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Title: VERITAS OBSERVATIONS OF THE BL LAC OBJECT PG 1553+113

We present results from VERITAS observations of the BL Lac object PG 1553+113 spanning the years 2010, 2011, and 2012. The time-averaged spectrum, measured between 160 and 560 GeV, is well described by a power law with a spectral index of 4.33 ± 0.09. The time-averaged integral flux above 200 GeV measured for this period was (1.69 ± 0.06) × 10{sup –11} photons cm{sup –2} s{sup –1}, corresponding to 6.9% of the Crab Nebula flux. We also present the combined γ-ray spectrum from the Fermi Large Area Telescope and VERITAS covering an energy range from 100 MeV to 560 GeV. The data are well fit by a power law with an exponential cutoff at 101.9 ± 3.2 GeV. The origin of the cutoff could be intrinsic to PG 1553+113 or be due to the γ-ray opacity of our universe through pair production off the extragalactic background light (EBL). Given lower limits to the redshift of z > 0.395 based on optical/UV observations of PG 1553+113, the cutoff would be dominated by EBL absorption. Conversely, the small statistical uncertainties of the VERITAS energy spectrum have allowed us to provide a robust upper limit on the redshift of PG 1553+113 of zmore » ≤ 0.62. A strongly elevated mean flux of (2.50 ± 0.14) × 10{sup –11} photons cm{sup –2} s{sup –1} (10.3% of the Crab Nebula flux) was observed during 2012, with the daily flux reaching as high as (4.44±0.71)×10{sup −11} photons cm{sup −2} s{sup −1} (18.3% of the Crab Nebula flux) on MJD 56048. The light curve measured during the 2012 observing season is marginally inconsistent with a steady flux, giving a χ{sup 2} probability for a steady flux of 0.03%.« less
Authors:
 [1] ; ; ; ;  [2] ;  [3] ;  [4] ; ;  [5] ; ;  [6] ;  [7] ;  [8] ;  [9] ; ;  [10] ;  [11] ;  [12] ;  [13] ;  [14] more »; « less
  1. Department of Physics and Astronomy, Barnard College, Columbia University, NY 10027 (United States)
  2. Department of Physics, Washington University, St. Louis, MO 63130 (United States)
  3. Department of Physics and Astronomy, University of California, Los Angeles, CA 90095 (United States)
  4. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)
  5. DESY, Platanenallee 6, 15738 Zeuthen (Germany)
  6. Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ 85645 (United States)
  7. Department of Physics and Astronomy and the Bartol Research Institute, University of Delaware, Newark, DE 19716 (United States)
  8. School of Physics, University College Dublin, Belfield, Dublin 4 (Ireland)
  9. Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439 (United States)
  10. Department of Physics and Astronomy, Iowa State University, Ames, IA 50011 (United States)
  11. Astronomy Department, Adler Planetarium and Astronomy Museum, Chicago, IL 60605 (United States)
  12. School of Physics, National University of Ireland Galway, University Road, Galway (Ireland)
  13. Department of Physics and Astronomy, Purdue University, West Lafayette, IN 47907 (United States)
  14. School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455 (United States)
Publication Date:
OSTI Identifier:
22364581
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 799; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; BL LACERTAE OBJECTS; COSMIC PHOTONS; CRAB NEBULA; ENERGY SPECTRA; GEV RANGE; INDEXES; MEV RANGE; OPACITY; ORIGIN; PAIR PRODUCTION; PROBABILITY; RED SHIFT; TELESCOPES; UNIVERSE; VISIBLE RADIATION