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

TL;DR

The paper conducts a multi-wavelength RM study of four low-redshift quasars using PS1, ZTF, VVV/VVVX, and WISE data to measure optical–NIR and optical–MIR lags, correcting for redshift and accretion-disk contamination. By combining lag measurements with SED fitting, it derives a graphite-to-silicate grain size ratio of $\frac{a_C}{a_S}\sim 0.4$ and confirms that dust torus sizes follow the $R_{dust}-L_{AGN}$ relation, with Ks and W1 lags tracing distinct dust populations. Structure-function analysis shows MIR variability is smoother and steeper, reflecting reprocessing in an extended torus. The work also presents evidence for polar dust in one source and demonstrates a robust, survey-driven approach to large-scale, multi-band RM that can inform future time-domain surveys.

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 Vía Láctea 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.

Figures (10)

• Figure 1: The optical, NIR and MIR light curves of four quasars between 2010 and 2024. These were collected from PS1 (g, r, i, z, y), ZTF (g, r), VVV/VVVX (Ks) and WISE (W1, W2). The optical light curves were calibrated by PyCALI.
• Figure 2: Spectrum fitting results for VVV J1845-2426 (LJT). In the top panel, observed spectrum (black line) is decomposed into power law continuum (orange line), Fe2 (cyan line), and emission lines (blue line). The emission lines include the broad (red line) and narrow (green line) components. The middle panel displays the residuals between the data and the model. The bottom left and right panels show the fitting results for the $H_{\gamma}$, $H_{\beta}$ and $H_{\alpha}$ emission lines, respectively.
• Figure 3: Light curves (left) and ICCF results (right) for targets VVV J1834-2925 (top), VVV J1833-2731 (middle) and VVV J1845-2426 (bottom), respectively. The CCF analysis results in the upper right corner correspond to optical-NIR and NIR-MIR correlations, respectively. All other CCFs represent optical-NIR and optical-MIR correlations. In the right panel, the orange lines represent the CCCDs, and CCF results appear in black.
• Figure 4: Javelin fits to the Optical, NIR and MIR light curves of VVV J1834-2925, VVV J1833-2731, and VVV J1845-2426.
• Figure 5: Comparison of rest-frame time lags before (x-axis) and after (y-axis) ADC subtraction. Black, blue, and red dots correspond to sources VVV J1834-2925, VVV J1833-2731, and VVV J1845-2426, respectively. The diagonal gray dashed line indicates unity correlation. The orange line represents the best fit for all the data points except the Ks-optical lag for VVV J1833-2731 (magenta diamond dot).