Optical Flux and Spectral Variability of the TeV blazar PG 1553+113

Abstract

We present the results of our optical (VRI) observations of the TeV blazar PG 1553+113 over eight nights in 2016 April. We monitored the blazar quasi-simultaneously in V and R bands each night and examined the light curves (LCs) for intraday flux and color variations using two of the most powerful tests; the power-enhanced F-test and the nested ANOVA test. The source was found to be significantly (> 99%) variable in both V and R bands only on April 13, while clear variations only in R band LCs were seen on April 8 and 12. No temporal variation was seen in the color during the observation period. We did not find any significant correlation between V-R color index and R magnitude on any observing night. We found a mean optical spectral index of $\sim 0.83 \pm 0.02 $ with a maximum variation of 0.21 by fitting a power law ($F_{\nu} \propto \nu^{-\alpha}$) in the optical (VRI) spectral energy distribution of PG 1553+113. We briefly discuss the possible physical processes responsible for the observed flux and spectral variability.