Intranight optical variability of low-mass Active Galactic Nuclei: A Pointer to blazar-like activity

Abstract

This study aims to characterise, for the first time, intranight optical variability (INOV) of low-mass active galactic nuclei (LMAGN) which host a black hole (BH) of mass $M_{BH}\sim 10^6{M}_{\odot }$, i.e., even less massive than the Galactic centre black hole Sgr A* and 2-3 orders of magnitude below the supermassive black holes (SMBH, $M_{BH}$ $\sim$ $10^8-10^9{M}_{\odot }$) which are believed to power quasars. Thus, LMAGN are a crucial subclass of AGN 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 LMAGN is found to exhibit INOV at a level statistically comparable to that observed for blazars (M${}_{BH}\gtrsim$ 10${}^{8-9}$ M${}_{\odot }$) and for the ${\hat{I}}^3$-ray detected Narrow-line Seyfert1 galaxies (M${}_{BH}\sim 10^7$ M${}_{\odot }$) which, too, are believed to have relativistic jets. This indicates that the blazar-level activity can even be sustained by central engines with black holes near the upper limit for Intermediate Mass Black Holes ($M_{BH}$ $\sim$ $10^3-10^6{M}_{\odot }$).

Accepted for publication in MNRAS Letters