2023MNRAS.518L..13G

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 M_BH ∼ 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, $M_{BH}\, \sim \, 10^8 \rm{-} 10^9 M_{\odot }$), 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 (M_BH ≳ 10^8–9M_⊙) and for the γ-ray-detected Narrow-line Seyfert1 galaxies (M_BH ∼ 10⁷M_⊙) 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 ($M_{BH}\, \sim \, 10^3 \rm{-}10^6 M_{\odot }$).