Correlation between optical flux and polarization variations in Flat Spectrum Radio Quasars on diverse timescales

Blazars are a special class of active galactic nuclei (AGN), whose radiation output is dominated by Doppler boosted non-thermal emission from their relativistic jets that are closely oriented to the observer’s line of sight (1995PASP..107..803U). Doppler boosting amplifies the measured radiation resulting in the observed flux hundreds of times larger than that emitted by the source. The emission from blazars is highly variable in all bands of the electromagnetic (EM) spectrum i.e. from radio to very high energy $\gamma$-rays (e.g. 1995ARA&A..33..163W; 1997ARA&A..35..445U; 2017ApJ...841..123P). These flux variations are observed on a wide range of timescales from minutes to years (e.g. 2004MNRAS.350..175S; 2020ApJ...890...72P; 2020A&A...634A..80R). The broadband spectral energy distribution (SED) of blazars consists of a two hump structure (1998MNRAS.299..433F; 2016ApJS..224...26M). The low energy hump is attributed to the synchrotron process and the high-energy hump is attributed to the inverse Compton (IC) process (2010ApJ...716...30A). The seed photons for IC scattering can be from within the jet (synchrotron-self Compton; SSC; 1981ApJ...243..700K; 1985ApJ...298..114M; 1989ApJ...340..181G; 1996ApJ...461..657B) or outside the jet such as accretion disk, broad-line region, and torus (external Compton; EC; 1987ApJ...322..650B; 1994ApJ...421..153S). In the flat spectrum radio quasar (FSRQ) category of blazars the high energy component of the SED is usually produced by the EC process, while in the BL Lacertae (BL Lac) category of blazars the high energy component of the SED is predominantly due to SSC process. However, some recent studies have also reported temporary SSC dominance on some FSRQs (e.g. 2018MNRAS.479.2037P; 2019A&A...630A..56P; 2020ApJ...891...68C; 2021ApJ...906....5A).

In addition to flux variations, FSRQs are also characterized by a high degree ($>$3%) of variable optical polarization. Optical polarization in quasars is known since their discovery about six decades ago (1966ApJ...146..964K) and subsequently optical polarization were measured for many sources (e.g. 1980ARA&A..18..321A; 1990A&AS...83..183M; 1998AJ....116.2119V; 2008ApJ...672...40H; 2016ApJ...833...77I). The observed polarized emission in optical as well as at other longer wavelengths from these sources provide strong evidence for synchrotron radiation process as a source of the low energy component of their broadband SED (e.g. 1978ApJ...220L..67S; 1990A&AS...83..183M; 1991ApJ...375...46I; 2001ApJ...562..208L; 2002ApJ...577...85M; 2011MNRAS.412..318M). Observations of polarized emission, therefore, provide valuable information on the magnitude and direction of the magnetic field within relativistic jets. Changes in the observed polarization position angle could be related to changes in the direction of the magnetic field in the jet along the observer’s line of sight. Moreover, as the flux variations in the optical and GeV $\gamma$-rays are connected (2009ApJ...697L..81B; 2012ApJ...749..191C; 2014ApJ...783...83L; 2015MNRAS.450.2677C; 2018MNRAS.479.2037P; 2020ApJ...891...68C; 2021ApJ...906....5A, however, see also 2019MNRAS.486.1781R; 2020MNRAS.498.5128R and references therein for no correlation between optical and $\gamma$-ray flux variations) flux and polarization variability observations at different wavelengths can provide important inputs on the connection between different emission regions in the jets of these sources. Also, observations with the Fermi Gamma-ray Space Telescope (hereinafter Fermi) have revealed a close association between the $\gamma$-ray flare and the rotation of the optical polarization position angle, which again can constrain the nature of the high energy $\gamma$-ray emission process (2008Natur.452..966M; 2010Natur.463..919A; 2010ApJ...710L.126M).