Unresolved Soft X-Ray Emission from the Galactic Disk
Abstract
The soft X-ray sky below 1 keV is spatially smooth after subtracting the local structure. In high galactic latitude regions, emissions from faint unresolved extragalactic point sources, i.e., the Cosmic X-ray Background (CXB), are responsible for ~40 % of the soft X-ray emission in the ROSAT R45 band 0.44-1.0 keV) (McCammom et al. 2002). Since the interstellar X-ray absorption column density is high enough 10^22 cm^-2) to block the extragalactic X-ray photons below 1 keV totally in the galactic midplane, it is naturally expected that the X-ray surface brightness in the R45 band decreases by ~40 %. However, the R45 band surface brightness reduces only by ~20 % or less from high galactic latitude regions to the midplane regions. This issue has been known as the “M band problem” (McCammom & Sanders 1990; Cox 2005). M band itself is a name of the energy band which is almost the same as the ROSAT R45 band. Masui et al. (2009) discovered the existence of an unresolved emission in its energy spectrum from a region located in the midplane for the first time and this excess emission is considered to be partly filling the decrease of the extragalactic component in the midplane. Spectral analysis revealed that this excess emission is represented well by a thin thermal emission with a temperature of abount 0.8 keV. If this excess emission is an answer for the M band problem, this should be observed in other midplane regions. We searched for this excess emission using archival data of Suzaku which has the lowest and stable background and therefore is optimum for faint soft X-ray emissions. Systematic analysis for over 100 observations with the galactic latitude of |b| < 5 was conducted and finally we detected excess emissions successfully from different 11 regions in the midplane (Mitsuishi et al. in prep). Temperatures ranges from 0.6 keV to 1.3 keV with different intensities. Our results suggests that these excess emissions are distributed in the whole galactic disk region such as CXB and the Galactic ridge X-ray emission. The origin for these excess emissions is also discussed in this conference.
- Publication:
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AAS/High Energy Astrophysics Division #13
- Pub Date:
- April 2013
- Bibcode:
- 2013HEAD...1330103M