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

TL;DR

This study presents the first characterization of intranight optical variability in low-mass active galactic nuclei, revealing blazar-like activity that suggests relativistic jets can be powered by black holes much less massive than typical quasars.

Contribution

It provides the first detailed analysis of INOV in LMAGN, demonstrating blazar-like variability levels in black holes as small as 10^6 solar masses.

Findings

LMAGN exhibit INOV levels comparable to blazars.

Blazar-like activity can be sustained by black holes near 10^3-10^6 solar masses.

INOV observed in LMAGN indicates presence of relativistic jets.

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 $\gamma$-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 ($M_{BH}$ $\sim$ $10^3 - 10^6 M_{\odot}$).