Characterizing the Optical Nature of the Blazar S5 1803+784 during Its 2020 Flare

Abstract

We report the results from our study of the blazar S5 1803+784 carried out using quasi-simultaneous B, V, R, and I observations from 2020 May to 2021 July on 122 nights. Our observing campaign detected a historically bright optical flare during MJD 59,063.5−MJD 59,120.5. We also found the source in its brightest (R mag = 13.617) and faintest (R mag = 15.888) states to date. On 13 nights, covering both flaring and nonflaring periods, we searched for intraday variability using the power-enhanced F-test and the nested ANOVA test. We found significant variability in 2 of these 13 nights. However, no such variability was detected during the flaring period. From correlation analysis, we observed that the emission in all optical bands were strongly correlated with a time lag of ∼0 days. To get insights into its dominant emission mechanisms, we generated the optical spectral energy distributions of the source on 79 nights and estimated the spectral indices by fitting a simple power law. The spectral index varied from 1.392 to 1.911 and showed significant variations with time and R-band magnitude. We detected a mild bluer-when-brighter (BWB) trend during the whole monitoring period and a much stronger BWB trend during the flare. We also carried out a periodicity search using four different methods and found no significant periodicity during our observation period. Based on the analysis during the flaring state of the source one can say that the emissions most likely originate from the jet rather than from the accretion disk.