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 a ranging from 0.4 to -1.7 (S-v proportional to v(alpha)). Two sources with alpha < -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 similar or equal to 1300-4500 Angstrom and the only emission feature apparent from the broad-band spectra is the C IV lambda 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 logS(v)-log v plane exhibits a break at around 3000 Angstrom where the spectrum changes from alpha(blue) = 0.11 +/- 0.16 at lambda > 3000 Angstrom to alpha(UV) = -0.66 +/- 0.15 at lambda < 3000 Angstrom. 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 alpha(blue) = 0.21 +/- 0.16 and aw = -0.87 +/- 0.20, i.e. a higher steepening. For z > 1.2, aw is more flat, -0.48 +/- 0.12, and there are too few spectral points longward of 3000 Angstrom to obtain abl,, and analyse the presence of the 3000-Angstrom break. A trend similar to that between alpha(UV) and z is found between alpha(UV) and luminosity at 2400 Angstrom, L-2400, 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 (alpha(UV)-z, alpha(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 alpha(UV)-L-2400 01' alpha(UV)-z needs to be intrinsic.