Discovery of VHE gamma-rays from the distant BL Lac 1ES 0347-121

TL;DR

This paper reports the detection of VHE gamma-rays from the distant BL Lac 1ES 0347-121, providing new constraints on the Extragalactic Background Light and modeling its spectral energy distribution.

Contribution

First detection of VHE gamma-rays from 1ES 0347-121, enabling constraints on EBL density and modeling of its broad-band SED.

Findings

VHE gamma-ray spectrum well described by a power law with index 3.10

No VHE flux variability detected during observations

EBL density limits close to the strongest previous constraints

Abstract

Aims: Our aim is to study the production mechanism for very-high-energy (VHE; >100GeV) gamma-rays in distant active galactic nuclei (AGN) and use the observed VHE spectrum to derive limits on the Extragalactic Background Light (EBL). We also want to determine physical quantities through the modeling of the object's broad-band spectral energy distribution (SED). Methods: VHE observations (~25h live time) of the BL Lac 1ES 0347-121 (redshift z=0.188) were conducted with the High Energy Stereoscopic System (H.E.S.S.) between August and December 2006. Contemporaneous X-ray and UV/optical observations from the SWIFT satellite are used to interpret the SED of the source in terms of a synchrotron self Compton (SSC) model. Results: An excess of 327 events, corresponding to a statistical significance of 10.1 standard deviations, is detected from 1ES 0347-121. Its photon spectrum, ranging from ~250GeV to ~3TeV, is well described by a power law with a photon index of Gamma = 3.10 +/- 0.23_stat +/- 0.10_sys. The integral flux above 250GeV corresponds to ~2% of the flux of the Crab Nebula above the same threshold. No VHE flux variability is detected within the data set. Conclusions: Constraints on the EBL density at optical to near-infrared wavelengths derived from the photon spectrum of 1ES 0347-121 are close to the strongest limits derived previously. The strong EBL limits confirm earlier findings, that the EBL density in the near-infrared is close to the lower limits from source counts. This implies that the universe is more transparent to VHE gamma-rays than previously believed. An SSC model provides a reasonable description of the contemporaneous SED.