Optical Continuum Reverberation Mapping of a Candidate IMBH in a Nearby Seyfert 1 Galaxy

TL;DR

This study used high-cadence multiband photometry to investigate optical continuum variability in a Seyfert 1 galaxy potentially hosting an intermediate-mass black hole, but found no significant short-term variability due to host galaxy light dilution.

Contribution

First high-cadence multiband photometric monitoring of a candidate IMBH in a Seyfert galaxy, highlighting challenges in detecting variability due to host galaxy contamination.

Findings

No significant variability detected (<1.4%) over 6-10 hours.

Host galaxy light dilution likely obscured AGN variability.

Future strategies include selecting targets with higher AGN-to-host flux ratio.

Abstract

To investigate the short-term variability and determine the size of the optical continuum emitting region of intermediate-mass black holes (IMBHs), we carried out high-cadence, multiband photometric monitoring of a Seyfert 1 galaxy J0249-0815 across two nights, together with a one-night single-band preliminary test. The presence of the broad Ha component in our target was confirmed by recent Paloma spectroscopic observations, 23 years after the Sloan Digital Sky Survey, ruling out the supernovae origin of the broad Ha line. The photometric experiment was primarily conducted utilizing four-channel imagers MuSCAT 3 and 4 mounted on 2 m telescopes within the Las Cumbres Observatory Global Telescope Network. Despite the expectation of variability, we observed no significant variation (<1.4%) on timescales of 6 to 10 hr. This nondetection is likely due to substantial host galaxy light diluting the subtle active galactic nucleus (AGN) variability. Additionally, we cannot rule out that the target was in a relatively quiescent state without intranight variability during our monitoring, owing to the stochastic nature of AGN variations. To enhance the possibility of detecting subtle variability signals and lag in future IMBH reverberation campaigns, it may be beneficial to select targets with a higher AGN-to-host flux ratio, and conduct dual-band preliminary tests and tailored simulations.