INTEGRAL SPI All-Sky View in Soft Gamma Rays: Study of Point Source and Galactic Diffuse Emissions

TL;DR

This study analyzes 4 years of INTEGRAL SPI data to map Galactic diffuse emission and point sources in soft gamma rays, revealing a stellar origin below 50 keV and characterizing the Galactic Ridge emission spectrum.

Contribution

It provides a comprehensive all-sky census of gamma-ray sources and diffuse emission, with detailed spectral and spatial analysis, including positron annihilation parameters.

Findings

Resolved point source emission accounts for over 68-91% of total emission depending on energy.

Galactic Ridge emission follows a power law with photon index ~1.55 above 50 keV.

Diffuse annihilation emission is detected over a large region, indicating complex spatial distribution.

Abstract

We have processed the data accumulated with INTEGRAL SPI during 4 years (~ 51 Ms) to study the Galactic ``diffuse'' emission morphology in the 20 keV to 8 MeV energy range. To achieve this objective, we have derived simultaneously an all-sky census of emitting sources and images of the Galactic Ridge (GR) emission. In the central radian, the resolved point source emission amounts to 88%, 91% and 68% of the total emission in the 25-50, 50-100 and 100-300 keV domains respectively. We have compared the GR emission spatial distribution to those obtained from CO and NIR maps, and quantified our results through latitude and longitude profiles. Below 50 keV, the SPI data are better traced by the latter, supporting a stellar origin for this emission. Furthermore, we found that the GR emission spectrum follows a power law with a photon index ~ 1.55 above 50 keV while an additional component is required below this energy. This component shows a cutoff around 30 keV, reinforcing a stellar origin, as proposed by Krivonos et al. (2007). The annihilation diffuse emission component is extracted simultaneously, leading to the determination of the related parameters (positronium flux and fraction). A specific discussion is devoted to the annihilation line distribution since a significant emission is detected over a region as large as ~80 degrees by ~10 degrees potentially associated with the disk or halo surrounding the central regions of our Galaxy.