The eROSITA X-ray telescope on SRG

Abstract: eROSITA (extended ROentgen Survey with an Imaging Telescope Array) is the primary instrument on the Spectrum-Roentgen-Gamma (SRG) mission, which was successfully launched on July 13, 2019, from the Baikonour cosmodrome. After the commissioning of the instrument and a subsequent calibration and performance verification phase, eROSITA started a survey of the entire sky on December 13, 2019. By the end of 2023, eight complete scans of the celestial sphere will have been performed, each lasting six months. At the end of this program, the eROSITA all-sky survey in the soft X-ray band (0.2--2.3\,keV) will be about 25 times more sensitive than the ROSAT All-Sky Survey, while in the hard band (2.3--8\,keV) it will provide the first ever true imaging survey of the sky. The eROSITA design driving science is the detection of large samples of galaxy clusters up to redshifts $z>1$ in order to study the large-scale structure of the universe and test cosmological models including Dark Energy. In addition, eROSITA is expected to yield a sample of a few million AGNs, including obscured objects, revolutionizing our view of the evolution of supermassive black holes. The survey will also provide new insights into a wide range of astrophysical phenomena, including X-ray binaries, active stars, and diffuse emission within the Galaxy. Results from early observations, some of which are presented here, confirm that the performance of the instrument is able to fulfil its scientific promise. With this paper, we aim to give a concise description of the instrument, its performance as measured on ground, its operation in space, and also the first results from in-orbit measurements.

• The paper details eROSITA’s novel survey capabilities, including its ability to detect galaxy clusters beyond redshift 1.
• It explains the instrument’s design, calibration processes, and stability in orbit to ensure high-precision X-ray imaging.
• Results imply transformative potential for studying cosmic structures, AGN evolution, and supermassive black holes.

Overview of the eROSITA X-ray Telescope on SRG Mission

The paper presents a comprehensive examination of the eROSITA (extended ROentgen Survey with an Imaging Telescope Array) X-ray telescope, the primary instrument aboard the Spectrum-Roentgen-Gamma (SRG) mission. The mission, a collaborative effort between Germany and Russia, aims to perform a highly sensitive all-sky survey in the X-ray domain, significantly enhancing our understanding of various astrophysical phenomena and cosmological structures.

Mission and Objectives

The eROSITA telescope, having commenced its survey operations in December 2019, is designed to conduct multiple scans of the sky, providing unprecedented sensitivity—up to 25 times more than the ROSAT All-Sky Survey in the soft X-ray band (0.2–2.3 keV). This sensitivity allows for the detection of significant populations of galaxy clusters beyond redshifts of 1, paving the way for in-depth studies of large-scale cosmic structures and providing empirical data to assess cosmological models, including those concerning dark energy. Additionally, eROSITA is expected to identify millions of AGNs and other exotic X-ray phenomena, such as X-ray binaries and supernova remnants. This capability is expected to advance the understanding of supermassive black hole evolution and various energetic processes in the cosmos.

Technical Overview and Performance

The eROSITA telescope comprises seven mirror modules designed in a Wolter-I geometry, offering a substantial field of view with excellent spatial resolution. The telescope operates optimally across a photon energy range of 0.2 to 8 keV, enhanced by the ART-XC telescope for higher energy imaging, providing the first true hard X-ray imaging survey. Calibrated to operate well in space conditions post-initial commissioning phase, the instrument performs with stability and precision, as demonstrated by initial observations during its early operational phases.

Ground Calibration and Operational Challenges

Ground calibration activities laid the foundation for the accurate operation of eROSITA, indicating strong alignment between ground performance metrics and in-orbit functionality. These calibrations included assessments of energy resolution and detector sensitivity, alongside a meticulous characterization of mirror assemblies. However, challenges such as unexpected light leaks in certain telescope modules and susceptibility to cosmic radiation-induced disturbances in camera electronics required strategic modifications and highlighted potential areas for further refinement of instrument operation protocols.

Survey Execution and Data Processing

The planned all-sky survey, consisting of eight six-month epochs, exploited a strategic operational configuration optimized for enhanced coverage and depth, particularly at polar caps. The data capturing processes involved sophisticated real-time telemetry handling and processing pipelines—eSASS, which meticulously translate raw data into scientifically usable formats. This workflow facilitates rapid response to transient events and systematic long-term data analysis.

Implications and Future Prospects

The anticipated outcomes from eROSITA's continuous operation will likely contribute significantly to the cosmology and high-energy astrophysics community by providing a rich legacy dataset that holds potential relevance for diverse research applications. Post-survey, eROSITA’s operational phase is expected to transition into targeted observations, further extending its scientific impact. The eROSITA's mission will substantially shape future X-ray astrophysics, establishing a formidable benchmark for survey sensitivity and precision.

By delivering substantial advancements in observational X-ray astronomy, the findings from eROSITA will enhance theoretical models and expand our empirical understanding of the universe's structure and evolution. The mission’s data exploitation strategy, focused on complementing open science principles, ensures that its legacy will extend beyond the proprietary period, supporting a global scientific endeavor in X-ray astronomy.