The shape of the blue/UVcontinuum of B3-VLA radio quasars: dependence on redshift, blue/UVluminosity 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 alpha ranging from 0.4 to -1.7 (S_nu ~ nu^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 ~= 1300-4500 Å 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 log S_nu -log nu plane exhibits a break at around 3000 Å where the spectrum changes from alpha_blue=0.11 +/- 0.16 at lambda >3000 Å to alpha_UV=-0.66 +/- 0.15 at lambda <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 alpha_blue=0.21+/-0.16 and alpha_UV=-0.87+/-0.20, i.e. a higher steepening. For z>1.2, alpha_UV is more flat, -0.48+/-0.12, and there are too few spectral points longward of 3000 Å to obtain alpha_blue and analyse the presence of the 3000-Å break. A trend similar to that between alpha_UV and z is found between alpha_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 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 or alpha_UV-z needs to be intrinsic.