# The SRG/eROSITA diffuse soft X-ray background II. spectra and morphology of the eROSITA bubbles in the western Galactic hemisphere

**Authors:** Michael C. H. Yeung, Martin G. F. Mayer, Andy Strong, Michael J. Freyberg, Gabriele Ponti, Konrad Dennerl, Junjie Mao, Manami Sasaki, Xueying Zheng, Jeremy S. Sanders, Yi Zhang, Jiejia Liu, Liyi Gu, Werner Becker, Frank Haberl, Teng Liu, Andrea Merloni, Peter Predehl  
**arXiv:** [2605.02998](https://arxiv.org/abs/2605.02998)  
**Date:** 2026-05-04  
**Summary page:** https://www.summarizepaper.com/en/paper/2605.02998v1/

## Key points

- The eROSITA bubbles (eROb) were discovered in 2020 during the SRG/eROSITA All-Sky Survey, standing out as large structures in the X-ray sky.
- Researchers aim to explore the bubbles further using eROSITA all-sky maps and spatially resolved spectra to understand their three-dimensional structure and hot gas properties.
- Analysis of high-quality spectra revealed two emission components within the western eRObub with different temperatures and emission measures.
- Gas properties suggest sub-solar abundances for the eRObub, consistent with Galactic halo expectations, while the North Polar Spur shows higher abundances possibly indicating a distinct origin.
- A cool shell surrounds the northern eRObub with an apparent temperature under collisional ionization equilibrium assumptions.
- Both eRObub have well-constrained horizontal sizes but uncertain vertical extents due to limitations in observing X-ray emissions.
- The study sheds light on physical properties of eROSITA bubbles, addressing questions about their geometry, emitting material characteristics, energetics for inflation, and implications for related structures like the North Polar Spur and Fermi Bubbles within our Galaxy.

## Summary

The eROSITA bubbles (eRObub) were first discovered in 2020 during the initial SRG/eROSITA All-Sky Survey. They stand out as some of the largest structures in the X-ray sky and have sparked interest among researchers. This study aims to delve deeper into these enigmatic bubbles by utilizing eROSITA all-sky maps and spatially resolved spectra to unravel their three-dimensional structure and characterize the hot gas properties within them. Through fitting spectra with a constant signal-to-noise ratio and analyzing high-quality spectra from specific regions, the researchers were able to scrutinize the gas properties in greater detail. The morphology of the eRObub was examined using a parametrized geometrical model that depicts a blast wave expanding into an idealized Galactic halo from its center. The analysis revealed that the western eRObub's interior is best described by two emission components with relatively uniform temperatures: a hotter component at $kT=0.60\pm0.02$ keV and a colder one at $kT=0.21^{+0.03}_{-0.01}$ keV. The latter exhibits about five times higher emission measure on average. Furthermore, spectroscopic findings suggest sub-solar abundances ($Z=0.2\pm0.1 Z_\odot$), consistent with expectations for the Galactic halo, without clear evidence of α-element enhancement. In contrast, the North Polar Spur displays higher abundances ($Z>0.5 Z_\odot$), hinting at a potential distinct origin from the eRObub. The study also confirms the presence of a cool shell surrounding the northern eRObub with an apparent temperature of $kT\sim0.18$-$0.2$ keV under collisional ionization equilibrium assumptions. Additionally, while no significant difference in X-ray emission was observed in regions overlapping with the Fermi Bubbles, it was noted that both eRObub exhibit well-constrained horizontal sizes (semi-minor axis $\sim 6$ kpc). However, the vertical extents remain uncertain due to insensitivity of observed X-ray emission to bubble cap existence and location. In conclusion, this comprehensive analysis sheds light on the physical properties of the eROSITA bubbles and addresses questions regarding their geometry, emitting material characteristics, energetics required for inflation, and implications for related structures such as the North Polar Spur and Fermi Bubbles within our Galaxy's context.