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

TL;DR

This study uses eROSITA, radio, and gamma-ray data to detect high Mach number shocks at the tips of both eROSITA bubbles, revealing their evolved nature and connection to past energetic outbursts from the Galactic center.

Contribution

It provides the first robust detection of shocks with Mach numbers greater than 3 at the tips of both eROSITA bubbles using multi-wavelength data analysis.

Findings

Detected >5σ signals at bubble tips in radio and gamma-rays.

Measured high Mach number shocks (3-5) at bubble edges.

Southern bubble is fainter and propagates into less dense medium.

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 $\gamma$-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 ($>5\sigma$) both bubble tips in both radio and $\gamma$-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.