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

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 $(3.69_{-0.44}^{+0.47})\times 10^6$ 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.

17 pages, 6 figures. Accepted for publication in the Astrophysical Journal

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• XMM-Newton Measurement of the Galactic Halo X-ray Emission using a Compact Shadowing Cloud
• Hydrodynamics of Clustered Clouds: Drafting, Survival, Condensation, and Ablation
• Gamma rays from reaccelerated cosmic rays in high velocity clouds colliding with the Galactic disk