Optical monitoring of Active Galactic Nuclei from ARIES

TL;DR

This paper reviews ARIES's contributions to optical monitoring of AGN variability since the 1990s, revealing mild intra-night variability in various AGN types and highlighting the need for further dedicated studies.

Contribution

It provides a historical overview of ARIES's pioneering optical variability studies of AGNs, demonstrating variability in radio-quiet and radio-loud quasars, and emphasizing the importance of future targeted observations.

Findings

Mild INOV occurs in radio-quiet QSOs with amplitudes up to 5%.

Most radio-loud quasars also exhibit mild INOV, except blazars.

Blazars, especially BL Lac objects, show pronounced INOV and bluer-when-brighter behavior.

Abstract

This overview provides a historical perspective highlighting the pioneering role which the fairly modest observational facilities of ARIES have played since the 1990s in systematically characterizing the optical variability on hour-like time scale (intra-night optical variability, or INOV) of several major types of high-luminosity Active Galactic Nuclei (AGN). Such information was previously available only for blazars. Similar studies have since been initiated in at least a dozen countries, giving a boost to AGN variability research. Our work has, in particular, provided strong indication that mild INOV occurs in radio-quiet QSOs (amplitude up to $\sim 3-5\%$ and duty cycle $\sim 10\%$) and, moreover, has demonstrated that similarly mild INOV is exhibited even by the vast majority of radio-loud quasars which possess powerful relativistic jets (even including many that are beamed towards us). The solitary outliers are blazars, the tiny strongly polarized subset of powerful AGN, which frequently exhibit a pronounced INOV. Among the blazars, BL Lac objects often show a bluer-when-brighter chromatic behavior, while the flat spectrum radio quasars seem not to. Quantifying any differences of INOV among the major subclasses of non-blazar type AGNs will require dedicated monitoring programs using $2-3$ metre class telescopes.