Gamma-ray blazar spectra with HESS II mono analysis: The case of PKS2155-304 and PG1553+113

Context. The addition of a 28 m Cherenkov telescope (CT5) to the H.E.S.S. array extended the experiment’s sensitivityto 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 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).\ud
Aims. The aims of this work are twofold: to demonstrate the monoscopic analysis of CT5 data with a low energy threshold, and to obtain accurate measurements of the spectral energy distributions (SED) of PKS 2155−304 and PG 1553+113 near their SED peaks at energies ≈100 GeV.\ud
Methods. 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 (CT1–5). 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 which are derived from this analysis.\ud
Results. Using the data from CT5, the energy spectra of PKS 2155−304 and PG 1553+113 were reconstructed down to conservative threshold 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 5.0σ statistical preference for non-zero curvature for PKS 2155−304 and 4.5σ 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.The support of the Namibian authorities and of the University\ud
of Namibia in facilitating the construction and operation of H.E.S.S. is\ud
gratefully acknowledged, as is the support by the German Ministry for Education\ud
and Research (BMBF), the Max Planck Society, the German Research\ud
Foundation (DFG), the French Ministry for Research, the CNRS-IN2P3 and the\ud
Astroparticle Interdisciplinary Programme of the CNRS, the UK Science and\ud
Technology Facilities Council (STFC), the IPNP of the Charles University, the\ud
Czech Science Foundation, the Polish Ministry of Science and Higher Education,\ud
the South African Department of Science and Technology and National\ud
Research Foundation, the University of Namibia, the Innsbruck University, the\ud
Austrian Science Fund (FWF), and the Austrian Federal Ministry for Science,\ud
Research and Economy, and by the University of Adelaide and the Australian\ud
Research Council. We appreciate the excellent work of the technical support\ud
staff in Berlin, Durham, Hamburg, Heidelberg, Palaiseau, Paris, Saclay, and in\ud
Namibia in the construction and operation of the equipment. This work bene-\ud
fited from services provided by the H.E.S.S. Virtual Organisation, supported by\ud
the national resource providers of the EGI Federation. The Fermi-LAT Collaboration\ud
acknowledges generous ongoing support from a number of agencies and\ud
institutes that have supported both the development and the operation of the LAT\ud
as well as scientific data analysis. These include the National Aeronautics and\ud
Space Administration and the Department of Energy in the United States, the\ud
Commissariat à l’Énergie Atomique and the Centre National de la Recherche\ud
Scientifique/Institut National de Physique Nucléaire et de Physique des Particules\ud
in France, the Agenzia Spaziale Italiana and the Istituto Nazionale di Fisica\ud
Nucleare in Italy, the Ministry of Education, Culture, Sports, Science and Technology\ud
(MEXT), High Energy Accelerator Research Organization (KEK) and\ud
Japan Aerospace Exploration Agency (JAXA) in Japan, and the K. A. Wallenberg\ud
Foundation, the Swedish Research Council and the Swedish National Space\ud
Board in Sweden. Additional support for science analysis during the operations\ud
phase is gratefully acknowledged from the Istituto Nazionale di Astrofisica in\ud
Italy and the Centre National d’Études Spatiales in France. This research has\ud
made use of NASA’s Astrophysics Data System. This research has made use of\ud
the SIMBAD database, operated at CDS, Strasbourg, France. This research made\ud
use of Enrico, a community-developed Python package to simplify Fermi-LAT\ud
analysis (Sanchez & Deil 2013). M. Razzano is funded by contract FIRB-2012-\ud
RBFR12PM1F from the Italian Ministry of Education, University and Research\ud
(MIUR).Peer-reviewedPublisher Versio