Joint Analysis of Optical, Near-Infrared And Mid-Infrared Variability of 4 Quasars at Redshift < 1

TL;DR

This study analyzes multi-wavelength variability in four quasars at redshift less than 1, measuring time lags between optical and infrared bands to infer dust torus properties and confirm the $R_{dust}-L_{AGN}$ relation.

Contribution

It provides the first systematic multi-wavelength variability analysis of these quasars, measuring dust torus sizes and grain ratios using combined optical and infrared data.

Findings

Measured Ks-optical and W1-optical lags for two quasars.

Derived a graphite-to-silicate grain size ratio of approximately 0.4.

Confirmed that dust torus sizes follow the established $R_{dust}-L_{AGN}$ relation.

Abstract

Amid rapid advances in time-domain astronomy, multi-wavelength (e.g., optical and infrared) time-domain studies of quasars remain scarce. Here we present a systematic analysis of four quasars initially selected by their Ks-band variability amplitudes in the VISTA Variables in the V\'{\i}a L\'actea Survey (VVV/VVVX). For these objects, we obtain complementary optical light curves from Pan-STARRS1 (PS1) and the Zwicky Transient Facility (ZTF), and W1-band light curves from the Wide-field Infrared Survey Explorer (WISE). We perform correlation analysis to study the time lags between different bands, which may be directly related to the size of the dust torus. After correcting for infrared flux contamination from the accretion disk and accounting for the redshift effect, we measure the Ks-optical and W1-optical lags for the targets VVV J1834-2925 and VVV J1845-2426. Using typical sublimation temperatures and reverberation time lags, we obtain a graphite-to-silicate grain size ratio of $\frac{a_C}{a_S}\sim$ 0.4. Through SED fitting, we determine the luminosities of these quasars and find that their dust torus sizes follow the established $R_{dust}-L_{AGN}$ relation reported in previous studies.