Feedback by massive stars and the emergence of superbubbles II. X-ray properties

TL;DR

This study uses 3D hydrodynamic simulations to analyze the X-ray properties of superbubbles created by massive stars, revealing that mixing and instabilities significantly influence their X-ray emission and luminosity evolution.

Contribution

It provides detailed simulation-based insights into the X-ray spectral evolution and luminosity dynamics of superbubbles, highlighting the effects of supernovae and hydrodynamic instabilities.

Findings

X-ray spectra are predominantly soft, below 1 keV.

Supernovae increase X-ray luminosity by 1-2 orders of magnitude temporarily.

Simulated luminosities are about ten times lower than observed in nearby galaxies.

Abstract

In a previous paper we investigated the energy transfer of massive stars to the interstellar medium as a function of time and the geometrical configuration of three massive stars via 3D-mesh-refining hydrodynamics simulations, following the complete evolution of the massive stars and their supernovae except non-thermal processes . We analysed our ISM simulation results with the help of spectra for plasma temperatures between 0.1 and 10 keV and computed the spectral evolution and the spatio-temporal distribution of the hot gas. Results. Despite significant input of high temperature gas from supernovae and fast stellar winds, the resulting thermal X-ray spectra are generally very soft, with most of the emission well below 1 keV. We show that this is due to mixing triggered by resolved hydrodynamic instabilities. Supernovae enhance the X-ray luminosity of a superbubble by 1-2 orders of magnitude for a time span of about 0.1 Myr; longer if a supernova occurs in a larger superbubble and shorter in higher energy bands. Peak superbubble luminosities of the order of 10^{36} erg/s are reproduced well. The strong decay of the X-ray luminosity is due to bubble expansion, hydrodynamic instabilities related to the acceleration of the superbubble's shell thanks to the sudden energy input, and subsequent mixing. We also find global oscillations of our simulated superbubbles, which produce spatial variations of the X-ray spectrum, similar to what we see in the Orion-Eridanus cavity. We calculated the fraction of energy emitted in X-rays and find that with a value of a few times 10^{-4}, it is about a factor of ten below the measurements for nearby galaxies.