INTEGRAL constraints on the Galactic hard X-ray background from the Milky Way anticenter

TL;DR

This study uses INTEGRAL IBIS data to set upper limits on the Galactic hard X-ray background near the Galactic Anticenter, constraining the origin of the Galactic ridge X-ray emission.

Contribution

It provides the first upper limit on hard X-ray emission in the Galactic Anticenter, challenging the cosmic-ray origin hypothesis of the Galactic ridge X-ray emission.

Findings

Upper limit exceeds expected stellar-origin GRXE by an order of magnitude.

Measured upper limit is inconsistent with cosmic-ray induced GRXE.

Results support a stellar origin for the Galactic ridge X-ray emission.

Abstract

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.