The ultraviolet/optical variability of steep-spectrum radio quasars: the change in accretion rate?

Context. The steep-spectrum radio quasars (SSRQs) are powerful radio sources, with thermal emission from accretion disk and jet nonthermal emission both likely contributing to the ultraviolet (UV)/optical luminosity; however, the former may play a dominant role. Since the UV/optical variability of SSRQs has been poorly studied, little is known about the mechanism of their variability. Aims. We investigate the mechanism of the UV/optical variability of SSRQs. Methods. A sample of eighteen SSRQs was established in SDSS Stripe 82 region in our previous works, in which the flux and spectral variability were studied. In this work, we construct the flux-flux diagram using SDSS u and i multi-epoch data for these eighteen SSRQs. The standard accretion disk model is used to fit the flux-flux variations, in order to explore the variability mechanism. Results. The model fit to flux-flux diagram are tuned with fixed black hole mass and varying accretion rate. We find that the flux-flux diagram of all our SSRQs can be qualitatively described by the standard accretion disk model with a change in the accretion rate. Although nonthermal jet power-law emission can also qualitatively reproduce the variability, the reasonable accretion rates and black hole masses required to fit the flux-flux variations suggest that a disk emission with variable accretion rate is a plausible description of the data.