Galactic Sneeze: Bubbles from a Black Hole express
In 2020, the eROSITA X-ray telescope discovered a pair of enormous bubbles looming above and below the disk of our Galaxy. These clouds of gas heated to millions of degrees are the aftermath of a colossal energy release from the center.
Astronomers examined the southeastern edge of one bubble, where matter slams into the sparse background. By breaking X-ray light into colors with spectroscopy, they found the shock wave racing at 700 km/s, surrounded by a near-vacuum: the gas is mostly hydrogen, with less than 10% of the heavy elements found in the Sun. Although the gas is scorching, an astronaut inside wouldn't feel the heat — the density is trillions of times lower than air, and particles zip by hardly colliding. This shock wave acts as a natural particle accelerator: it revs up particles to such speeds that they become cosmic rays bombarding Earth. Understanding these bubbles links the history of the galaxy to the mystery of the most energetic particles in the universe.
Original abstract
as written by the authors arXiv:2603.20740
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.
The shock wave's speed (700 km/s) is such that it could travel from Earth to the Moon in 9 minutes.