Multiband optical variability of the TeV blazar PG 1553 + 113 in 2019

ABSTRACT We report the flux and spectral variability of PG 1553 + 113 on intra-night (IDV) to short-term time-scales using BVRI data collected over 91 nights from 28 February 8 November 2019 employing 10 optical telescopes: three in Bulgaria, two each India Serbia, one Greece, Georgia, Latvia. monitored blazar quasi-simultaneously for 16 V R bands V, R, I examined light curves (LCs) intra-day colour variations powerful tests: power-enhanced F-test nested ANOVA test. The source was found be significantly (>99 per cent) variable 4 out 27 R-band, 1 V-band, 6 I-band. No temporal colours were observed IDV time-scale. During course these observations total variation R-band 0.89 mag observed. also investigated energy distribution (SED) B-, V-, R-, I-band data. indices range 0.878 ± 0.029 1.106 0.065 by fitting a power law (Fν∝ν−α) SEDs 113. that follows bluer-when-brighter trend time-scales. discuss possible physical causes variability.