The Origin of the X-ray Emission from the High-velocity Cloud MS30.7-81.4-118

TL;DR

This study investigates the origin of X-ray emission from the high-velocity cloud MS30.7, analyzing XMM-Newton data and testing various physical models, suggesting magnetic reconnection as a plausible source of the observed X-ray enhancement.

Contribution

The paper provides the first detailed analysis of the X-ray emission from MS30.7 and evaluates multiple physical mechanisms, proposing magnetic reconnection as a likely explanation.

Findings

X-ray enhancement is ~100 pc across, concentrated north and west of the densest cloud regions.

Single-temperature plasma model fits but underpredicts emission around 1 keV, indicating hotter plasma or recombination effects.

Magnetic reconnection is a plausible mechanism, with potential to constrain magnetic fields near the Magellanic Stream.

Abstract

A soft X-ray enhancement has recently been reported toward the high-velocity cloud MS30.7-81.4-118 (MS30.7), a constituent of the Magellanic Stream. In order to investigate the origin of this enhancement, we have analyzed two overlapping XMM-Newton observations of this cloud. We find that the X-ray enhancement is $\sim$6' or $\sim$100 pc across, and is concentrated to the north and west of the densest part of the cloud. We modeled the X-ray enhancement with a variety of spectral models. A single-temperature equilibrium plasma model yields a temperature of $(3.69^{+0.47}_{-0.44}) \times 10^6$ K and a 0.4-2.0 keV luminosity of $7.9 \times 10^{33}$ erg s$^{-1}$. However, this model underpredicts the on-enhancement emission around 1 keV, which may indicate the additional presence of hotter plasma ($T \gtrsim 10^7$ K), or that recombination emission is important. We examined several different physical models for the origin of the X-ray enhancement. We find that turbulent mixing of cold cloud material with hot ambient material, compression or shock heating of a hot ambient medium, and charge exchange reactions between cloud atoms and ions in a hot ambient medium all lead to emission that is too faint. In addition, shock heating in a cool or warm medium leads to emission that is too soft (for reasonable cloud speeds). We find that magnetic reconnection could plausibly power the observed X-ray emission, but resistive magnetohydrodynamical simulations are needed to test this hypothesis. If magnetic reconnection is responsible for the X-ray enhancement, the observed spectral properties could potentially constrain the magnetic field in the vicinity of the Magellanic Stream.