Gamma-ray continuum emission from the inner Galactic region as observed with INTEGRAL/SPI

TL;DR

This study uses 16 Ms of INTEGRAL/SPI data to disentangle diffuse Galactic ridge emission from point sources in the 18 keV–1 MeV band. By combining SPI imaging with an IBIS/ISGRI source catalog and HI/CO sky templates, the authors identify three main spectral components: a diffuse power-law with index $1.7$, a positron annihilation component including the 511 keV line and positronium continuum, and a second component from detectable sources. They find the positron-annihilation emission is strongly concentrated in the inner Galaxy ($|l|<10$), while the hard power-law shows a broader longitude distribution and is not easily explained by interstellar processes, suggesting unresolved sources such as AXPs may dominate above ~100 keV. The results indicate that detectable sources account for most of the $20-60$ keV emission but their influence wanes with energy, and a broadband keV–TeV spectrum supports a population-synthesis or alternative-diffuse-mechanism interpretation for the hard tail; a quantitative population analysis is proposed for future work.

Abstract

The diffuse continuum emission from the Galactic plane in the energy range 18-1000 keV has been studied using 16 Ms of data from the SPI instrument on INTEGRAL. With such an exposure we can exploit the imaging properties of SPI to achieve a good separation of point sources from the various diffuse components. Using a candidate-source catalogue derived with IBIS on INTEGRAL and a number of sky distribution models we obtained spectra resolved in Galactic longitude. We can identify spectral components of a diffuse continuum of power law shape with index about 1.7, a positron annihilation component with a continuum from positronium and the line at 511 keV, and a second, roughly power-law component from detected point sources. Our analysis confirms the concentration of positron annihilation emission in the inner region (|l|<10), the disk (10<|l|<30) being at least a factor 7 weaker in this emission. The power-law component in contrast drops by only a factor 2, showing a quite different longitude distribution and spatial origin. Detectable sources constitute about 90% of the total Galactic emission between 20 and 60 keV, but have a steeper spectrum than the diffuse emission, their contribution to the total emission dropping rapidly to a small fraction at higher energies. In the SPI energy range the flux is lower than found by OSSE, probably due to the more complete accounting for sources by SPI. The power-law emission is difficult to explain as of interstellar origin, inverse Compton giving at most 10%, and instead a population of unresolved point sources is proposed as a possible origin, AXPs with their spectra hardening above 100 keV being plausible candidates. We present a broadband spectrum of the emission.

Figures (8)

• Figure 1: Sky distribution of exposure, at 1 MeV. Galactic coordinates centred on $l=b=0$. Units:$10^7$ cm$^2$ s. Green: 0.5--1.4, yellow: 1.4--4, orange: 4--13, red: 13--35
• Figure 2: Time dependence of background for 48 - 58 keV determined from the fitting procedure. Relative units. Time axis in days since 1 Jan 2000
• Figure 3: Spectrum of diffuse emission summed over components (blue), and sum of all sources (red), for the inner region of the Galaxy ($330^\circ<l<30^\circ, |b|<10^\circ$). 3$\sigma$ source threshold.
• Figure 4: Spectrum of diffuse emission summed over model components, for the inner region of the Galaxy ($330^\circ<l<30^\circ, |b|<10^\circ$). 3$\sigma$ source threshold. Also shown is the fit of SPI data to power-law plus positronium plus line, using the full energy response. Note that the fit lies below the data points because of the non-diagonal energy reponse of SPI. Magenta point: diffuse emission measured by IBIS/ISGRI terrier04.
• Figure 5: Spectrum of diffuse emission summed over model components, for the central part of the Galaxy ($350^\circ<l<10^\circ, |b|<10^\circ$). See Fig 4 caption for details of fit.