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) $\gamma$-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 10$\sigma$. The measured VHE $\gamma$-ray flux amounts to ${\rm d}N/{\rm d}E$ = (5.63 $\pm$ 0.89) $\times$ 10-13 $(E/{\rm TeV})^{-(2.94\pm0.20)}$ cm-2 s-1 TeV-1, above a spectral energy threshold of 225 GeV. No significant variation of the VHE $\gamma$-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 $\gamma$-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 $
u F_{
u}$ = 14 nWm-2 sr-1 at 1.5$~\mu$m as presented in Aharonian et al. (2006a) suggests an intrinsic VHE power output peak of the source at above 3 TeV.