| Abstract
| - The characteristic properties of blazars (rapid variability, strong polarization and high brightness) are widely attributed to a powerful relativistic jet oriented close to our line of sight. Despite the spectral energy distributions being strongly jet-dominated, a ‘big blue bump’ has been recently detected in the 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 2, 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 , while 3C 273 harbours a rapidly spinning black hole (a= 0.97) with . We also find that the black hole masses in these high-redshift sources must be ≳5 × 109 M⊙.
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