performed by ROSAT in 1990 at energies 0.1-2.4 keV.

The development and construction of the eROSITA X-ray instrument was led by the Max Planck Institute for Extraterrestrial Physics (MPE), with contributions from the Dr. Karl Remeis Observatory Bamberg, the University of Hamburg Observatory, the Leibniz Institute for Astrophysics Potsdam (AIP), and the Institute for Astronomy and Astrophysics of the University of Tübingen, with the support of DLR and the Max Planck Society. The Argelander Institute for Astronomy of the University of Bonn and the Ludwig-Maximilians-Universität Munich also participated in the science preparation for eROSITA.

The eROSITA data are processed using the eSASS software system developed by the German eROSITA consortium.

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