  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. 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 PKS 2155-304 (z = 0.116) and PG
1553+113 (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). Multiple observational campaigns of PKS 2155-304 and PG 1553+113
were conducted during 2013 and 2014 using the full H.E.S.S. II instrument. A
monoscopic analysis of the data taken with the new CT5 telescope was developed
along with an investigation into the systematic uncertainties on the spectral
parameters. The energy spectra of PKS 2155-304 and PG 1553+113 were
reconstructed down to energies of 80 GeV for PKS 2155-304, which transits near
zenith, and 110 GeV for the more northern PG 1553+113. The measured spectra,
well fitted in both cases by a log-parabola spectral model (with a 5.0 sigma
statistical preference for non-zero curvature for PKS 2155-304 and 4.5 sigma
for PG 1553+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 PKS 2155-304 was found to
show significant curvature. For PG 1553+113, however, no significant detection
of curvature in the intrinsic spectrum could be found within statistical and
systematic uncertainties.

High Energy Astrophysical Phenomena;Instrumentation and Methods for Astrophysics