Accretion Discs in Blazars
  The characteristic properties of blazars (rapid variability, strong
polarization, high brightness) are widely attributed to a powerful relativistic
jet oriented close to our line of sight. Despite the spectral energy
distributions (SEDs) being strongly jet-dominated, a "big blue bump" has been
recently detected in sources known as flat spectrum radio quasars (FSRQs).
These new data provide a unique opportunity to observationally test coupled
jet-disc accretion models in these extreme sources. In particular, as energy
and angular momentum can be extracted by a jet magnetically coupled to the
accretion disc, the thermal disc emission spectrum may be modified from that
predicted by the standard model for disc accretion. We compare the
theoretically predicted jet-modified accretion disc spectra against the new
observations of the "big blue bump" in FSRQs. We find mass accretion rates that
are higher, typically by a factor of two, than predicted by standard accretion
disc theory. Furthermore, our results predict that the high redshift blazars
PKS 0836+710, PKS 2149-307, B2 0743+25 and PKS 0537-286 may be predominantly
powered by a low or moderate spin (a < 0.6) black hole with high mass accretion
rates mdot_a ~ 50 - 200 msol/yr, while 3C 273 harbours a rapidly spinning black
hole (a = 0.97) with mdot_a ~ 20 msol/yr. We also find that the black hole
masses in these high redshift sources must be > 5 * 10^9 msol.
