What powers the most relativistic jets? - II. Flat-spectrum radio quasars

Abstract

Flat-spectrum radio quasars (FSRQs) are the most powerful relativistic jets seen from supermassive black holes (BHs) accreting via a radiatively efficient thin disc. Their high energy emission is well modelled by highly relativistic electrons in the jet Compton upscattering an external source of seed photons, primarily from the broad-line region. Strong Doppler boosting by the jet bulk motion makes these FSRQs readily detectable by the Fermi Large Area Telescope. We combine jet spectral models with scaling relations for the jet physical parameters as a function of mass and accretion rate. This does not match well to the gamma-ray loud narrow-line Seyfert 1s, assuming their low BH masses are reliable, but is able to predict much of the spectral evolution observed along the Blazar sequence. We use these models in conjunction with cosmological simulations of efficiently accreting BH number densities, and find that they overpredict the observed number of FSRQs by two orders of magnitude if all of these objects produce an FSRQ jet. We can better reproduce the observed numbers if jets are only produced by high-spin BHs and BH spin is built from chaotically aligned accretion episodes so that high-spin BHs are rare. However, this does not reproduce the observed redshift and mass accretion rate distributions of the FSRQs. This may indicate a redshift dependence in accretion mode, with sustained alignment accretion episodes being more prevalent at higher redshift, or that there is some other trigger for FSRQ jets.