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

Context. The eROSITA bubbles (eRObub) were discovered in 2020 in the first SRG/eROSITA All-Sky Survey. They are some of the most extended structures in the X-ray sky. In projection, the eRObub encloses the γ-ray Fermi bubbles, and the northern eRObub appears to be bounded by the Loop I/North Polar Spur. Aims. Using eROSITA all-sky maps and spatially resolved spectra, we infer the three-dimensional structure and measure the hot gas properties of the eRObub. Methods. We fitted spectra binned to a constant signal-to-noise ratio and spectra with a high signal-to-noise ratio from custom regions to examine the gas properties in more detail. We fitted the morphology of eRObub using a parametrised geometrical model that describes a blast wave propagating from the Galactic Centre into an idealised Galactic halo. Results. The interior of the western eRObub is best characterised by two emission components with relatively uniform temperatures: a hotter component at kT = 0.60 ± 0.02 keV, and a colder component at kT = 0.21^+0.03 _−0.01 keV, where the emission measure of the colder component is higher by about five times on average. Our spectra suggest sub-solar abundances (Z = 0.2 ± 0.1 Z, consistent with expectations for the Galactic halo. We found no conclusive evidence of α-element enhancement. In contrast, the abundances of the North Polar Spur are higher (Z > 0.5 Z_sun ). At face value, this disfavours a common origin. We spectrally confirmed an apparent cool shell at kT ∼ 0.18–0.2 keV surrounding the northern eRObub, assuming collisional ionisation equilibrium. We found no noticeable difference in X-ray emission in regions overlapping with the Fermi bubbles. Our geometrical model suggests that the horizontal size of the two eRObub is well constrained (semi-minor axis of ∼ 6 kpc), but their vertical extent is uncertain because the observed X-ray emission is almost insensitive to the existence and location of a bubble cap. Additionally, a tilt (∼ 30◦) towards l ∼ 220◦ is needed to reproduce the projected image of the northern eRObub, whereas the southern bubble requires a slight tilt.