The shape of the blue/UV continuum of B3‐VLA radio quasars: dependence on redshift, blue/UV luminosity and radio power

We present UBVR photometry of a sample of 73 radio quasars, about 80 per cent complete, with redshifts 0.4–2.8. From these data the shape of the spectral energy distribution (SED) in the rest‐frame blue/ultraviolet is analysed, using the individual sources as well as through broad‐band composite SEDs. The SEDs of the individual sources are generally well fitted with power laws, with slopes α ranging from 0.4 to –1.7 ( S ν ∝ν α ). Two sources with α < −1.6 were excluded from the general study for having very red SEDs, significantly deviating with respect to the remaining sources. The composite SEDs cover the range ≃ 1300–4500 Å and the only emission feature apparent from the broad‐band spectra is the C iv λ 1549 line, in agreement with expectations from line equivalent width measurements of radio‐loud quasars from the literature. The shape of the composites in the log S ν –log ν plane exhibits a break at around 3000 Å where the spectrum changes from α blue =0.11 ± 0.16 at λ >3000 Å to α UV =−0.66 ± 0.15 at λ <3000 Å. Although the broad‐band spectral points are expected to include some masked contamination from emission lines/bumps, the break cannot be explained by line/bump emission, and most likely reflects an intrinsic trend in the continuum. The continuum shape is shown to depend on redshift. For the quasars with z <1.2 we find α blue =0.21±0.16 and α UV =−0.87±0.20, i.e. a higher steepening. For z >1.2, α UV is more flat, −0.48±0.12, and there are too few spectral points longward of 3000 Å to obtain α blue and analyse the presence of the 3000‐Å break. A trend similar to that between α UV and z is found between α UV and luminosity at 2400 Å, L 2400 , with luminous quasars exhibiting a harder spectrum. The data show an intrinsic correlation between L 2400 and the radio power at 408 MHz, not related to selection effects or independent cosmic evolution. The correlations α UV – z , α UV – L 2400 and L 2400 – z appear to be consistent with accretion disc models with approximately constant black hole mass and accretion rates decreasing with time. If the trends L 2400 – z and P 408 – z are predominantly related to a selection bias, rather than cosmic evolution, only one of the correlations α UV – L 2400 or α UV – z needs to be intrinsic.