AGN variability using optical and WISE photometry

The growth of super-massive black holes (SMBH) and their connection to galaxy evolution is one of the pressing topics in modern astrophysics. Active galactic nuclei (AGN) are powered by the accretion of matter onto the SMBH, hence are a crucial tool to understand the formation and growth of SMBHs over cosmic time. AGN can exhibit flux variation on timescales ranging from minutes to years over the entire electromagnetic spectrum. The underlying physics of the variability is not clearly understood. It is suggested that on short timescales, disk instabilities play a significant role, while on longer timescales, the fueling of gas into the nuclear regions and regulation through feedback processes dominates. AGN feedback is expected to have a significant impact on the star formation of the host galaxy. A major contribution to the ultraviolet (UV) - near-infrared (NIR) part of the Spectral Energy Distribution (SED) of an AGN is produced is by the innermost regions of the accretion disk. The dusty region at larger distances from the accretion disk, often referred as the 'dusty torus' absorbs the light of the accretion disk and re-emits it in the mid-infrared (MIR), dominating the SED at wavelengths longer than ~ 1.0 micron. The variability of AGN not only places size limits on the accretion disk but also inform about gas inflows that power the AGN. To be explored further, a combination of observations in the time-domain and across multiple wavelengths is necessary. In this talk, I will present a statistical analysis of AGN variability using optical and MIR (WISE) light curves and focus on a MIR variable AGN J030654.88+010833.6 discovered in WISE data in Stripe 82 field. The extreme MIR variability shown in WISE W1 and W2 filters is not seen in the optical light curve taken over overlapping time. It has a bright optical, IR, radio, and a faint ultraviolet source associated with it. SED fitting using photometric observations identifies this source as a ULIRG undergoing intense star formation at z = 0.32. This object provides a rare opportunity to put important constraints on the growth of SMBHs and their connection to galaxy evolution at late cosmic times.