Intranight optical variability of low-mass active galactic nuclei: a pointer to blazar-like activity

Abstract

This study aims to characterize, for the first time, intranight optical variability (INOV) of low-mass active galactic nuclei (LMAGNs) which host a black hole (BH) of mass MBH ∼ 106M , i.e. even less massive than the Galactic centre BH Sgr A∗ and 2–3 orders of magnitude below the supermassive black holes (SMBHs, MBH ∼ 108–109M ), which are believed to power quasars. Thus, LMAGNs are a crucial subclass of AGNs filling the wide gap between SMBH and stellar-mass BHs of Galactic X-ray binaries. We have carried out a 36-session campaign of intranight optical monitoring of a well-defined, representative sample of 12 LMAGNs already detected in X-ray and radio bands. This set of LMAGNs is found to exhibit INOV at a level statistically comparable to that observed for blazars (MBH 108–9 M ) and for the γ -ray-detected Narrow-line Seyfert1 galaxies (MBH ∼ 107 M ) which, too, are believed to have relativistic jets. This indicates that the blazar-level activity can even be sustained by central engines with BHs near the upper limit for intermediate-mass BHs (MBH ∼ 103–106M ).