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The impact of the eROSITA bubbles on Galactic cosmic-ray transport

Benedikt Schroer

Abstract

We propose that the observed spectral hardening in Galactic cosmic ray fluxes is governed by macroscopic Galactic outflows, such as the eROSITA bubbles, rather than microphysical variations in their scattering properties. Employing a phenomenological transport model, we show that an advective outflow boundary naturally reproduces the $300\,$GV hardening in secondary-to-primary ratios. Global fits to precision AMS-02 data yield an effective local inner halo boundary of $\sim 5\,$kpc and an outflow speed of $\sim 360\,$km/s, in striking agreement with independent multi-wavelength kinematic constraints of the eROSITA outflows. This interpretation provides a testable alternative to breaks in the effective diffusion coefficient, without increasing the number of free parameters.

The impact of the eROSITA bubbles on Galactic cosmic-ray transport

Abstract

We propose that the observed spectral hardening in Galactic cosmic ray fluxes is governed by macroscopic Galactic outflows, such as the eROSITA bubbles, rather than microphysical variations in their scattering properties. Employing a phenomenological transport model, we show that an advective outflow boundary naturally reproduces the GV hardening in secondary-to-primary ratios. Global fits to precision AMS-02 data yield an effective local inner halo boundary of kpc and an outflow speed of km/s, in striking agreement with independent multi-wavelength kinematic constraints of the eROSITA outflows. This interpretation provides a testable alternative to breaks in the effective diffusion coefficient, without increasing the number of free parameters.

Paper Structure

This paper contains 8 sections, 15 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Phenomenological Model
  3. Grammage
  4. Results and Discussion
  5. Justification of 1D Geometry and Model Uncertainties
  6. Conclusions
  7. Solution of the Spatial Part
  8. Analytical Scaling

Figures (3)

  • Figure 1: Boron-over-Carbon and Beryllium-over-Carbon ratios as a function of rigidity compared to AMS-02 data. Error bars represent statistical and systematic uncertainties summed in quadrature.
  • Figure 2: Left panel: Fluxes of H, He, C and O as a function of rigidity compared to AMS-02. Right panel: Same for DAMPE data as a function of total kinetic energy, divided by charge to align rigidity features. The model predictions and data of the H, He, C and O fluxes are multiplied with $f=1$, $6.3$, $233$ and $333$ respectively for illustration purposes in the left panel ($15$, $4.2$, $233$, $333$ in the right panel). Dashed lines indicate the effect of moving the softening to higher rigidities by dividing $D_2/(H_2-H_1)$ by a factor of two.
  • Figure 3: Sketch illustrating the escape of CRs from the Galaxy. The propagation volume is depicted as a cylinder around the Sun with height $9\,$kpc, following self-generated halo models chernyshov+22chernyshov+24 at $1\,$TV. CRs can escape along two surfaces: The intersection $A_b$ of the bubble with the propagation volume with $u_2$ and the top surface of the halo $A_h$ with $v_A$.