Imaging the Gamma-Ray Sky with SPI aboard INTEGRAL

TL;DR

This paper demonstrates the first simultaneous imaging of diffuse and point-like gamma-ray emission using SPI on INTEGRAL, employing a new deconvolution algorithm to produce detailed sky maps and analyze emission components.

Contribution

It introduces the adaptation of the MREM deconvolution algorithm for SPI data, enabling detailed imaging of gamma-ray sky emission and revealing unresolved or diffuse components.

Findings

Detection of 26Al gamma-ray line emission from Galactic center and Cygnus region

Transition observed from point-source to diffuse emission dominance across energy bands

Evidence for unresolved or diffuse gamma-ray emission above 100 keV

Abstract

The spectrometer SPI on INTEGRAL allows for the first time simultaneous imaging of diffuse and point-like emission in the hard X-ray and soft gamma-ray regime. To fully exploit the capabilities of the instrument, we implemented the MREM image deconvolution algorithm, initially developed for COMPTEL data analysis, to SPI data analysis. We present the performances of the algorithm by means of simulations and apply it to data accumulated during the first 2 mission years of INTEGRAL. Skymaps are presented for the 1809 keV gamma-ray line, attributed to the radioactive decay of 26Al, and for continuum energy bands, covering the range 20 keV - 3 MeV. The 1809 keV map indicates that emission is clearly detected by SPI from the inner Galactic radian and from the Cygnus region. The continuum maps reveal the transition between a point-source dominated hard X-ray sky to a diffuse emission dominated soft gamma-ray sky. From the skymaps, we extract a Galactic ridge emission spectrum that matches well SPI results obtained by model fitting. By comparing our spectrum with the cumulative flux measured by IBIS from point sources, we find indications for the existence of an unresolved or diffuse emission component above ~100 keV.