We investigate the effect of a disc-driven jet on the accretion growth of cosmological supermassive black holes (SMBHs). The presence of a jet enhances the mass growth rate because for a given luminosity, the mass accretion rate,Ṁ a , is higher (or equivalently, the radiative efficiency r is lower for a fixedṀ a ) than that predicted by standard accretion disc theory. As jets carry away very little of the accreting matter, a larger proportion of the rest mass can reach the black hole during episodes of jet activity. We show quantitatively that the conditions required to grow a rapidly spinning black hole to a mass ≈10 9 M by redshift z ≈ 6, whilst satisfying the observational constraint r 0.1, are considerably less restrictive for jet-enhanced disc accretion than for standard disc accretion, which requires implausibly high super-Eddington accretion rates. Furthermore, jet-enhanced accretion growth offers a viable explanation for the observed correlation between black hole mass and radio loudness of quasars.

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 highmass accretion ratesṀ a ≈ 50-200 M yr −1 , while 3C 273 harbours a rapidly spinning black hole (a = 0.97) withṀ a ≈ 20 M yr −1 . We also find that the black hole masses in these high-redshift sources must be 5 × 10 9 M .

We revisit theoretical and observational constraints on geometrically-thin disk accretion in Sagittarius A ⋆ (Sgr A ⋆ ). We show that the combined effects of mass outflows and electron energization in the hot part of the accretion flow can deflate the inflowing gas from a geometrically-thick structure. This allows the gas to cool and even thermalize on an inflow timescale. As a result, a compact, relatively cool disk may form at small radii. We show that magnetic coupling between the relativistic disk and a steady-state jet results in a disk that is less luminous than a standard relativistic disk accreting at the same rate. This relaxes the observational constraints on thin-disk accretion in Sgr A ⋆ (and by implication, other Low-Luminosity Active Galactic Nulcei, LLAGN). We find typical cold gas accretion rates of a few × 10 −9 M ⊙ yr −1 . We also find that the predicted modified disk emission is compatible with existing near-infrared (NIR) observations of Sgr A ⋆ in its quiescent state provided that the disk inclination angle is > ∼ 87 • and that the jet extracts more than 75% of the accretion power.

It has been proposed that the radio spectra of radio-quiet quasars is produced by free-free emission in the optically thin part of an accretion disc wind. An important observational constraint on this model is the observed X-ray luminosity. We investigate this constraint using a sample of PG radio-quiet quasars for which XMM-Newton EPIC spectra are available. Comparing the predicted and measured luminosities for 0.5, 2 and 5 keV, we conclude that all of the studied PG quasars require a large hydrogen column density absorber, requiring these quasars to be close to or Compton-thick. Such a large column density can be directly excluded for PG 0050+124, for which a high-resolution RGS spectrum exists. Further constraint on the column density for a further 19 out of the 21 studied PG quasars comes from the EPIC spectrum characteristics such as hard X-ray power-law photon index and the equivalent width of the Fe Kalpha line; and the small equivalent width of the C IV absorber present in UV spectra. For 2 sources: PG 1001+054 and PG 1411+442 we cannot exclude that they are indeed Compton-thick, and the radio and X-ray luminosity are due to a wind originating close to the super-massive black hole. We conclude that for 20 out of 22 PG quasars studied free-free emission from a wind emanating from the accretion disc cannot mutually explain the observed radio and X-ray luminosity.Comment: Accepted for publication in MNRAS, 10 pages, 5 figure

We explore an accretion model for low luminosity AGN (LLAGN) that attributes the low radiative output to a low mass accretion rate,Ṁ a , rather than a low radiative efficiency. In this model, electrons are assumed to drain energy from the ions as a result of collisionless plasma microinstabilities. Consequently, the accreting gas collapses to form a geometrically thin disk at small radii and is able to cool before reaching the black hole. The accretion disk is not a standard disk, however, because the radial disk structure is modified by a magnetic torque which drives a jet and which is primarily responsible for angular momentum transport. We also include relativistic effects. We apply this model to the well known LLAGN M87 and calculate the combined disk-jet steady-state broadband spectrum. A comparison between predicted and observed spectra indicates that M87 may be a maximally spinning black hole accreting at a rate of ∼10 −3 M yr −1 . This is about 6 orders of magnitude below the Eddington rate for the same radiative efficiency. Furthermore, the total jet power inferred by our model is in remarkably good agreement with the value independently deduced from observations of the M87 jet on kiloparsec scales.