[Paper Review] Recent H.E.S.S. results on extra-galactic sources

M. Cerruti|arXiv (Cornell University)|Oct 10, 2011

Astrophysics and Cosmic Phenomena3 citations

TL;DR

This paper presents recent Very High Energy (VHE) gamma-ray detections of five extra-galactic sources—SHBL J001355.9-185406, 1RXS J101015.9-311909, 1ES 1312-423, AP Lib, and HESS J1943+213—using the H.E.S.S. Cherenkov telescope array, with fluxes ranging from 0.004 to 0.02 times the Crab nebula flux and redshifts up to z ≈ 0.58. The results demonstrate improved sensitivity from advanced analysis techniques and lay the groundwork for deeper exploration with H.E.S.S. II, which will lower the energy threshold and enhance detection of faint, distant sources.

ABSTRACT

Since the beginning of scientific operations in 2003, more than 20 extra-galactic sources have been detected with H.E.S.S. (High Energy Stereoscopic System) at very high energies (VHE): apart from the starburst galaxy NGC 253, all of them are active galactic nuclei (AGNs). BL Lac objects are by far the dominant AGN sub-class in the H.E.S.S. extra-galactic sky, but two radio-galaxies and one flat-spectrum radio quasar (FSRQ) have been detected as well. The study of extra-galactic VHE emitters will be improved in the near future by the completion of a fifth 24m-diameter telescope (H.E.S.S. II). In this talk, a review of the AGNs seen by H.E.S.S. will be given, including the latest results achieved, namely the detection of VHE emission from the blazars 1RXS J101015.9-311909, SHBL J001355.9-185406, 1ES 1312-423, AP Lib and the BL Lac candidate HESS J1943+213.

Motivation & Objective

• To report the latest detections of extra-galactic Very High Energy (VHE) gamma-ray sources by the H.E.S.S. Cherenkov telescope array.
• To evaluate the performance of enhanced analysis techniques in detecting faint VHE sources with shorter observation times.
• To characterize the spectral energy distributions (SEDs) and emission mechanisms of newly detected AGNs, particularly BL Lacs and FSRQs.
• To assess the potential origin of HESS J1943+213, a newly detected VHE source with a soft spectrum and no clear stellar counterpart.
• To prepare for the enhanced sensitivity and lower energy threshold of H.E.S.S. II, which will improve the study of the VHE extra-galactic sky.

Proposed method

• Utilization of the High Energy Stereoscopic System (H.E.S.S.), a ground-based array of four 12m-diameter atmospheric Cherenkov telescopes, to detect VHE gamma rays above 100 GeV.
• Application of advanced analysis techniques, including improved reconstruction algorithms beyond the standard Hillas method, to enhance sensitivity and reduce observation time for faint sources.
• Spectral and flux measurements of detected sources using time-integrated photon indices and fluxes in units of Crab nebula flux.
• Cross-identification of VHE sources with multi-wavelength data, including Fermi-LAT, X-ray, radio, and infrared observations, to constrain source origin and SED modeling.
• Redshift estimation and host galaxy flux modeling to set lower limits on redshift for unidentified sources such as HESS J1943+213.
• Exclusion of alternative origins (e.g., gamma-ray binaries, pulsar wind nebulae) through absence of orbital variability, extended X-ray emission, or massive stellar companions.

Experimental results

Research questions

• RQ1 What is the significance and flux of the VHE gamma-ray emission from the BL Lac object SHBL J001355.9-185406, and how does it compare to the Crab nebula?
• RQ2 How does the spectral energy distribution (SED) of AP Lib, a low-energy-peaked BL Lac, support the external inverse Compton emission model?
• RQ3 What evidence supports the extra-galactic origin of HESS J1943+213, and why are alternative origins like gamma-ray binaries or pulsar wind nebulae unlikely?
• RQ4 How do improved analysis techniques enable the detection of faint VHE sources like 1ES 1312-423 and 1RXS J101015.9-311909 with shorter observation times?
• RQ5 What is the expected impact of H.E.S.S. II, with its fifth 24m telescope, on the detection of fainter and more distant extra-galactic VHE sources?

Key findings

• H.E.S.S. detected VHE emission from SHBL J001355.9-185406 at 5σ significance, with a flux of approximately 1% of the Crab nebula flux, at a redshift of z = 0.095.
• The source 1RXS J101015.9-311909 was detected at 7σ significance with a flux of 0.008 Crab units and a photon index of 3.1 ± 0.5, consistent with a high-energy-peaked BL Lac.
• 1ES 1312-423 was detected at 7σ with a flux of 0.004 Crab units, despite being located 2° off-axis from the central field of view, demonstrating improved sensitivity of new analysis techniques.
• AP Lib, a low-energy-peaked BL Lac at z = 0.049, was detected at 7σ with a flux of 0.02 Crab units and a spectral index of 2.5 ± 0.2, with its broad SED consistent with external inverse Compton emission.
• HESS J1943+213 was detected at 7.9σ with a flux of 0.02 Crab units and a soft spectrum (photon index 3.1 ± 0.3), with no plausible stellar counterpart, supporting an extreme high-energy-peaked blazar origin and a redshift lower limit of z > 0.14.
• The development of enhanced analysis techniques has significantly improved the detection of faint VHE sources, enabling shorter observation times and expanding the known VHE extra-galactic source population.