Emission from hadronic and leptonic processes in galactic jet-driven bubbles

TL;DR

This study models multiwavelength emission from galactic bubbles driven by explosive outbursts, comparing hadronic, leptonic, and hybrid cosmic ray compositions, revealing differences in brightness across various energy bands.

Contribution

It provides the first detailed 3D MHD simulations of galactic bubbles with comprehensive spectral analysis for different cosmic ray compositions.

Findings

Hadronic and hybrid bubbles are brighter in X-rays than leptonic ones.

Hadronic systems produce more high-energy gamma rays due to pion decay.

Emission spectra are broadly similar across compositions, with key differences in brightness at specific energies.

Abstract

We investigate the multiwavelength emission from hadronic and leptonic cosmic rays (CRs) in bubbles around galaxies, analogous to the Fermi bubbles of the Milky Way. The bubbles are modeled using 3D magnetohydrodynamical (MHD) simulations, and are driven by a 0.3 Myr intense explosive outburst from the nucleus of Milky Way-like galaxies. We compute their non-thermal emission properties at different stages throughout their evolution, up to 7 Myr, by post-processing the simulations. We compare the spectral and spatial signatures of bubbles with hadronic, leptonic and hybrid hadro-leptonic CR compositions. These each show broadly similar emission spectra, comprised of radio synchrotron, inverse Compton and non-thermal bremsstrahlung components. However, hadronic and hybrid bubbles were found to be brighter than leptonic bubbles in X-rays, and marginally less bright at radio frequencies, and in $\gamma$-rays between $\sim$ 0.1 and a few 10s of GeV, with a large part of their emission being driven by secondary electrons formed in hadronic interactions. Hadronic systems were also found to be slightly brighter in high-energy $\gamma$-rays than their leptonic counterparts, owing to the $\pi^0$ decay emission that dominates their emission between energies of 100s of GeV and a few TeV.