The mid-infrared variability of the SDSS optical quasars

TL;DR

This study examines mid-infrared variability in SDSS optical quasars using WISE and NEOWISE data, finding no strong correlations with optical properties or radio luminosity, suggesting dust emission smoothing and weak jet contributions.

Contribution

It provides new insights into quasar mid-infrared variability and its lack of correlation with optical and radio properties, using a large quasar sample and structure function analysis.

Findings

No correlation between mid-infrared and optical variability amplitudes.

Steeper structure function slopes in W1 and W2 bands compared to previous studies.

Weak or no correlation between mid-infrared emission and radio luminosity.

Abstract

Based on the Seventh Data Release (DR7) quasar catalog from the Sloan Digital Sky Survey, we investigate the variability of optical quasars in W1, W2, W3 and W4 bands of the Wide-field Infrared Survey Explorer (WISE) and the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE). Adopting the structure function method, we calculate the structure function ($\rm\delta t$=1 yr) which shows no obvious correlations with the bolometric luminosity, the black hole mass and the Eddington ratio. The ensemble structure functions in W1 and W2 bands show that the SF slopes are steeper than those in previous studies which may be caused by different cadence and observational epoch number. We further investigate the relation of variability amplitude $\sigma_m$ between mid-infrared band and optical band, but no obvious correlation is found. No correlation is found between W1-W2 and g-r color. We think the mid-infrared emission of quasars may be smoothed out by the extended dust distribution, thus leading to no obvious correlation. For the radio-loud quasar sub-sample, we further analyze the relation between the variability amplitude in the mid-infrared band and the radio luminosity at 6 cm, but no obvious correlations are found, which indicate the mid-infrared emission contributed from the synchrotron radiation of the relativistic jet is very weak.