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 \raisebox { -2.15 pt } { $ \stackrel { < } { \scriptstyle \sim } $ } 0.6 ) black hole with high mass accretion rates \dot { M } _ { a } \approx 50 - 200 { M } _ { \odot } { yr } ^ { -1 } , while 3C 273 harbours a rapidly spinning black hole ( a = 0.97 ) with \dot { M } _ { a } \approx 20 { M } _ { \odot } { yr } ^ { -1 } .
We also find that the black hole masses in these high redshift sources must be \raisebox { -2.15 pt } { $ \stackrel { > } { \scriptstyle \sim } $ } 5 \times 10 ^ { 9 } { M% } _ { \odot } .