Study of variability in long-term multiwavelength optical lightcurves of blazar AO 0235+164

TL;DR

This study provides a comprehensive long-term optical variability analysis of blazar AO 0235+164 over 44 years, revealing significant correlated multiwavelength variations, intraday variability, and insights into its physical mechanisms.

Contribution

It offers the first extensive 44-year multiwavelength optical variability dataset for AO 0235+164, including intraday variability and black hole mass estimation, enhancing understanding of blazar behavior.

Findings

Significant variability across all wavebands over 44 years.

Highly correlated variations in different wavebands without time lag.

Frequent intraday variability observed on 22 nights.

Abstract

We present a long-term and intraday variability study on optical multiwaveband ($U\!BV\!RI$) data from the blazar AO 0235+164 collected by various telescopes for $\sim$44 years (1975--2019). The blazar was found to be significantly variable over the years in all wavebands with a variation of about six magnitudes between its low and active states. The variations in the different wavebands are highly correlated without any time lag. We did not observe any significant trend in color variation with time, but we observed a bluer-when-brighter trend between the $B-I$ color index and the $R$-magnitude. Optical $BV\!R$-band spectral energy distributions always show a convex shape. Significant intraday variability was frequently seen in the quasi-simultaneous observations of AO\,0235+164 made on 22 nights in $R$ and $V$-bands by the CASLEO and CAHA telescopes during 1999--2019. We also estimated the central supermassive black-hole mass of $7.9\times10^7 M_{\odot}$ by analyzing the broad Mg II emission line in AO 0235+164's spectrum. We briefly explore the probable physical scenarios responsible for the observed variability.