On the correct intrinsic VHE properties of the BL Lac H 2356-309

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.

💡 Research Summary

The paper presents a thorough re‑examination of the very‑high‑energy (VHE; >100 GeV) γ‑ray spectrum of the high‑frequency‑peaked BL Lac object H 2356‑309 (redshift z = 0.165) using the full HESS data set accumulated between 2004 and 2007 (≈52 h of exposure). After correcting for attenuation by the diffuse extragalactic background light (EBL) with up‑to‑date models (e.g., Franceschini 2008, Domínguez 2011), the intrinsic VHE spectrum is found to be remarkably hard, with a photon index Γ≈1.9–2.0 that remains essentially flat over a decade in energy. The spectral energy distribution (SED) therefore peaks at or above 0.6–1 TeV, a location that is significantly higher than the ∼100 GeV peak inferred by the HESS collaboration in their original publication.

The authors demonstrate that the HESS‑published SSC (synchrotron‑self‑Compton) model, which was built on a single‑zone assumption and anchored to the relatively faint GeV flux measured by Fermi‑LAT, cannot reproduce the de‑absorbed VHE data. Quantitatively, the model under‑predicts the measured VHE flux by more than 3σ, and a χ² test rejects the fit with high confidence. The discrepancy arises because the original model places the inverse‑Compton peak at too low an energy, leading to an over‑estimated EBL absorption when the spectrum is shifted to the observed VHE band.

The paper argues that H 2356‑309 occupies an intermediate regime between the two previously identified subclasses of BL Lac objects: the “TeV‑peaked” (Fermi‑faint) sources whose IC peaks lie well above 1 TeV, and the “100 GeV‑peaked” (Fermi‑bright) sources whose peaks sit near the lower end of the HESS sensitivity. Its hard VHE spectrum combined with a modest GeV flux makes it a challenging case for the canonical single‑zone SSC framework. The authors suggest that more sophisticated scenarios—such as multi‑zone SSC, inclusion of external Compton components, or a non‑standard electron energy distribution (e.g., a broken power‑law with a very hard high‑energy tail)—are required to reconcile the synchrotron and γ‑ray components.

In conclusion, the intrinsic VHE properties of H 2356‑309 are at odds with the earlier HESS interpretation and highlight the critical importance of accurate EBL correction and flexible modeling in blazar physics. The source provides a valuable benchmark for testing theories of particle acceleration and radiative processes in relativistic jets, and it underscores the need for coordinated GeV–TeV observations to capture the full shape of the high‑energy SED.