VERITAS Observations of the BL Lac Object PG 1553+113

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 $4.33\pm 0.09$. 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 ${\hat{I}}^3$-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 $101.9\pm 3.2\mathrm{GeV}$. The origin of the cutoff could be intrinsic to PG~1553+113 or be due to the ${\hat{I}}^3$-ray opacity of our universe through pair production off the extragalactic background light (EBL). Given lower limits to the redshift of $\mathrm{z}>0.395$ 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 $z\le 0.62$. 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 $\ddot{I}{‡}^2$ probability for a steady flux of 0.03\%.

in press in ApJ, 2014