[Paper Review] Evidence for VHE emission from SNR G22.7-0.2 region with H.E.S.S

Hélène Laffon, B. Khélifi+8 more|arXiv (Cornell University)|Oct 31, 2011

Gamma-ray bursts and supernovae3 citations

TL;DR

H.E.S.S. observations provide evidence for very high energy (VHE) gamma-ray emission from the supernova remnant G22.7-0.2, detected as a 5.8σ excess source (HESS J1832-093) with a flux of (1.7 ± 0.9_stat ± 0.3_syst) × 10⁻¹³ cm⁻² s⁻¹ above 1 TeV. The emission is spatially coincident with molecular clouds in 13 CO data, suggesting a hadronic origin via pion decay, though a distant X-ray point source with high absorption may also contribute.

ABSTRACT

Observations of the supernova remnant G22.7-0.2 in the field of view of W41 with the H.E.S.S. telescope array have resulted in evidence for a very high energy (VHE) gamma-ray emission from the direction of the object. The emission region is spatially coincident with molecular clouds visible in 13CO data, suggesting a hadronic origin of the TeV emission, although other scenarios brought by X-ray observations with the XMM-Newton satellite are also discussed. The latest results obtained on this SNR with H.E.S.S. and associated multi-wavelength information are presented here.

Motivation & Objective

• To investigate the origin of very high energy (VHE) gamma-ray emission in the direction of the supernova remnant G22.7-0.2.
• To determine whether the VHE emission arises from hadronic interactions with molecular clouds or alternative mechanisms such as inverse Compton scattering or pulsar wind nebulae.
• To assess the multi-wavelength counterparts of the VHE source, particularly in X-rays and radio, to constrain the emission mechanism.
• To evaluate the energetic feasibility of the hadronic scenario using the supernova's kinetic energy budget.
• To determine the nature and distance of a nearby X-ray point source detected by XMM-Newton, which may be associated with the VHE emission.

Proposed method

• H.E.S.S. telescope array conducted 50 hours of live-time observations from 2004 to 2010, applying standard run selection and Hillas-based image analysis with a 80 photo-electron threshold.
• Background estimation used the ring-background model for source detection and the reflected background model for spectral analysis.
• Source position and intrinsic size were determined via two-dimensional Gaussian fitting to the excess map, with PSF correction applied.
• A circular region of 0.15° radius centered on the source was used to extract the energy spectrum, minimizing contamination from the nearby W41 source.
• The energy spectrum was fitted with a power-law model, and flux was derived above 1 TeV for comparison with the Crab nebula.
• XMM-Newton X-ray observations (7 ks usable) were analyzed to identify point-like sources, with spectral fitting using instrumental background models and column density corrections.

Experimental results

Research questions

• RQ1 Is the VHE gamma-ray emission from G22.7-0.2 spatially coincident with molecular clouds, supporting a hadronic origin?
• RQ2 What is the nature of the hard X-ray point source located 30” from the H.E.S.S. excess position, and could it be responsible for the VHE emission?
• RQ3 Can the observed VHE flux be explained by hadronic interactions between cosmic rays and molecular clouds, given the supernova's kinetic energy budget?
• RQ4 Why is there no Fermi-LAT source detected at the H.E.S.S. position, despite the proximity of 2FGL J1834.3-0848?
• RQ5 Does the high column density (NH ≈ 6.9 × 10²² cm⁻²) toward the X-ray source suggest it is distant, potentially obscuring a Pulsar Wind Nebula (PWN) or other extended emission?

Key findings

• A VHE gamma-ray source, HESS J1832-093, was detected at a significance of 5.8σ, with a best-fitted position at l = 22.48° ± 0.02°, b = -0.16° ± 0.02° in galactic coordinates.
• The source is slightly extended, with an intrinsic size of 0.06° ± 0.02°, consistent with the angular resolution of 0.11° at the source position.
• The integral flux above 1 TeV is (1.7 ± 0.9_stat ± 0.3_syst) × 10⁻¹³ cm⁻² s⁻¹, equivalent to 0.7% of the Crab nebula flux above the same energy.
• The VHE emission is spatially coincident with 13 CO molecular cloud enhancements, supporting a hadronic origin via pion decay from cosmic-ray interactions.
• An X-ray point source with a hard spectrum (Γ = 1.3⁺⁰.⁶₋₀.⁵) and high column density (NH = 6.9⁺².⁸₋₂.₁ × 10²² cm⁻²) was detected 30” from the H.E.S.S. position, but no pulsations were found.
• The high column density suggests the X-ray source may be distant, possibly obscuring a faint Pulsar Wind Nebula, though no extended X-ray emission was detected in the short observation.