The Host Galaxy Fluxes of Active Galaxy Nuclei Are Generally Overestimated by the Flux Variation Gradient Method

TL;DR

The paper demonstrates that the flux variation gradient (FVG) method overestimates host galaxy fluxes in active galactic nuclei due to intrinsic color variations, confirmed both theoretically and observationally.

Contribution

It reveals the limitations of the FVG method caused by AGN color variability and provides a cautionary perspective for its application in time-domain astronomy.

Findings

FVG method overestimates host galaxy flux, especially for brighter AGN.

Theoretical simulations show the overestimation due to intrinsic color variation.

Observational data confirm the overestimation compared to other methods.

Abstract

In terms of the variable nature of normal active galaxy nuclei (AGN) and luminous quasars, a so-called flux variation gradient (FVG) method has been widely utilized to estimate the underlying non-variable host galaxy fluxes. The FVG method assumes an invariable AGN color, but this assumption has been questioned by the intrinsic color variation of quasars and local Seyfert galaxies. Here, using an up-to-date thermal fluctuation model to simulate multi-wavelength AGN variability, we theoretically demonstrate that the FVG method generally overestimates the host galaxy flux; that is, it is more significant for brighter AGN/quasars. Furthermore, we observationally confirm that the FVG method indeed overestimates the host galaxy flux by comparing it to that estimated through other independent methods. We thus caution that applying the FVG method should be performed carefully in the era of time-domain astronomy.