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The addition of a 28 m Cherenkov telescope (CT5) to the H.E.S.S. arrayextended the experiment's sensitivity to lower energies, providing newopportunities for studies of Active Galactic Nuclei (AGNs) with soft intrinsicspectra and at high redshifts. The high-frequency peaked BL Lac objects PKS2155-304 (z = 0.116) and PG 1553+113 (0.43 < z < 0.58) are among the brightestobjects in the gamma-ray sky, both showing clear signatures of gamma-rayabsorption at E > 100 GeV interpreted as being due to interactions with theextragalactic background light (EBL). Multiple observational campaigns of PKS2155-304 and PG 1553+113 were conducted during 2013 using the full H.E.S.S. IIarray (CT1-5). To achieve the lowest energy threshold, a monoscopic analysis ofthe data taken with CT5 was developed along with an investigation into thesystematic uncertainties on the spectral parameters which are derived from thisanalysis. The energy spectra were reconstructed down to energies of 80 GeV forPKS 2155-304, which transits near zenith, and 110 GeV for the more northern PG1553+113. The measured spectra, well fitted in both cases by a log-parabolaspectral model (with a 5 sigma statistical preference for non-zero curvaturefor PKS 2155-304 and 4.5 sigma for PG 1553+113), were found consistent withspectra derived from contemporaneous Fermi-LAT data, indicating a sharp breakin the observed spectra of both sources at E ~ 100 GeV. When corrected for EBLabsorption, the intrinsic spectrum of PKS 2155-304 was found to showsignificant curvature. For PG 1553+113, however, no significant detection ofcurvature in the intrinsic spectrum could be found within statistical andsystematic uncertainties.

Gamma-ray blazar spectra with H.E.S.S. II mono analysis: the case of PKS 2155-304 and PG 1553+113