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 \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 \pm 3.2 \mathrm { 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 \negthinspace > \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 \chi ^ { 2 } probability for a steady flux of 0.03 % .