The near-infrared to ultraviolet continuum of radio-loud versus radio-quiet quasars

Starting from a sample of Sloan Digital Sky Survey quasars appearing also in the Two-Micron All-Sky Survey, we study the continuum properties of ∼1000 objects observed in eight bands, from near-infrared to ultraviolet. We construct the mean spectral energy distribution (SED) and compare and contrast the continua of radio-loud and radio-quiet quasar (RLQ and RQQ, respectively) objects. The SEDs of the two populations are significantly different, in the sense that RLQs are redder with power-law spectral indices 〈αRLQ〉=−0.55 ± 0.04 and 〈αRQQ〉=−0.31 ± 0.01 in the spectral range between 1014.5 and 1015.35 Hz. This difference is discussed in terms of different extinctions, different disc temperatures, or slopes of the non-thermal component.