After observing IC131, the second most X-ray luminous giant HII region (GHR) in M33 with the Chandra X-ray observatory for more than 420ks, it became pretty soon obvious, that this object is very unusual compared to other regions of similar size.
X-ray emission is detected in IC131 only in the south eastern part of IC131, which we named IC131-se, and is limited to an elliptical region of about 220 pc in extent. This region appears to be confined towards the west by a hemispherical shell of warm ionized gas and only fills about half that volume (see Fig. 1).
Figure 1: (a) Multicolor image, showing Hα in red, [OIII]5007Å in green, and X-rays (0.35-8.0\,keV) in blue. (b): High resolution (2") Chandra X-ray images (red: 0.35-1.1\,keV, green: 1.1-2.6\,keV, blue: 2.6-8.0~keV). (c): Optical false color composite consisting of MMT broadband r, g, and u imagery. Positions of three spectroscopically classified WR stars (see labels for their spectral type) and 21 unclassified stars within the bubble (green circles) are taken from
Massey et al. 2006. A zoom into the region surrounded by the white dashed box is shown in panel (d) and reveals a possible optical counterpart to the bright X-ray point source marked by a red cross in panels (a) and (b). Contours in panels a)-c) represent IR emission at 3.6μm (red, 0.42-1.50mJy/sr) and 24μm (white, 31.9-39mJy/sr).
Although the X-ray spectrum shown in
Fig. 2 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=4.3keV 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 0.35-8keV energy band amounts to L
X=6.8 (8.7)x10
35erg/s.
Figure 2:
In the left panel the X-ray spectrum of the diffuse emission is fitted with a soft (dotted line) and hard (dashed line) thermal plasma model, whereas the right panel shows the best-fit of a non-thermal power law model. Based on the fit statistics and the derived parameters both models provide an equally good fit to the data.
Among other known HII regions IC131-se is extreme with respect to 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 2keV (~40%-53%).
A thermal plasma of ~4keV poses serious challenges to theoretical models, as it is not clear how high electron temperatures can be produced in HII regions in view of mass-proportional and collisionless heating. If the gas is non-thermal or has non-thermal contributions, it is not clear if the same mechanisms which create non-thermal X-rays or accelerate CRs in SNRs can be applied to much larger scales of 200pc. 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.
The point source presented in
Figure 1d shows signs of variability. It appears that this object (CXOJ013315.10+304453.0) is a high mass X-ray binary whose optical counter part is a B2-type star with a mass of ~9M
☉.
The full details of this study can be found in
Tüllmann et al. 2009 (ApJ 707, 1361).