[Paper Review] Latest results on Galactic sources as seen in VHE gamma-rays

M. Renaud|ArXiv.org|May 8, 2009

Astrophysics and Cosmic Phenomena7 citations

TL;DR

This paper reviews the latest Very High Energy (VHE) gamma-ray observations of Galactic sources by the H.E.S.S. experiment, focusing on shell-type supernova remnants and pulsar wind nebulae. It investigates the nature of VHE emission—leptonic vs. hadronic—and finds that unresolved sources explain a significant fraction of the diffuse Galactic gamma-ray background, with next-generation telescopes expected to detect ~500 such sources.

ABSTRACT

As of early 2009, latest results on Galactic sources (mainly shell-type and plerionic supernova remnants), as observed in the very-high-energy gamma-ray domain, are reviewed. A particular attention is given to those obtained with the H.E.S.S experiment during its Galactic Plane Survey which now covers the inner part of the Milky Way. From the well identified gamma-ray sources to those without any obvious counterpart and the putative Galactic diffuse emission, this observational window fully deserves to be celebrated during this International Year of Astronomy, as a new mean to image the Galaxy and reveal sites of particle acceleration, potentially at the origin of Galactic cosmic rays.

Motivation & Objective

• To review the latest VHE gamma-ray observations of Galactic sources from the H.E.S.S. Galactic Plane Survey.
• To investigate the origin of VHE gamma-ray emission in shell-type supernova remnants and pulsar wind nebulae, distinguishing between leptonic and hadronic emission mechanisms.
• To assess the contribution of unresolved or 'dark' sources to the putative Galactic diffuse VHE gamma-ray emission.
• To evaluate the implications of the log N(>S)–log S distribution for source population models and future detection prospects.
• To project the expected number of detectable VHE Galactic sources with upcoming instruments like CTA and AGIS.

Proposed method

• Utilized data from the H.E.S.S. Galactic Plane Survey covering the inner Galaxy (ℓ ∈ [-90°, 60°], |b| < 3°), with high sensitivity and angular resolution.
• Analyzed broadband spectral energy distributions from radio to VHE gamma-rays to constrain particle acceleration mechanisms.
• Applied Monte Carlo simulations to account for statistical and systematic errors in the log N(>S)–log S distribution.
• Compared H.E.S.S. data with previous Milagro measurements to assess the contribution of resolved sources to the diffuse emission.
• Modeled electron and proton injection spectra, using EGRET data to constrain inverse-Compton emission and test for deviations from standard π⁰-decay spectra.
• Evaluated source morphology and extent to distinguish between resolved sources and diffuse emission, especially in regions with non-uniform survey coverage.

Experimental results

Research questions

• RQ1 What is the dominant emission mechanism (leptonic vs. hadronic) in VHE gamma-ray emission from shell-type supernova remnants?
• RQ2 To what extent do unresolved or 'dark' VHE sources contribute to the Galactic diffuse gamma-ray emission?
• RQ3 How does the spatial distribution of VHE sources (uniform disk vs. spiral arm-enhanced) inform the origin of Galactic cosmic rays?
• RQ4 What fraction of the VHE diffuse emission can be attributed to unresolved point-like or extended sources?
• RQ5 How many new VHE Galactic sources are expected to be detected by next-generation IACTs like CTA and AGIS?

Key findings

• Five shell-type supernova remnants—Cas A, RX J1713.7-3946, RX J0852-4622, RCW 86, and SN 1006—have been detected in VHE gamma-rays, with morphologies matching X-ray shells.
• The correlation between X-ray and VHE gamma-ray emission supports a leptonic origin, but requires implausibly low magnetic fields (~few tens of μG) in simple models, suggesting potential hadronic contributions.
• H.E.S.S. resolved five VHE sources in the region previously detected as diffuse emission by Milagro, explaining roughly 20% of the diffuse flux.
• First studies of the H.E.S.S. second survey catalogue suggest that at least 10% of the VHE Galactic diffuse emission arises from unresolved sources.
• The log N(>S)–log S distribution shows a slope of ~−0.6 at high fluxes, indicating a spiral-arm-enhanced source distribution, consistent with young stellar populations.
• Extrapolation to a sensitivity of 1 mCrab—achievable with CTA and AGIS—projects the detection of approximately 500 VHE Galactic sources.