The Impact of Cosmic Ray Transport on the $\gamma$-Ray Luminosity of Diffuse Gas

Abstract

Observations of $\gamma$-rays from diffuse gas provide the opportunity to study the distribution of high energy particles in different astrophysical environments. In the circumgalactic medium (CGM) and the intracluster medium (ICM), it is expected that relativistic cosmic rays collide with thermal particles and produce $\gamma$-rays through pion decay. The $\gamma$-ray luminosity of a plasma depends on where cosmic rays are: if they are in denser gas, they produce more $\gamma$-rays. In this work, we study how different cosmic-ray transport mechanisms impact the $\gamma$-ray luminosity of a turbulent, multiphase medium formed from an initially diffuse medium. Two quantities set the luminosity: the average cosmic-ray energy density and the correlation of cosmic-ray energy and gas density. Overall, cosmic rays must escape cold dense regions in order to produce less $\gamma$-ray emission and be consistent with observations. Our simulations with fast transport mechanisms (either diffusion or streaming) are degenerate: they each produce a lower $\gamma$-ray luminosity than slow transport simulations by two orders of magnitude. This result means that fast transport (particularly in dense clumps) is necessary for simulations to agree with the dearth of observations of $\gamma$-ray emission from diffuse gas like the CGM and ICM. We also show the significant difference in luminosity is the result of cosmic-ray reacceleration. This reacceleration is different from the turbulent reacceleration described by Ptuskin (1988). Instead, condensing, cold clouds drive a significant increase in the average cosmic-ray energy and, as a result, the $\gamma$-ray luminosity.