Optical flux and spectral characterization of the blazar PG 1553+113 based on the past 15 years of data

We study flux and spectral variability of the high energy peaked TeV blazar PG 1553 + 113 on diverse timescales using the data collected from 2005 to 2019 which also includes the intensive intra-night monitoring of the target. Additionally, we recorded the brightest flare of the blazar PG 1553 + 113 during April 2019 when the source attained an R-band magnitude of 13.2. Analyzing the spectral evolution of the source during the flare gave a clockwise spectral hysteresis loop and a time lag with V-band variations leading to the R-band ones. Various statistical tests, fitting procedures and cross-correlation techniques are applied to search for periodicity and examine the color-magnitude relationship. We find a median period of (2.21 +/- 0.04) years along with the secondary period of about 210 days. Finally, we briefly discuss various physical mechanisms which are capable of explaining our findings.