Detection of VHE gamma-ray emission from the distant blazar 1ES 1101-232 with HESS and broadband characterisation

Context: The blazar 1ES 1101-232 was observed with the High Energy Stereoscopic System (HESS) of Atmospheric Cherenkov Telescopes (ACT) in 2004 and 2005, for a live time of 43 h. VHE (E > 10¹¹ eV) γ-rays were detected for the first time from this object.
Aims: VHE observations of blazars are used to investigate the inner parts of the blazar jets, and also to study the extragalactic background light (EBL) in the near-infrared band.
Methods: Observations in 2005 were conducted in a multiwavelength campaign, together with the RXTE satellite and optical observations. In 2004, simultaneous observations with XMM-Newton were obtained.
Results: 1ES 1101-232 was detected with HESS with an excess of 649 photons, at a significance of 10σ. The measured VHE γ-ray flux amounts to dN/dE = (5.63 ± 0.89) × 10^-13 (E/TeV)^{-(2.94±0.20)} cm^-2 s^-1 TeV^-1, above a spectral energy threshold of 225 GeV. No significant variation of the VHE γ-ray flux on any time scale was found. 1ES 1101-232 exhibits a very hard spectrum, and at a redshift of z = 0.186, is the blazar with the highest confirmed redshift detected in VHE γ-rays so far.
Conclusions: The data allow the construction of truly simultaneous spectral energy distributions of the source, from the optical to the VHE band. Using an EBL model with ν F_ν = 14 nWm^-2 sr^-1 at 1.5~μm as presented in Aharonian et al. (2006a) suggests an intrinsic VHE power output peak of the source at above 3 TeV.