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Context. The steep-spectrum radio quasars (SSRQs) are powerful radio sources,with thermal emission from accretion disk and jet nonthermal emission likelyboth contributing in the Ultraviolet (UV)/optical luminosity, however theformer may play a dominant role. While the UV/optical variability of SSRQs hasbeen poorly studied, little is known on the mechanism of their variability.Aims. We investigate the mechanism of the UV/optical variability of SSRQs.Methods. A sample of eighteen SSRQs has been established in SDSS Stripe 82region in our previous works, in which the flux and spectral variability havebeen studied. In this work, we construct the flux-flux diagram using SDSS u andi multi-epoch data for these eighteen SSRQs. The standard accretion disk modelis used to fit the flux-flux variations, in order to explore the variabilitymechanism. Results. The model fit to flux-flux diagram are tuned with fixedblack hole mass and varying accretion rate. We found that the flux-flux diagramof all our SSRQs can be qualitatively described by the standard accretion diskmodel with change of the accretion rate. Although non-thermal jet power-lawemission can also qualitatively reproduce the variability, the reasonableaccretion rates and black hole masses required to fit the flux-flux variationssuggest that a disk emission with variable accretion rate is a plausibledescription of the data.

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