The properties of the Galactic hard X-ray and soft γ-ray background based on 20 years of INTEGRAL/IBIS observations

Abstract

We present results of a study of the Galactic hard X-ray and soft γ-ray background emission performed with the IBIS telescope aboard the INTEGRAL observatory using data obtained over more than 20 years of operations. The study of the Galactic background at energies between 10 keV and a few MeV is problematic due to the contribution of point sources, high instrumental background and large-scale extent of the emission, which leads to the need of utilizing complex model-dependent methods. Using the unique properties of the IBIS coded-mask telescope, we developed a model-independent approach to study diffuse continuum emission near the Galactic plane in the 25−60, 60−80, and 80−200 keV bands. The comparison of the 25−60 keV longitude profile with the near infrared intensity shows excellent agreement, confirming the stellar origin of the Galactic Ridge X-ray Emission (GRXE). The Galactic X-ray background is significantly detected from the direction of the Galactic bulge up to 200 keV. We built broad-band spectra of the Galactic background in three large regions, the Galactic bulge and two spiral arms at $l\approx \pm {20}^{\circ }$. The spectral analysis reveals two distinct components with a minimum at about 80 keV. The low-energy (≲60 keV) component, associated with the GRXE, is consistent with a one-dimensional accretion flow model of intermediate polars with an average white dwarf mass of about 0.7 $M_{\odot }$. The high-energy part of the spectrum, dominating above ∼60 keV and attributed to the γ-ray background, is consistent with a power-law model with photon index $\Gamma =1.55$. The total 30−80 keV flux budget of $1.5\times {10}^{-9}$ erg s⁻¹ cm⁻² observed within the effective IBIS field of view (≈286 deg²) in the Galactic bulge region, consists of 2/3 of GRXE and 1/3 of γ-ray background. Finally, we provide the Python code of the IBIS/ISGRI background model, which can be used to measure the X-ray intensity of the Galactic background in different parts of the Milky Way.