[Paper Review] H.E.S.S. Galactic Plane Survey unveils a Milagro Hotspot

H. E. S. S. Collaboration, :+4 more|arXiv (Cornell University)|Oct 12, 2007

Astronomical Observations and Instrumentation1 references5 citations

TL;DR

This paper presents the detection of a new very high-energy (VHE) gamma-ray source, HESS J1908+063, via the H.E.S.S. Galactic Plane Survey, which confirms the previously reported Milagro source MGRO J1908+06. The source is extended (FWHM ≈ 0.5°), has a hard spectrum (photon index = 2.08 ± 0.10), and emits at 14% of the Crab Nebula flux above 1 TeV, establishing a firm association between the H.E.S.S. and Milagro detections for the first time at low Galactic latitudes.

ABSTRACT

We report here on a new VHE source, HESS J1908+063, disovered during the extended H.E.S.S. survey of the Galactic plane and which coincides with the recently reported MILAGRO unidentied source MGRO J1908+06. The position, extension and spectrum measurements of the HESS source are presented and compared to those of MGRO J1908+06. Possible counterparts at other wavelenghts are discussed. For the first time one of the low-lattitude MILAGRO sources is confirmed.

Motivation & Objective

• To confirm the association of the Milagro source MGRO J1908+06 with a VHE gamma-ray source detected by H.E.S.S. in the Galactic plane.
• To measure the precise position, spatial extension, and energy spectrum of the H.E.S.S. source HESS J1908+063.
• To search for multi-wavelength counterparts at radio, GeV, and molecular cloud wavelengths to constrain the physical origin of the VHE emission.
• To assess the likelihood of association with known objects such as SNR G40.5-0.5, the EGRET source GRO J1908+0556, or nearby molecular clouds.
• To evaluate whether the extended morphology of the VHE source can be explained by cosmic-ray interactions or unresolved sources.

Proposed method

• H.E.S.S. performed deep observations of the Galactic plane between 30° and 60° galactic longitude from 2005 to 2007, with a total live time of 14.9 hours and a mean energy threshold of ~300 GeV.
• A two-stage analysis was applied: tight image-size cuts (200 p.e.) for source morphology and looser cuts (80 p.e.) for spectral analysis to maximize energy range and signal-to-noise.
• Background estimation used a ring of radius 1.0° centered on the source, four times larger than the on-source region, to account for the extended source size.
• Source position and extension were fitted with a 2D Gaussian convolved with the instrument’s point spread function (PSF), yielding a best-fit position at l = 40.45° ± 0.06° and b = -0.80° ± 0.05°.
• Spectral analysis used forward-folding maximum likelihood fitting to derive the differential energy spectrum above 300 GeV, assuming a power-law model.
• Multi-wavelength data from EGRET, radio, and CO surveys were overlaid to compare source positions and search for associations.

Experimental results

Research questions

• RQ1 Is the Milagro source MGRO J1908+06 a real VHE gamma-ray source, and can it be confirmed by an independent experiment?
• RQ2 What is the precise spatial extent and spectral shape of the VHE emission at the position of MGRO J1908+06?
• RQ3 What is the most likely astrophysical counterpart of HESS J1908+063 among known radio, GeV, or molecular sources?
• RQ4 Does the extended morphology of the H.E.S.S. source suggest emission from a diffuse region, such as cosmic-ray interactions with molecular clouds, rather than a compact object?
• RQ5 Can the flux and spectrum of HESS J1908+063 be reconciled with the flux measured by EGRET at GeV energies?

Key findings

• HESS J1908+063 was detected at a post-trials significance of 5.7σ, confirming the Milagro source MGRO J1908+06 as a real VHE gamma-ray source.
• The source has a FWHM spatial extent of 0.5°, corresponding to a source radius of 0.21° ± 0.07°, indicating an extended morphology.
• The differential energy spectrum is well described by a power law with a photon index of 2.08 ± 0.10 (statistical) and 2.08 ± 0.20 (systematic), with a flux of (3.23 ± 0.45) × 10⁻¹² cm⁻² s⁻¹ at 1 TeV.
• The integrated flux above 1 TeV is 14% of the Crab Nebula flux, indicating a bright VHE source.
• The H.E.S.S. source position is consistent with the EGRET source GRO J1908+0556 within 1σ error, suggesting a possible multi-wavelength association.
• The association with SNR G40.5-0.5 is not excluded, but the larger size of the VHE emission compared to the SNR shell suggests additional contributions from unresolved sources or molecular cloud interactions.