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 (Sν∝να). 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 Sν-log ν plane exhibits a break at around 3000 Å where the spectrum changes from αblue=0.11 ± 0.16 at λ >3000 Å to αUV=−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 αblue=0.21±0.16 and αUV=−0.87±0.20, i.e. a higher steepening. For z>1.2, αUV is more flat, −0.48±0.12, and there are too few spectral points longward of 3000 Å to obtain αblue and analyse the presence of the 3000-Å break. A trend similar to that between αUV and z is found between αUV and luminosity at 2400 Å, L2400, with luminous quasars exhibiting a harder spectrum. The data show an intrinsic correlation between L2400 and the radio power at 408 MHz, not related to selection effects or independent cosmic evolution. The correlations αUV-z, αUV-L2400 and L2400-z appear to be consistent with accretion disc models with approximately constant black hole mass and accretion rates decreasing with time. If the trends L2400-z and P408-z are predominantly related to a selection bias, rather than cosmic evolution, only one of the correlations αUV-L2400 or αUV-z needs to be intrinsic.