1 Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85740 Garching bei München, Germany
2 Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia
3 Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
4 Astronomy Division, Department of Physics, 90014 University of Oulu, Finland
Received: 8 September 2011
Accepted: 16 November 2011
We present results of a study of the Galactic ridge X-ray emission (GRXE) in hard X-rays with the IBIS telescope onboard INTEGRAL in the region near the Galactic anticenter (GA) at l = 155°. We assumed a conservative 2σ upper limit on the flux from the GA in the 25–60 keV energy band of 1.25 × 10-10 erg s-1 cm-2 (12.8 mCrab) per IBIS field of view, or 6.6 × 10-12 erg s-1 cm-2 (0.7 mCrab) per degree longitude in the 135° < l < 175° region. This upper limit exceeds the expected GRXE intensity in the GA direction by an order of magnitude, given the near-infrared (NIR) surface brightness of the Milky Way in this region and the standard hard X-ray-to-NIR intensity ratio for the GRXE, assuming stellar origin. Based on the CGRO/EGRET surface brightness of the Galaxy above 100 MeV as a tracer of the cosmic-ray (CR) induced gamma-ray background, the expected GRXE flux in GA exceeds the measured 2σ upper limit by a factor of 4. Therefore, the non-detection of hard X-ray emission from the GA does not contradict the stellar nature of the GRXE, but is inconsistent with CR origin.
Key words: Galaxy: structure / X-rays: diffuse background
Based on observations with INTEGRAL, an ESA project with instruments and science data centre funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Switzerland, Spain), Czech Republic and Poland, and with the participation of Russia and the USA.
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© ESO, 2012