VERITAS observations of the BL Lac object PG 1553+113

Connolly, M.P.
Gillanders, Gary
Moriarty, P.
Lang, Mark
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
American Astronomical Society
Publisher DOI
Attribution-NonCommercial-NoDerivs 3.0 Ireland