Abstract
Context. The High Energy Stereoscopic System Galactic plane survey (HGPS) is to date the most comprehensive census of Galactic γ-ray sources at very high energies (VHE; 100 GeV ≤ E ≤ 100 TeV). As a consequence of the limited sensitivity of this survey, the 78 detected γ-ray sources comprise only a small and biased subsample of the overall population. The larger part consists of currently unresolved sources, which contribute to large-scale diffuse emission to a still uncertain amount. Aims. We study the VHE γ-ray source population in the Milky Way. For this purpose population-synthesis models are derived based on the distributions of source positions, extents, and luminosities. Methods. Several azimuth-symmetric and spiral-arm models are compared for spatial source distribution. The luminosity and radius function of the population are derived from the source properties of the HGPS data set and are corrected for the sensitivity bias of the HGPS. Based on these models, VHE source populations are simulated and the subsets of sources detectable according to the HGPS are compared with HGPS sources. Results. The power-law indices of luminosity and radius functions are determined to range between −1.6 and −1.9 for luminosity and −1.1 and −1.6 for radius. A two-arm spiral structure with central bar is discarded as spatial distribution of VHE sources, while azimuth-symmetric distributions and a distribution following a four-arm spiral structure without bar describe the HGPS data reasonably well. The total number of Galactic VHE sources is predicted to be in the range from 800 to 7000 with a total luminosity and flux of (1.6 − 6.3) × 1036 ph s−1 and (3 − 15) × 10−10 ph cm−2 s−1, respectively. Conclusions. Depending on the model, the HGPS sample accounts for (68 − 87)% of the emission of the population in the scanned region. This suggests that unresolved sources represent a critical component of the diffuse emission measurable in the HGPS. With the foreseen jump in sensitivity of the Cherenkov Telescope Array, the number of detectable sources is predicted to increase by a factor between 5 and 9.
Highlights
The past two decades have witnessed the birth and explosive development of teraelectronvolt astronomy
The larger part consists of currently unresolved sources, which contribute to large-scale diffuse emission to a still uncertain amount
This procedure allows the prediction of the number of sources detectable with future instruments and to estimate the amount of unresolved sources that contribute to the diffuse emission measurements
Summary
The past two decades have witnessed the birth and explosive development of teraelectronvolt astronomy. A characterisation of the overall population of sources can be achieved by population synthesis with the simulation of synthetic source samples and comparison with observations in the range of detectability of the data set This procedure is customarily followed for the study of object classes such as pulsars (Gonthier et al 2018). Rather than aiming to derive properties of a specific class of objects, this approach characterises the overall population of sources at a certain wavelength This procedure allows the prediction of the number of sources detectable with future instruments (e.g. the Cherenkov Telescope Array CTA; Funk et al 2008) and to estimate the amount of unresolved sources that contribute to the diffuse emission measurements. An assessment of the entire Galactic source population can disentangle the two components of unresolved γ-ray-source emission and diffuse emission from propagating cosmic rays and allow for the study of cosmic-ray propagation properties in the H.E.S.S. and HAWC data sets
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