Microvariability in BL Lac: Zooming into the Innermost Blazar Regions

Abstract

In this work, we present the results of our multi-band microvariability study of the famous blazar BL Lac. We performed microvariablity observations of the source in the optical VRI bands for 4 nights in the year 2016. We studied the intranight flux and spectral variability of the source in detail with an objective to characterize microvariability in the blazars, a frequently observed phenomenon in blazars. The results show that the source often displays a fast flux variability with an amplitude as large as ~ 0.2 magnitude within a few hours, and that the color variability in the similar time scales can be characterized as "bluer-when-brighter" trend. We also observed markedly curved optical spectrum during one of the nights. Furthermore, the correlation between multi-band emission shows that in general the emission in all the bands are highly correlated; and in one of the nights V band emission was found to lead the I band emission by ~ 20 minutes. The search for characteristic timescale using auto-correlation function and the structure function analyses reveals characteristic timescale of ~ 48 minutes in one of the R band observations. We try to explain the observed results in the context of the passage of shock waves through the relativistic outflows in blazars.