Veritas observations of the BL LAC object PG 1553+113

E. Aliu, A. Archer, T. Aune, A. Barnacka, B. Behera, M. Beilicke, W. Benbow, K. Berger, R. Bird, J. H. Buckley, V. Bugaev, K. Byrum, J. V. Cardenzana, M. Cerruti, X. Chen, L. Ciupik, M. P. Connolly, W. Cui, H. J. Dickinson, J. DummJ. D. Eisch, M. Errando, A. Falcone, S. Federici, Q. Feng, J. P. Finley, P. Fortin, L. Fortson, A. Furniss, N. Galante, G. H. Gillanders, S. Griffin, S. T. Griffiths, J. Grube, G. Gyuk, N. Håkansson, D. Hanna, J. Holder, G. Hughes, T. B. Humensky, C. A. Johnson, P. Kaaret, P. Kar, M. Kertzman, Y. Khassen, D. Kieda, H. Krawczynski, F. Krennrich, S. Kumar, M. J. Lang, A. Madhavan, S. McArthur, A. McCann, K. Meagher, J. Millis, P. Moriarty, D. Nieto, A. O'Faoláin De Bhróithe, R. A. Ong, M. Orr, A. N. Otte, N. Park, J. S. Perkins, M. Pohl, A. Popkow, H. Prokoph, E. Pueschel, J. Quinn, K. Ragan, J. Rajotte, L. C. Reyes, P. T. Reynolds, G. T. Richards, E. Roache, G. H. Sembroski, K. Shahinyan, D. Staszak, I. Telezhinsky, J. V. Tucci, J. Tyler, A. Varlotta, V. V. Vassiliev, S. P. Wakely, A. Weinstein, R. Welsing, A. Wilhelm, D. A. Williams, B. Zitzer

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Abstract

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) × 10-11 photons cm-2 s-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 z ≤ 0.62. A strongly elevated mean flux of (2.50 ± 0.14) × 10-11 photons cm-2 s-1 (10.3% of the Crab Nebula flux) was observed during 2012, with the daily flux reaching as high as (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 χ2 probability for a steady flux of 0.03%.

Original languageEnglish (US)
Article number7
JournalAstrophysical Journal
Volume799
Issue number1
DOIs
StatePublished - Jan 20 2015

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    Aliu, E., Archer, A., Aune, T., Barnacka, A., Behera, B., Beilicke, M., Benbow, W., Berger, K., Bird, R., Buckley, J. H., Bugaev, V., Byrum, K., Cardenzana, J. V., Cerruti, M., Chen, X., Ciupik, L., Connolly, M. P., Cui, W., Dickinson, H. J., ... Zitzer, B. (2015). Veritas observations of the BL LAC object PG 1553+113. Astrophysical Journal, 799(1), [7]. https://doi.org/10.1088/0004-637X/799/1/7