Multiband optical variability on diverse time-scales of the TeV blazar TXS 0506 + 056, the first cosmic neutrino source

Abstract

We report the first extensive optical flux and spectral variability study of the TeV blazar TXS 0506+056 on intranight to long-term time-scales using BVRI data collected o ver 220 nights between 2017 January 21 to 2022 April 9 using eight optical ground-based telescopes. In our search for intraday variability (IDV), we have employed two statistical analysis techniques, the nested ANOVA test and the power enhanced F-test. We found the source was variable in 8 nights out of 35 in the R-band and in 2 of 14 in the V-band yielding duty cycles (DC) of 22.8 per cent and 14.3 per cent, respectively. Clear colour variation in V − R was seen in only 1 out of 14 observing nights, but no IDV was found in the more limited B, I, and B − I data. During our monitoring period the source showed a 1.18 mag variation in the R-band and similar variations are clearly seen at all opticalwavelengths. We extracted the optical (BVRI ) SEDs of the blazar for 44 nights when observations were carried out in all four of those wavebands. The mean spectral index (α) was determined to be 0.897 ± 0.171.