eROSITA Science Book: Mapping the Structure of the Energetic Universe

Abstract: eROSITA is the primary instrument on the Russian SRG mission. In the first four years of scientific operation after its launch, foreseen for 2014, it will perform a deep survey of the entire X-ray sky. In the soft X-ray band (0.5-2 keV), this will be about 20 times more sensitive than the ROSAT all sky survey, while in the hard band (2-10 keV) it will provide the first ever true imaging survey of the sky at those energies. Such a sensitive all-sky survey will revolutionize our view of the high-energy sky, and calls for major efforts in synergic, multi-wavelength wide area surveys in order to fully exploit the scientific potential of the X-ray data. The design-driving science of eROSITA is the detection of very large samples (~10^5 objects) of galaxy clusters out to redshifts z>1, in order to study the large scale structure in the Universe, test and characterize cosmological models including Dark Energy. eROSITA is also expected to yield a sample of around 3 millions Active Galactic Nuclei, including both obscured and un-obscured objects, providing a unique view of the evolution of supermassive black holes within the emerging cosmic structure. The survey will also provide new insights into a wide range of astrophysical phenomena, including accreting binaries, active stars and diffuse emission within the Galaxy, as well as solar system bodies that emit X-rays via the charge exchange process. Finally, such a deep imaging survey at high spectral resolution, with its scanning strategy sensitive to a range of variability timescales from tens of seconds to years, will undoubtedly open up a vast discovery space for the study of rare, unpredicted, or unpredictable high-energy astrophysical phenomena. In this living document we present a comprehensive description of the main scientific goals of the mission, with strong emphasis on the early survey phases.

• The paper outlines the primary contribution of extending ROSAT’s survey sensitivity with eROSITA, enabling detection of ~100,000 galaxy clusters and 3 million AGNs.
• It details an eight-survey strategy over four years that combines advanced imaging techniques with continuous X-ray monitoring to capture both static and transient phenomena.
• The paper emphasizes the mission’s potential to enhance cosmological studies by integrating multi-wavelength data to analyze dark energy and cosmic structure formation.

Overview of the eROSITA Science Book

The "eROSITA Science Book: Mapping the Structure of the Energetic Universe" presents a detailed account of the scientific objectives and operational strategy for the eROSITA telescope, which is part of the Spektrum-Roentgen-Gamma (SRG) mission. Developed under the leadership of the Max-Planck Institute for Extraterrestrial Physics, eROSITA aims to conduct the most comprehensive all-sky X-ray survey in the 0.5-10 keV energy range. This paper elaborates on the technical specifications, scientific potential, and expected advancements in astrophysical research prompted by the eROSITA mission.

Scientific Goals and Methodology

The primary scientific goal of eROSITA is to construct a detailed picture of the X-ray Universe, focusing on galaxy clusters and AGN. By extending the ROSAT all-sky survey's sensitivity by a factor of 30, eROSITA aims to identify ~100,000 galaxy clusters at redshifts beyond z > 1. This large sample is pivotal for investigating the large-scale structure of the Universe and conducting cosmological tests related to Dark Energy and the growth of cosmic structures. eROSITA is also expected to detect about 3 million AGNs, offering new insights into supermassive black hole growth and the Universe's accretion history.

The mission strategy includes eight all-sky surveys over four years, followed by several years of pointed observations. This strategy allows for continuous monitoring of the entire X-ray sky and facilitates studies of both static and transient phenomena.

Technical Capabilities

eROSITA's novel imaging technique employs seven identical mirror modules and associated CCD cameras to achieve a high angular resolution and a broad energy band coverage. The design offers significant improvements over previous missions like ROSAT, with enhanced field-of-view and sensitivity. The telescope's ability to detect point sources as faint as ~10⁻¹⁴ erg/s/cm² spectral flux across the soft and hard X-ray energy bands is vital for the thorough identification of cosmic structures.

Key Scientific Objectives

• Galaxy Clusters and Cosmology: The comprehensive detection of galaxy clusters will permit precise constraints on cosmological parameters and the study of Dark Energy's influence on cosmic expansion and structure formation. The paper describes the utility of such data in representing one of the first Stage IV Dark Energy probes.
• Acting Galactic Nuclei (AGN) and Normal Galaxies: eROSITA will enhance our understanding of AGN evolution and provide a substantial catalog for investigating the environmental conditions affecting supermassive black hole growth. By offering extensive scope over multiple redshift intervals, eROSITA will further refine our understanding of AGN feedback processes on the formation and evolution of galaxies.
• Transient Events and Variable X-ray Sources: With the capability to observe vast regions of the sky multiple times, eROSITA will serve as a valuable tool for cataloging transient sources and studying phenomena such as stellar flares, neutron star and black hole outbursts, and tidal disruption events. The mission’s continuous monitoring aspect will allow the investigation of these phenomena over a variety of timescales.
• Synergies with Multi-Wavelength Surveys: The paper emphasizes the importance of multi-wavelength data synergy, including wide-area optical/IR surveys, for achieving a comprehensive analysis of the diverse astrophysical phenomena detected by eROSITA. The integration of these data can substantially enhance source identification and facilitate accurate photometric redshift estimations.

Implications and Future Prospects

The eROSITA mission is expected to open new discovery avenues in X-ray astronomy by vastly improving our high-energy vision of the Universe. The insights gained will significantly influence both theoretical and observational cosmology and enhance our understanding of the energetic processes shaping the Universe. It also anticipates stimulating follow-up studies with future complementary space missions, which can further elucidate the nature and dynamics of discovered X-ray sources.

The comprehensive scope of the eROSITA science program elucidated in this paper underscores its potential as a major contributor to astronomical research in the coming decade, setting a new benchmark in X-ray studies of the Universe.