The stability of the optical flux variation gradient for 3C120

TL;DR

This study confirms that the optical flux variation gradient method reliably measures host galaxy contribution in Seyfert galaxies, showing no significant color variability over various timescales despite luminosity changes.

Contribution

It demonstrates that analyzing fluxes instead of magnitudes avoids artifacts in color variability studies of AGN, validating the flux variation gradient method for Seyfert galaxies.

Findings

No significant color variability detected over 1 to 1800 days.

Flux variation gradients provide consistent host galaxy estimates.

Luminosity changes do not affect the flux ratio in 3C120.

Abstract

New $B$- and $V$-band monitoring in 2014 $-$ 2015 reveals that the Seyfert 1 Galaxy, 3C120, has brightened by a magnitude of $1.4$, compared to our campaign that took place in 2009 $-$ 2010. This allowed us to check for the debated luminosity and time-dependent color variations claimed for SDSS quasars. For our 3C120 data, we find that the $B/V$ flux ratio of the variable component in the bright epoch is indistinguishable from the faint one. We do not find any color variability on different timescales ranging from about $1$ to $1800$ days. We suggest that the luminosity and time-dependent color variability is an artifact caused by analyzing the data in magnitudes instead of fluxes. The flux variation gradients of both epochs yield consistent estimates of the host galaxy contribution to our 7.5" aperture. These results confirm that the optical flux variation gradient method works well for Seyfert galaxies.