2009ApJ...707.1361T

We present the first X-ray analysis of the diffuse hot ionized gas and the point sources in IC131, after NGC604 the second most X-ray luminous giant H II region (GHR) in M33. The X-ray emission is detected only in the south eastern part of IC131 (named IC131-se) and is limited to an elliptical region of ∼200 pc in extent. This region appears to be confined toward the west by a hemispherical shell of warm ionized gas and only fills about half that volume. Although the corresponding X-ray spectrum has 1215 counts, it cannot conclusively be told whether the extended X-ray emission is thermal, non-thermal, or a combination of both. A thermal plasma model of kT_e= 4.3 keV or a single power law of Γ ≃ 2.1 fit the spectrum equally well. If the spectrum is purely thermal (non-thermal), the total unabsorbed X-ray luminosity in the 0.35-8 keV energy band amounts to L_X= 6.8(8.7)x10³⁵ erg/s. Among other known H II regions IC131-se seems to be extreme regarding the combination of its large extent of the X-ray plasma, the lack of massive O stars, its unusually high electron temperature (if thermal), and the large fraction of L_X emitted above 2 keV (∼40%-53%). A thermal plasma of ∼4 keV poses serious challenges to theoretical models, as it is not clear how high electron temperatures can be produced in H II regions in view of mass-proportional and collisionless heating. If the gas is non-thermal or has non-thermal contributions, synchrotron emission would clearly dominate over inverse Compton emission. It is not clear if the same mechanisms which create non-thermal X-rays or accelerate cosmic rays in supernova remnants can be applied to much larger scales of 200 pc. In both cases the existing theoretical models for GHRs and superbubbles do not explain the hardness and extent of the X-ray emission in IC131-se. We also detect a variable source candidate in IC131. It seems that this object (CXO J013315.10+304453.0) is a high mass X-ray binary whose optical counterpart is a B2-type star with a mass of ∼9 M_☉.