Tidal disruption events and other nuclear transients discovered with eROSITA

Grotova, Iuliia (2026): Tidal disruption events and other nuclear transients discovered with eROSITA. Dissertation, LMU München: Fakultät für Physik

Abstract

Time-domain X-ray astronomy provides a powerful window into some of the most energetic and short-lived phenomena in the Universe. The eROSITA telescope aboard the Spectrum-Roentgen-Gamma satellite offers an unprecedented opportunity to explore the transient and variable extragalactic X-ray sky, thanks to its sensitivity, sky coverage, and survey cadence. These capabilities make eROSITA particularly well suited for detecting variable extragalactic X-ray sources. Among these, a small but scientifically valuable subset arises from rare phenomena such as tidal disruption events (TDEs), in which stars are torn apart by the gravitational forces of supermassive black holes (SMBH). Identifying such transient signals within the vast population of X-ray sources is essential for advancing our understanding of these extreme physical processes. This thesis is dedicated to exploring the variable X-ray sky with eROSITA, with a primary focus on the identification and analysis of stellar tidal disruption event candidates. Chapter 1 provides an overview of the eROSITA instrument as well as transients and variables present in the X-ray sky, which includes both galactic and extragalactic sources. The chapter also includes an overview of TDEs, starting with a theoretical introduction and followed by a discussion of the observational properties of TDEs in various bands and previously discovered TDEs with eROSITA. Chapter 2 describes the systematic selection of X-ray extragalactic transients found in the first two eROSITA all-sky surveys (eRASS) that are not associated with known active galactic nuclei (AGN) prior to eROSITA observations. The presented catalog eRO-ExTra includes 304 extragalactic eROSITA transients and variables, with more than 90% of sources associated with reliable optical counterparts. Each source is characterized through archival X-ray data, long-term eROSITA light curves, and X-ray spectral modeling. eRO-ExTra delivers the first systematic view of the extragalactic non-AGN transient sky with eROSITA, offering a valuable dataset for investigating rare nuclear variability phenomena associated with massive black holes. Chapters 3 and 4 relate specifically to TDE science, including canonical candidates as well as extreme scenarios that define the lower and upper bounds of the black hole mass range for TDEs: from a candidate around an intermediate-mass black hole to one associated with a SMBH above the Hills mass. Chapter 3 presents a systematic population study of TDEs from the eRO-ExTra catalog. The selected TDE sample contains 30 canonical TDEs and one off-nuclear TDE candidate and is the largest uniformly X-ray–selected TDE sample to date. From this sample, we derive the TDE X-ray luminosity function and infer the volumetric TDE rate. A multiwavelength analysis reveals that a subset of events shows optical, mid-infrared, or radio variability consistent with reprocessing or stream–stream collisions. This work demonstrates the statistical power of eROSITA for TDE studies and provides a foundation for future research in upcoming X-ray surveys. Chapter 4 presents the discovery and analysis of J0936, an exceptional X-ray transient identified by eROSITA in a Fanaroff-Riley Type II (FRII) radio galaxy. The source exhibits an ultra-soft flare and a months-later re-brightening. The chapter shows how its X-ray properties differ significantly from those of typical FR II galaxies and instead resemble a partial, repeated TDE. With an inferred SMBH mass above the canonical Hills mass, its origin likely requires a rapidly spinning black hole or an off-nuclear SMBH. J0936 thus represents one of the most exotic and massive TDE candidates known, expanding the range of environments in which TDEs can occur. Chapter 5 summarizes the thesis and outlines prospective directions for future research on TDEs in X-rays and across other wavelengths. It highlights the potential applications of the methodologies developed in this work and identifies outstanding questions for further exploration in the study of TDEs.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 530 Physik
• Fakultäten: Fakultät für Physik
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 26. Februar 2026
• 1. Bericht­erstatter:in: Nandra, Kirpal