[Paper Review] AGN observations with a less than 100 GeV threshold using H.E.S.S. II

D. Zaborov, C. Romoli+4 more|arXiv (Cornell University)|Sep 22, 2015

Gamma-ray bursts and supernovae3 citations

TL;DR

This paper presents the first VHE gamma-ray observations of AGN PKS 2155–304 and PG 1553+113 using H.E.S.S. II's CT5 mono-telescope mode, achieving a sub-100 GeV energy threshold. The analysis reveals spectral curvature in both sources, with PG 1553+113 detected down to 100 GeV and a photon index of 2.95 ± 0.23 at 141 GeV, demonstrating the unique sensitivity of CT5 mono for low-energy VHE AGN studies.

ABSTRACT

The recent addition of the 28 m Cherenkov telescope (CT5) to the H.E.S.S. array extended the experiment's sensitivity towards low energies. The lowest energy threshold is obtained using monoscopic observations with CT5, providing access to gamma-ray energies below 100 GeV. This is particularly beneficial for studies of Active Galactic Nuclei (AGN) with soft spectra and located at redshifts >= 0.5. Stereoscopic measurements with the full array (CT1-5) provide a better background rejection than CT5 Mono, at a cost of a higher threshold. We report on the analysis employing the CT5 data for AGN observations with a < 100 GeV threshold. In particular, the spectra of PKS 2155-304 and PG 1553+113 are presented.

Motivation & Objective

• To extend very high-energy gamma-ray observations of AGN below 100 GeV using the newly added 28 m CT5 telescope in the H.E.S.S. array.
• To study AGN with soft spectra and high redshifts (z ≥ 0.5) where extragalactic background light absorption is significant.
• To validate the performance of monoscopic CT5 data analysis for low-energy VHE gamma-ray astronomy.
• To provide complementary data to Fermi-LAT at energies below 100 GeV, particularly for spectral curvature and absorption features.

Proposed method

• Utilized monoscopic data from the 28 m CT5 telescope in H.E.S.S. II, which provides a lower energy threshold than the full stereoscopic array.
• Applied a modified Model reconstruction algorithm adapted for monoscopic events, using likelihood fitting of Cherenkov shower images to a semi-analytical shower model.
• Implemented three cut configurations—Loose, Standard, and Safe—controlled by the NSBGoodness variable to balance threshold, background rejection, and systematic control.
• Performed background subtraction using the on-off method with exclusion radii and validated with significance maps and θ² distributions.
• Cross-checked results using the ImPACT method for image pixel-wise fitting, ensuring robustness of spectral measurements.
• Analyzed data with strict run quality cuts, including pointing stability, temperature, humidity, trigger rate, and pixel functionality.

Experimental results

Research questions

• RQ1 Can the H.E.S.S. CT5 mono mode achieve a reliable energy threshold below 100 GeV for VHE gamma-ray observations?
• RQ2 What is the spectral shape of PKS 2155–304 and PG 1553+113 at energies below 100 GeV, particularly regarding spectral curvature?
• RQ3 How do the measured spectra compare with Fermi-LAT and earlier H.E.S.S. I data in the overlapping energy range?
• RQ4 To what extent does the monoscopic CT5 analysis provide a robust and systematic measurement despite higher background levels?
• RQ5 Can the observed spectral curvature in both AGN be consistently explained by extragalactic background light absorption at high redshifts?

Key findings

• PKS 2155–304 was detected with a significance of 10σ in the 100–136 GeV energy bin, with a reconstructed energy threshold of approximately 80 GeV.
• The spectrum of PG 1553+113 was measured from 100 GeV to 500 GeV using CT5 mono data, showing a log-parabolic shape with a photon index of 2.95 ± 0.23 at a decorrelation energy of 141 GeV.
• A 27.4σ excess was observed within the on-source region (θ² < 0.015 deg²) for PG 1553+113, confirming high significance detection.
• The measured differential flux for PG 1553+113 at 141 GeV is (1.48 ± 0.07) × 10⁻⁹ cm⁻² s⁻¹ TeV⁻¹, consistent with earlier H.E.S.S. and Fermi-LAT measurements.
• The ImPACT cross-check analysis confirmed the spectral results, showing excellent agreement with the Model reconstruction method.
• The results are preliminary but demonstrate the feasibility and scientific value of monoscopic CT5 observations for probing the VHE gamma-ray sky below 100 GeV.