X-ray variability in the eROSITA era

Bogensberger, David (2023): X-ray variability in the eROSITA era. Dissertation, LMU München: Fakultät für Physik

Abstract

The bright X-ray emission from Black Hole Transients (BHTs) and Active Galactic Nuclei (AGNs) originates from the region close to the black hole event horizon. Due to the comparably small size of this region, the observed flux often varies significantly on short timescales. Probing the variability properties of these systems helps us identify and understand the physical mechanisms at work in these extreme environments. To this end, we set out to investigate the X-ray variability observed during the eROSITA all-sky surveys. Due to the unique characteristics of eROSITA, particularly its varying exposure times per observation, various variability methodologies had to be adapted to yield accurate results. Therefore, I determined thresholds for selecting variable sources for an extensive range of number of bins and mean count rates. I also defined a new method of estimating the normalised intrinsic variance of a source that is significantly more accurate than previous methods, especially at low count rates. Additionally, I determined the size of the sampling errors and the excess noise in periodograms. X-ray sources located close to the South Ecliptic Pole (SEP) were observed most frequently by eROSITA. Therefore, this field is the ideal choice for long-term variability analysis. I used the variability methods I defined to identify 453 unique significantly variable sources within $3\degree$ of the SEP in the first three all-sky surveys. Next, I distinguished sources as being either likely galactic or likely extragalactic and observed and fitted the optical spectra of thousands of eROSITA-selected sources. Finally, I investigated some of the significantly variable sources, identifying unique effects. I also investigated the intriguing and unique properties of the 2018 outburst of the black hole transient Swift J1658.2-4242. It prominently features several flip-flops, which have the appearance of top-hat functions in a light curve. These flip-flops had a larger flux difference between the dim and bright states than has ever been observed. X-ray binary light curves often feature quasi-periodic oscillations, which are grouped into three types; A, B, and C. For the first time, I identified a direct transition between QPO types A and C that does not involve a type B QPO in between. I found that transitions between flip-flop states only occur at multiples of a fundamental period of 2.8 ks, which provides a significant constraint for models seeking to describe this phenomenon. By fitting the X-ray spectra in the bright and dim states, I found that the 77% difference in source flux is mainly caused by a change in the temperature of the inner accretion disc.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: black holes, active galactic nuclei, X-ray binaries, variability, eROSITA
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 530 Physik
• Fakultäten: Fakultät für Physik
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 1. März 2023
• 1. Bericht­erstatter:in: Nandra, Kirpal