INTEGRAL constraints on the Galactic hard X-ray background from the
  Milky Way anticenter
  We present results of a study of the Galactic ridge X-ray emission (GRXE) in
hard X-rays with the IBIS telescope on board INTEGRAL in the region near the
Galactic Anticenter (GA) at l=155 deg. We assumed a conservative 2 sigma upper
limit on the flux from the GA in the 25-60 keV energy band of 1.25E-10
erg/s/cm^2 (12.8 mCrab) per IBIS field of view, or 6.6E-12 erg/s/cm^2 (0.7
mCrab) per degree longitude in the 135 deg. < l < 175 deg. 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 2 sigma 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.
