The diffuse soft X-ray background

Yeung, Michael Chun Hei (2025): The diffuse soft X-ray background: the local and the eROSITA bubbles. Dissertation, LMU München: Fakultät für Physik

Abstract

The diffuse soft X-ray background (SXRB) holds important information about astrophysical processes from a few AU to kpc around us. In this thesis, I go from the solar wind’s interaction with the inflowing interstellar medium (ISM) at AU scales, to the local hot bubble (LHB) in 100 pc scales, and eventually arrive at the possibly >~10 kpc giant X-ray structure stemming from the inner part of the Galaxy, called the eROSITA bubbles (eRObub), using the eROSITA All-Sky Survey data (eRASS). The Sun dwells in the LHB which is a low-density region containing ~10^6 K gas and emits thermally in the soft X-rays. The determination of the LHB hot gas properties, however, is often ambiguous because of a variable foreground caused by the charge exchange process between the solar wind ions and the inflowing neutral ISM (SWCX). I tackle this problem by choosing sight lines towards giant molecular clouds on the LHB surface, which isolate the foreground and provide calibration points for the LHB density. By the repeating surveys, the spectral analysis reveals a monotonically increasing SWCX contribution that correlates with solar activity, and the SWCX’s dependence on ecliptic latitudes, expected from the ionisation properties of the solar wind. On the other hand, the LHB seems to exhibit a temperature variation in the order of 20 eV. The LHB projected density appears to be consistent with a constant of n_e ~ 4x10^{-3} cm^{-3}, which would argue for a scenario where the hot gas indeed extends to the local bubble wall. Moving beyond isolated sight lines, I spatially and spectrally decompose the whole western Galactic hemisphere into conventional SXRB components and focus on the LHB aspect. I observe an apparent temperature dichotomy between the northern and southern hemispheres at |b|>30 deg, which could be set up by the most recent supernova explosions in the LHB. The LHB emission measure (EM) generally increases towards the Galactic poles, which is evidence of a larger LHB extent away from the Galactic plane due to less pressure resisting expansion. Comparison with local dust maps shows clear anti-correlation between N_H and EM, supporting a scenario where the hot gas fills cavities in the local bubble. I verify a known and discover a new tunnel of hot gas, potentially linking the LHB to the nearby superbubbles. In this light, I discuss the possibility of a wider interconnected network of hot ISM throughout the Milky Way. The resulting LHB EM map is used to produce a new 3D LHB model to create a latest view of the hot solar neighbourhood by combining known superbubbles in the literature. The last project explores the morphology and the spectrum of the eRObub as a follow-up to their recent discovery. The 3D morphology of the eRObub is explored using a geometrical model of a blast wave propagating in an ideal Galactic halo from the Galactic centre, which indicates a eRObub horizontal extent of ~6 kpc and demonstrates the large degeneracy revolving the vertical extent due to projection effects inherent in our location on the Galactic plane. The spectral analysis of the eRObub reveals an anomalous Fe XVII 3d/3s line ratio, which is best explained by two temperature components at kT = 0.25±0.03 and 0.56^{+0.04}_{-0.02} keV. We point out the existence of a bright shell surrounding the northern bubble, which is cooler and does not require a two-temperature model to fit. We rule out non-equilibrium ionisation as a possible explanation and discuss the possibility of the cool shell being a foreground structure. The eRObub spectra show a low abundance (Z ~ 0.2 Z_sun) which is consistent with shock-heating of the Galactic halo without significant metal enrichment. The North Polar Spur appears to be significantly higher in metallicity (Z > 0.5 Z_sun) than the eRObub, lending support for a star-formation-related and a separate origin from the eRObub.

• 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: 20. Oktober 2025
• 1. Bericht­erstatter:in: Becker, Werner