# Lack of thermal energy in superbubbles: hint of cosmic rays?

## TL;DR

This study investigates how cosmic rays influence the thermal and X-ray properties of interstellar superbubbles, revealing that even small CR energy fractions can significantly lower bubble temperatures and potentially explain observed temperature discrepancies.

## Contribution

The paper introduces a combined analytic and simulation approach to demonstrate the impact of cosmic rays on superbubble dynamics and thermal profiles, highlighting conditions for efficient CR energy transfer.

## Key findings

- CRs can significantly lower superbubble temperatures by a factor of 2-10.
- CR energy transfer is efficient when Mach number exceeds 12 and acceleration time is short.
- CR-influenced bubbles exhibit lower X-ray temperatures, addressing the temperature discrepancy problem.

## Abstract

Using analytic methods and $1$-D two-fluid simulations, we study the effect of cosmic rays (CRs) on the dynamics of interstellar superbubbles (ISBs) driven by multiple supernovae (SNe)/stellar winds in OB associations. In addition to CR advection and diffusion, our models include thermal conduction and radiative cooling. We find that CR injection at the reverse shock or within a central wind-driving region can affect the thermal profiles of ISBs and hence their X-ray properties. Even if a small fraction ($10-20\%$) of the total mechanical power is injected into CRs, a significant fraction of the ram pressure at the reverse shock can be transferred to CRs. The energy transfer becomes efficient if (1) the reverse shock gas Mach number exceeds a critical value ($M_{\rm th}\gtrsim 12$) and (2) the CR acceleration time scale $\tau_{\rm acc}\sim \kappa_{\rm cr}/v^2$ is shorter than the dynamical time, where $\kappa_{\rm cr}$ is CR diffusion constant and $v$ is the upstream velocity. We show that CR affected bubbles can exhibit a volume averaged hot gas temperature $1-5\times10^{6}$ K, lower by a factor of $2-10$ than without CRs. Thus CRs can potentially solve the long-standing problem of the observed low ISB temperatures.