VERITAS observations of the BL Lac object PG 1553+113
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Aliu, E,Archer, A,Aune, T,Barnacka, A,Behera, B,Beilicke, M,Benbow, W,Berger, K,Bird, R,Buckley, JH,Bugaev, V,Byrum, K,Cardenzana, JV,Cerruti, M,Chen, X,Ciupik, L,Connolly, MP,Cui, W,Dickinson, HJ,Dumm, J,Eisch, JD,Errando, M,Falcone, A,Federici, S,Feng, Q,Finley, JP,Fortin, P,Fortson, L,Furniss, A,Galante, N,Gillanders, GH,Griffin, S,Griffiths, ST,Grube, J,Gyuk, G,Hakansson, N,Hanna, D,Holder, J,Hughes, G,Humensky, TB,Johnson, CA,Kaaret, P,Kar, P,Kertzman, M,Khassen, Y,Kieda, D,Krawczynski, H,Krennrich, F,Kumar, S,Lang, MJ,Madhavan, A,McArthur, S,McCann, A,Meagher, K,Millis, J,Moriarty, P,Nieto, D,de Bhroithe, AO,Ong, RA,Orr, M,Otte, AN,Park, N,Perkins, JS,Pohl, M,Popkow, A,Prokoph, H,Pueschel, E,Quinn, J,Ragan, K,Rajotte, J,Reyes, LC,Reynolds, PT,Richards, GT,Roache, E,Sembroski, GH,Sha (2015) 'VERITAS OBSERVATIONS OF THE BL LAC OBJECT PG 1553+113'. Astrophysical Journal, 799 .
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) x 10(-11) photons cm(-2) s(-1), corresponding to 6.9% of the Crab Nebula flux. We also present the combined gamma-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 gamma-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