Discovery of very high energy γ-ray emission from the BL Lacertae object H2356-309 with the HESS Cherenkov telescopes

The extreme synchrotron BL Lac object H2356-309, located at a redshift of z = 0.165, was observed from June to December 2004 with a total exposure of approximate to 40 h live-time with the HESS (High Energy Stereoscopic System) array of atmospheric-Cherenkov telescopes (ACTs). Analysis of this data set yields, for the first time, a strong excess of 453 gamma-rays (10 standard deviations above background) from H2356-309, corresponding to an observed integral flux above 200 GeV of I(> 200 GeV) = (4.1 +/- 0.5) x 10(-12) cm(-2) s(-1) (statistical error only). The differential energy spectrum of the source between 200 GeV and 1.3 TeV is well-described by a power law with a normalisation (at 1 TeV) of N-0 = (3.00 +/- 0.80(stat) +/- 0.31(sys)) x 10(-13) cm(-2) s(-1) TeV-1 and a photon index of Gamma = 3.09 +/- 0.24(stat) +/- 0.10(sys). H2356-309 is one of the most distant BL Lac objects detected at very-high-energy-rays so far. Results from simultaneous observations from ROTSE-III (optical), RXTE (X-rays) and NRT (radio) are also included and used together with the HESS data to constrain a single-zone homogeneous synchrotron self-Compton (SSC) model. This model provides an adequate fit to the HESS data when using a reasonable set of model parameters.