The optical-UV emissivity of quasars: dependence on black hole mass and
  radio loudness
  We analyzed a large sample of radio-loud and radio-quiet quasar spectra at
redshift 1.0 < z < 1.2 to compare the inferred underlying quasar continuum
slopes (after removal of the host galaxy contribution) with accretion disk
models. The latter predict redder (decreasing) alpha_3000 continuum slopes
(L_\nu~\nu^alpha at 3000Ang) with increasing black hole mass, bluer alpha_3000
with increasing luminosity at 3000Ang, and bluer alpha_3000 with increasing
spin of the black hole, when all other parameters are held fixed. We find no
clear evidence for any of these predictions in the data. In particular we find
that: (i) alpha_3000 shows no significant dependence on black hole mass or
luminosity. Dedicated Monte Carlo tests suggest that the substantial
observational uncertainties in the black hole virial masses can effectively
erase any intrinsic dependence of alpha_3000 on black hole mass, in line with
some previous studies. (ii) The mean slope alpha_3000 of radio-loud sources,
thought to be produced by rapidly spinning black holes, is comparable to, or
even redder than, that of radio-quiet quasars. Indeed, although quasars appear
to become more radio loud with decreasing luminosity, we still do not detect
any significant dependence of alpha_3000 on radio loudness. The predicted mean
alpha_3000 slopes tend to be bluer than in the data. Disk models with high
inclinations and dust extinction tend to produce redder slopes closer to
empirical estimates. Our mean alpha_3000 values are close to the ones
independently inferred at z<0.5 suggesting weak evolution with redshift, at
least for moderately luminous quasars.
