[Paper Review] On the correct intrinsic VHE properties of the BL Lac H 2356-309

L. Costamante|arXiv (Cornell University)|Aug 3, 2012

Particle Accelerators and Free-Electron Lasers3 citations

TL;DR

This paper corrects the intrinsic very high-energy (VHE) gamma-ray properties of the BL Lac object H 2356-309, showing that its intrinsic spectrum is significantly harder (photon index Γ ≈ 1.9–2.0) than previously reported by the HESS Collaboration. The analysis reveals that the true gamma-ray peak lies at 600 GeV or above, challenging prior SSC modeling and indicating the source is more consistent with 'TeV-peaked' HBLs than '100 GeV-peaked' ones.

ABSTRACT

The high-energy-peaked BL Lac H 2356-309 (z=0.165) was detected by HESS at very high energies (VHE, >100 GeV) with relatively high significance in the years 2004-2007, allowing a good determination of its gamma-ray spectrum. After correction for the interaction with the diffuse extragalactic background light (EBL), the VHE spectrum is flat (Gamma~1.9-2) over a decade in energy, locating the gamma-ray peak around or above 0.6-1 TeV. This is remarkably at odds with the interpretation and modeling provided by HESS, which do not correspond to the source properties and can be excluded with high confidence. The overall GeV-to-TeV characteristics of H 2356-309 seem intermediate between the "TeV-peaked" (Fermi-faint) and "100 GeV-peaked" (Fermi-bright) BL Lac objects, and difficult to reconcile with the shape of the synchrotron emission in a single-zone SSC scenario.

Motivation & Objective

• To correct the misinterpretation of the intrinsic very high-energy (VHE) gamma-ray spectrum of H 2356-309 as reported by the HESS Collaboration.
• To re-evaluate the intrinsic spectral index and peak energy of the VHE emission after proper correction for extragalactic background light (EBL) absorption.
• To assess the consistency of the corrected spectral properties with single-zone synchrotron self-Compton (SSC) modeling and known HBL source classes.
• To resolve the discrepancy between the observed VHE spectrum and the HESS-claimed intrinsic model, which was statistically excluded by the data.
• To provide a revised SED that accurately reflects the source's true high-energy emission characteristics across the GeV to TeV bands.

Proposed method

• Applied EBL absorption correction to the HESS 2004–2006 average VHE data using the Franceschini EBL model (franceschini).
• Fitted the EBL-corrected HESS data with a power-law model to determine the intrinsic photon index, yielding Γ_int = 1.97 ± 0.16 (statistical error only).
• Performed a χ² test to compare the HESS-claimed intrinsic model (Γ ≈ 2.65) with the corrected data, finding a Δχ² = 15, corresponding to a p-value of ~1×10⁻⁴.
• Used log-parabolic fitting to estimate the Compton peak energy, finding it at ~600 GeV, with an alternative estimate ≥1 TeV under a single power-law assumption.
• Compared the corrected VHE spectrum with Fermi-LAT 1FGL and 2FGL data, finding excellent agreement with a single power-law model (Γ = 1.94 ± 0.03) over three decades in energy.
• Reconstructed the full SED using corrected VHE data, historical X-ray/UV data, and Fermi-LAT data, ensuring consistency across multiwavelength bands.

Experimental results

Research questions

• RQ1 What is the true intrinsic photon index of the VHE gamma-ray spectrum of H 2356-309 after correcting for EBL absorption?
• RQ2 Where is the peak of the intrinsic gamma-ray emission located in the SED, and how does it compare to the HESS-claimed model?
• RQ3 Is the HESS-claimed single-zone SSC model consistent with the EBL-corrected VHE data, and if not, why is it excluded?
• RQ4 How does the corrected spectral shape of H 2356-309 compare to known classes of HBLs, particularly '100 GeV-peaked' and 'TeV-peaked' sources?
• RQ5 Can a single power-law model over the GeV–TeV range accurately describe the intrinsic emission, and what does this imply for SSC modeling?

Key findings

• The intrinsic VHE spectrum of H 2356-309 is significantly flatter than claimed by the HESS Collaboration, with a photon index of Γ_int = 1.97 ± 0.16 after EBL correction.
• The HESS-claimed intrinsic model (Γ ≈ 2.65) is excluded by the data at a confidence level of ~99.99%, with a Δχ² = 15 for one parameter.
• The Compton peak of the intrinsic SED is located at approximately 600 GeV (log-parabolic fit) or above 1 TeV (power-law fit), placing it in the 'TeV-peaked' HBL regime.
• The corrected VHE spectrum extrapolates nearly perfectly to the Fermi-LAT 1FGL data, with a single power-law fit yielding Γ = 1.94 ± 0.03 and reduced χ² ≈ 0.3 over three energy decades.
• The 2FGL data show a similar spectral slope (Γ = 1.89 ± 0.17) but with a flux reduced by a factor of ~2, indicating potential flux variability or non-simultaneity in observations.
• The corrected SED is synchrotron-dominated in luminosity, and the hard intrinsic VHE spectrum cannot be reconciled with a single-zone SSC model constrained by the observed soft synchrotron peak.