Exploring the connection between ultraviolet/optical variations and radio emission in radio-quiet quasars: clues about the origin of radio emission

Abstract

ABSTRACT Radio emission in radio-quiet quasars (RQQs) has long been a mystery and its physical origin remains unclear. In previous work, we have found that quasars that are more variable in ultraviolet (UV)/optical have stronger X-ray emission, indicating a link between disc turbulence and X-ray corona heating. In this work, for the first time, we investigate the relation between UV/optical variability and the radio emission in RQQs selected from the Sloan Digital Sky Survey Stripe 82 survey and the Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) survey. We median stack the FIRST images and detect clear signals from RQQs in the co-added images of individually radio non-detected sources. Controlling the effects of other parameters, including redshift, black hole mass, bolometric luminosity and Eddington ratio, we find more variable RQQs, which are known to be relatively brighter in X-ray, and show tentatively weaker radio emission, contrary to the linear X-ray/radio correlation if the radio emission is from, or driven by, the corona. This discovery also suggests that if the radio emission in RQQs is driven by AGN activity (such as a weak jet), the underlying driving process is independent of the disc turbulence, which drives UV/optical variability and probably also corona heating. Alternatively, the radio emission could be a result of star formation in the host galaxies.