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