Southern eROSITA bubble as a forward shock and the low-metallicity CGM. South-east side story

Abstract

Unlike the complicated X-ray and radio structure observed in the North Polar Spur area, the South-Eastern part of the eROSITA bubbles can be reasonably well described as a propagating forward shock, plausibly created by the transient energy release at the Galactic Center. In this model, the physical radius of the bubble is $R_{\rm b}\sim 7-8\,{\rm kpc}$ and the age of the outburst is $t_{\rm age}\sim 5-8\,{\rm Myr}$. The visible segment of the shock front (located at a distance of $\sim 10-12\,{\rm kpc}$ above the Galactic Disk and at a similar distance from the Sun) is currently expanding with the velocity $\sim 700\,{\rm km\,s^{-1}}$ through the gas with density $n_e\sim 3\times 10^{-4}\,{\rm cm^{-3}}$, and the abundance of heavy elements in this gas is $Z\lesssim 0.1 \times Z_\odot$. Unlike constraints derived from the line-of-sight-integrated quantities, these are effectively in situ measurements of the circumgalactic medium (CGM) properties. Given the simplifying assumptions used in deriving the density, we assign a factor of 2 systematic uncertainty to the final estimate. An eventual decisive test for the shock properties can be provided by the velocity measurements of the X-ray-emitting gas with soft X-ray bolometers. The extended forward shock propagating through low-metallicity gas is a favorable site to accelerate very high-energy cosmic rays, which might contribute to the observed proton-rich galactic cosmic ray component at PeV energies.

Figures (15)

• Figure 1: X-ray image (0.7-1.05 keV band) of the eROSITA Bubbles in stereographic projection. The Southern bubble has a simpler morphology than the Northern one, resembling a shell characteristic of a propagating quasi-spherical shock. If this is the case, the spectrum could provide constraints on the shock velocity and the distance to the shock. The two panels on the right show the regions used for the extraction of spectra. The green and blue areas in the middle panel correspond to the "shell" and "background regions, respectively. They are also outlined in the left panel. The right panel shows the same regions after a narrow range of the galactic HI column density ($N_{\rm H}=10^{19}-2.5\times 10^{20}\,{\rm cm^{-2}}$) is considered. This is done in order to ensure that no bias is introduced by $N_{\rm H}$ variations between the shell and the background regions. Finally, the region outlined by the gray dashed line in the Northern part of the left panel is simply a reflection of the Southern shell to the upper hemisphere to indicate a possible (symmetric) location of the NE counterpart of the SE shell.
• Figure 2: Radial profiles across the Southern bubble in several energy bands. Statistical uncertainties, associated with photon counting noise (at the level of 1-3 percent of the flux), are not shown. The data are accumulated in the same wedge as shown in Fig. \ref{['f:sbub_image']} and using narrow radial bins. The left panel shows the observed profiles in narrow bands (the instrumental background is removed). The softest band (520-610 eV), dominated by OVII line, to which the widespread diffuse Galaxy emission makes an important contribution, shows a largely linear trend with radii. On the contrary, the harder bands, which are presumably dominated by the lines characteristic of hotter plasma (e.g., O VIII, Fe XVII, Ne IX, and Ne X), all show a non-monotonic behavior with a clear bump peaking at 26-28 degrees from the adopted center. The right panel shows the surface brightness profile in a broader band of 0.7-1.05 keV. The top blue histogram shows the observed profile, including the contribution of CXB (dashed horizontal line) and that of the Galaxy. The bottom histogram shows the same profile after subtraction of the background level, determined from the regions outside the Bubble. The peak in this band is $I_X\sim 3.5\times 10^{-5}\, {\rm counts\,s^{-1}\,arcmin^{-2}}$. For comparison, the red curve shows the expected surface brightness of a uniform spherical shell with the outer radius of $R_s=32$ degrees and the inner radius $0.8R_s$. The normalization of the curve was set to approximately match the observations. Two pairs of vertical dotted lines show radial ranges used for spectra extraction.
• Figure 3: Observed spectra of the bright shell and the background region (see the right panel in Fig. \ref{['f:rprof']}). The spectra are normalized per unit solid angle (per square arcminute) and per one (out of seven) eROSITA telescope units. The difference between these spectra is attributed to the shell emission.
• Figure 4: Background-subtracted spectrum of the bright shell region (see the right panel in Fig. \ref{['f:sbub_image']} for the definition of the spectra extraction regions). Left: Comparison of the spectrum with the best-fitting NPSHOCK with final temperature $0.63\,{\rm keV}$ and the inonization parameter $\tau\sim 2\times 10^{11}\,{\rm cm^{-3}s}$. This model performs significantly better than other simple models like APEC. Right: Comparison of the same spectrum with the predictions of the 1D hydrodynamic model. The red curve shows the predicted spectrum in the shell between $0.7R_s$ and $R_s$ for C2L6e41 model, with the downstream temperature and density motivated by the spectral analysis of the observed spectrum. The best-fitting normalization (shown in the plot) is a factor $\sim 1.6$ higher than the initial model.
• Figure 5: Sketch of the Southern eROSITA Bubble based on the morphological and spectral analysis. The observer's position is shown with a green box. The Southern Bubble outer boundary is approximated by a sphere/shell (red lines) with a radius of $\sim 7.4\,{\rm kpc}$. This sphere is plausibly a part of a more complicated structure sketched by the gray lines. The observed boundary of the bubble is at a distance of $\sim 12\,{\rm kpc}$. The outer boundary of the Fermi bubbles is shown with a green line. The blue lines show a simple reflection of the SB model to the North.