The extreme synchrotron BL Lac object H 2356 - 309 , located at a redshift of z = 0.165 , was observed from June to December 2004 with a total exposure of \approx 40 h live-time with the H.E.S.S .
( 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 H 2356 - 309 , corresponding to an observed integral flux above 200 GeV of I ( > 200 GeV ) = ( 4.1 \pm 0.5 ) \times 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 \pm 0.80 _ { \mathrm { stat } } \pm 0.31 _ { \mathrm { sys } } ) \times 10 ^ { -13 } cm ^ { -2 } s ^ { -1 } TeV ^ { -1 } and a photon index of \Gamma = 3.09 \pm 0.24 _ { \mathrm { stat } } \pm 0.10 _ { \mathrm { sys } } .
H 2356 - 309 is one of the most distant BL Lac objects detected at very-high-energy \gamma -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 H.E.S.S .
data to constrain a single-zone homogeneous synchrotron self-Compton ( SSC ) model .
This model provides an adequate fit to the H.E.S.S .
data when using a reasonable set of model parameters .