Gamma-ray blazar spectra with H.E.S.S. analysis: the case of

Context. The addition of a 28 m Cherenkov telescope (CT5) to the H.E.S.S. array extended the experiment’s sensitivity to lower energies. The lowest energy threshold is obtained using monoscopic analysis of data taken with CT5, providing access to gamma-ray energies below 100 GeV for small zenith angle observations. Such an extension of the instrument’s energy range is particularly beneficial for studies of active galactic nuclei with soft spectra, as expected for those at a redshift ≥ 0.5. The high-frequency peaked BL Lac objects PKS2155 − 304 ( z = 0 . 116) PG1553 + (0 . 43 < z < 0 . 58) are among the brightest objects in the gamma-ray sky, both showing clear signatures of gamma-ray absorption at E > 100 GeV interpreted as being due to interactions with the extragalactic background light (EBL). fitted in both cases by a log-parabola spectral model (with a 5.0 σ statistical preference for non-zero curvature for PKS2155 − 304 and 4.5 σ for PG1553 + 113), were found consistent with spectra derived from contemporaneous Fermi -LAT data, indicating a sharp break in the observed spectra of both sources at E ≈ 100 GeV. When corrected for EBL absorption, the intrinsic H.E.S.S. II mono and Fermi -LAT spectrum of PKS2155 − 304 was found to show significant curvature. For PG1553 + 113, however, no significant detection of curvature in the intrinsic spectrum could be found within statistical and systematic uncertainties.