Joint Analysis of Optical, Near-infrared, and Mid-infrared Variability of Four Quasars at Redshift <1

Abstract Amid rapid advances in time-domain astronomy, multiwavelength (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 Vía Láctea Survey (VVV/VVVX). For these objects, we obtain complementary optical light curves from Pan-STARRS1 and the Zwicky Transient Facility and W1-band light curves from the Wide-field Infrared Survey Explorer. 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfrac> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>a</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>C</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>a</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>S</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:mfrac> <mml:mo>∼</mml:mo> </mml:math> 0.4. Through spectral energy distribution 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.