VERITAS Observations of the BL Lac Object PG 1553+113
Creators: name: Aliu, E.
name: Archer, A.
name: Aune, T.
name: Barnacka, A.
name: Behera, B.
name: Beilicke, M.
name: Benbow, W.
name: Berger, K.
name: Bird, R.
name: Buckley, J. H.
name: Bugaev, V.
name: Byrum, K.
name: Cardenzana, J. V
name: Cerruti, M.
name: Chen, X.
name: Ciupik, L.
name: Connolly, M. P.
name: Cui, W.
name: Dickinson, H. J.
name: Dumm, J.
name: Eisch, J. D.
name: Errando, M.
name: Falcone, A.
name: Federici, S.
name: Feng, Q.
name: Finley, J. P.
name: Fortin, P.
name: Fortson, L.
name: Furniss, A.
name: Galante, N.
name: Gillanders, G. H.
name: Griffin, S.
name: Griffiths, S. T.
name: Grube, J.
name: Gyuk, G.
name: Håkansson, N.
name: Hanna, D.
name: Holder, J.
name: Hughes, G.
name: Humensky, T. B.
name: Johnson, C. A.
name: Kaaret, P.
name: Kertzman, P. Kar M.
name: Khassen, Y.
name: Kieda, D.
name: Krawczynski, H.
name: Krennrich, F.
name: Kumar, S.
name: Lang, M. J.
name: Madhavan, A.
name: McArthur, S.
name: McCann, A.
name: Meagher, K.
name: Millis, J.
name: Moriarty, P.
name: Nieto, D.
name: de Bhróithe, A. O'Faoláin
name: Ong, R. A.
name: Orr, M.
name: Otte, A. N.
name: Park, N.
name: Perkins, J. S.
name: Pohl, M.
name: Popkow, A.
name: Prokoph, H.
name: Pueschel, E.
name: Quinn, J.
name: Ragan, K.
name: Rajotte, J.
name: Reyes, L. C.
name: Reynolds, P. T.
name: Richards, G. T.
name: Roache, E.
name: Sembroski, G. H.
name: Shahinyan, K.
name: Staszak, D.
name: Telezhinsky, I.
name: Tucci, J. V.
name: Tyler, J.
name: Varlotta, A.
name: Vassiliev, V. V.
name: Wakely, S. P.
name: Weinstein, A.
name: Welsing, R.
name: Wilhelm, A.
name: Williams, D. A.
name: Zitzer, B.
Publisher: name: arXiv
Year: 2014
Subjects: subject: High Energy Astrophysical Phenomena (astro-ph.HE)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: 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 $4.33 \pm 0.09$. The time-averaged integral flux above $200\,$GeV measured for this period was $(1.69 \pm 0.06) \times 10^{-11} \, \mathrm{ph} \, \mathrm{cm}^{-2} \, \mathrm{s}^{-1}$, 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 $\rm {101.9 \pm 3.2 \, \mathrm{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 $\rm z \negthinspace &gt; \negthinspace 0.395$ 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 $z \negthinspace \leq \negthinspace 0.62$. A strongly-elevated mean flux of $(2.50 \pm 0.14) \times 10^{-11} \, \mathrm{ph} \, \mathrm{cm}^{-2} \, \mathrm{s}^{-1}$ (10.3\% of the Crab Nebula flux) was observed during 2012, with the daily flux reaching as high as $(4.44 \pm 0.71) \times 10^{-11} \, \mathrm{ph} \, \mathrm{cm}^{-2} \, \mathrm{s}^{-1}$ (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 $χ^2$ probability for a steady flux of 0.03\%.
description: in press in ApJ, 2014