Abstract
The BL Lac type object AP Lib has been targeted for ~8h of observations with the High Energy Stereoscopic System (H.E.S.S.) in June and July 2010. The Fermi source 1FGL J1517.8-2423, associated with AP Lib (Abdo et al., 2010, ApJS, 188, 405), has a hard gamma-ray photon index of 2.1+/-0.1. This, combined with its proximity (z=0.049), makes AP Lib an excellent candidate for very high energy (E>100 GeV; VHE) gamma-ray emission. Initial analysis of the H.E.S.S. data-set, using published advanced analysis methods, results in a detection of AP Lib with a statistical significance exceeding 6 standard deviations above 300 GeV, corresponding to ~2% of the flux of the Crab nebula (note that the mirror reflectivity of the system is currently being upgraded and this preliminary flux determination might, therefore, have a larger systematic uncertainty). No flux variability is detected in the H.E.S.S. data-set nor in 11 month of Fermi data (Abdo et al., 2010, ApJS, 188, 405). From the broad band spectral energy distribution AP Lib has been classified as an RBL (Ciliegi, P. et al., ApJ 439, 80). X-ray observations by the Swift satellite show a hard energy spectrum (Tavecchio et al., 2010, MNRAS, 401, 1570) which, together with the H.E.S.S. detection at VHE energies, indicate an unusually broad high energy component stretching from X-rays to VHE gamma-rays. H.E.S.S. is an array of four imaging atmospheric Cherenkov telescopes for the detection of gamma-ray sources above 100 GeV located in the Khomas highlands in Namibia. It was constructed and is operated by researchers from Germany, France, UK, South Africa, Poland, the Czech Republic, the Republic of Irelanda, Australia, Austria, Sweden, Armenia, and the host country, Namibia.
