Mach $>3$ shocks at the tips of both eROSITA bubbles

TL;DR

The study addresses uncertainties about the southern eROSITA bubble and the shock strength in the RB system by stacking radio and γ-ray data along edges traced in the eROSITA map. Using an edge-aligned approach with the angular distance $\psi$ and foreground subtraction, the authors detect >5σ nonthermal emission at the bubble tips in both bands and infer radio spectral indices of $\alpha \approx 0.67$ in the north and $\alpha \approx 0.72$ in the south, corresponding to Mach numbers in the range $3$--$5$. The southern bubble appears an order of magnitude fainter in radio and propagates into a denser upstream, yet the edge spectra remain broadly similar to the northern RB. The results strongly suggest that the RBs are older, evolved counterparts of the Fermi bubbles, produced by an earlier collimated high-energy outburst from the Galactic center, supporting a common GC-driven outburst history.

Abstract

eROSITA substantiated earlier indications that Loop-I is the northern part of an extended bipolar Galactic-bubble structure, but the southern bubble was not established in nonthermal emission and the shock strength was not robustly measured in either bubble. After using eROSITA data to map the bubble edges, we analyzed edge-adjacent radio and $γ$-ray data to remove foregrounds, test if the southern bubble can be detected in nonthermal emission, and measure the corresponding high-latitude spectra of both bubbles. Data were stacked parallel to the eROSITA bubble edges traced by an edge detector, in the same method used previously to pick up weak signals in the smaller, nested Fermi bubbles; the detected brightness jumps were then used to measure the spectrum. We detect ($>5σ$) both bubble tips in both radio and $γ$-rays, and find a radio spectrum corresponding to high, Mach $3$-$5$ shocks. The southern bubble is fainter, by $\sim$an order of magnitude in radio, its edge propagating into a medium roughly half as dense. The results indicate that these eROSITA bubbles are older, evolved counterparts of the Fermi bubbles, arising from an earlier collimated high-energy outburst from the Galactic center.

Figures (3)

• Figure 1: eROSITA $0.6$--$1\hbox{keV}$ spherical-projection images of north (top row) and south (bottom) RBs, centered on Galactic coordinates $(l,b)=(0,\pm70^{\circ})$. Superimposed on right panels are detected edges based on short (dot-dashed cyan curve) and long (dot-long-dashed yellow) gradients, the edge section used for binning (dotted red), its equal-angle distance-transform bins (shaded regions), and the Galactic center (blue arrows).
• Figure 2: Brightness profiles across north (top panel) and south (middle) RB edges, and the inferred radio spectra in the near downstream (bottom), in the nominal analysis. The X-rays (circles) and radio (triangles of different colors and orientations designating different bands, see legend) profiles are shown (including error bars), along with nominal upstream ($\psi>0$, hatched) and downstream regions (shaded) and linear fits to the upstream radio brightness (dotted lines); edge detectors pick up the steepest X-ray gradient (vertical dot-dashed red lines). The spectrum is inferred from the excess brightness $I_\nu-F_\nu$ in the four radio channels (error bars with dotted lines to guide the eye) in the north (up red triangles) and south (down blue triangles); best linear fits are shown (solid curves with shaded $1\sigma$ uncertainty range).
• Figure 3: Same as Fig. \ref{['fig:Radio']} for the four $\gamma$-ray channels (see legends). Linear (dotted red lines) and piecewise linear (dashed) fits are demonstrated for the $I_\nu(\psi)$ profiles in the high, $3$--$10\hbox{GeV}$ energy channel. In the bottom panel, down triangles depicting the spectrum of the south bubbles are slightly shifted horizontally for visibility.