New Global 3D MHD Simulations of Black Hole Disk Accretion and Outflows

It is widely accepted that quasars and other active galactic nuclei (AGN) are powered by accretion of matter onto a central supermassive black hole. While numerical simulations have demonstrated the importance of magnetic fields in generating the turbulence believed necessary for accretion, so far they have not produced the high mass accretion rates required to explain the most powerful sources. We describe new global 3D simulations we are developing to assess the importance of radiation and non-ideal MHD in generating magnetized outflows that can enhance the overall rates of angular momentum transport and mass accretion.Comment: 2 pages, including 1 colour figure. To appear in proceedings of IAU Symposium 259: "Cosmic Magnetic Fields: From Planets, To Stars and Galaxies", Tenerife, Nov 200

Soft-excess in ULX spectra: the chilled-disk scenario

Soft X-ray spectra of ULXs show small deviations from a power-law model, that can be attributed to reprocessing in a fast, ionized outflow, or to thermal emission from a cool disk. If it is thermal emission, the cool peak temperature can be explained by an inner disk that radiates only a small fraction of the gravitational power, transferring the rest to an upscattering medium which is then responsible for the dominant power-law component. This scenario does not require intermediate-mass black holes: we use a phenomenological model to show that the observed X-ray luminosities and spectra of ULXs are consistent with typical masses ~ 50-100 Msun.Comment: To appear in the proceedings of "The Multicoloured Landscape of Compact Objects and their Explosive Progenitors: Theory vs Observations", Cefalu', Sicily, June 11-24, 2006 (AIP

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.Comment: Accepted for publication (17 August 2009) in MNRA

Dense, thin clouds and reprocessed radiation in the central regions of Active Galactic Nuclei

The primary radiation generated in the central continuum-forming region of Active Galactic Nuclei can be reprocessed by very dense, small-scale clouds that are optically-thin to Thomson scattering. In spite of the extreme conditions expected to prevail in this innermost, central environment, the radiative clouds can survive and maintain cool temperatures relative to the ambient emitting region by means of magnetic confinement. Motivated by these ideas, we present a detailed quantitative study of such clouds, explicitly describing the physical properties they can attain under thermal and radiative equilibrium conditions. We also discuss the thermal stability of the gas in comparison to that of other reprocessing material thought to reside at larger distances from the central source. We construct a model to predict the emergent spectra from a source region containing dense clouds which absorb and reemit the primary radiation generated therein. Our predicted spectra show the following two important results: (i) the reprocessed flux emitted at optical/UV energies is insufficient to account for the blue bump component in the observed spectra; and (ii) the amount of line radiation that is emitted is at least comparable to (and in many cases dominates) the continuum radiation. The lines are extremely broad and tend to accumulate in the extreme ultraviolet, where they form a peak much more prominent than that which is observed in the optical/UV. This result is supported by current observations, which indicate that the spectral energy distribution of radio-quiet AGN may indeed reach a maximum in the EUV band.Comment: 14 pages, 5 figures, latex, uses epsf and rotate, accepted for publication in M

On the origin of radio core emission in radio-quiet quasars

We present a model for the radio emission from radio-quiet quasar nuclei. We show that a thermal origin for the high brightness temperature, flat spectrum point sources (known as radio ``cores'') is possible provided the emitting region is hot and optically-thin. We hence demonstrate that optically-thin bremsstrahlung from a slow, dense disk wind can make a significant contribution to the observed levels of radio core emission. This is a much more satisfactory explanation, particularly for sources where there is no evidence of a jet, than a sequence of self-absorbed synchrotron components which collectively conspire to give a flat spectrum. Furthermore, such core phenomena are already observed directly via milli-arcsecond radio imaging of the Galactic microquasar SS433 and the active galaxy NGC1068. We contend that radio-emitting disk winds must be operating at some level in radio-loud quasars and radio galaxies as well (although in these cases, observations of the radio cores are frequently contaminated/dominated by synchrotron emission from jet knots). This interpretation of radio core emission mandates mass accretion rates that are substantially higher than Eddington. Moreover, acknowledgment of this mass-loss mechanism as an AGN feedback process has important implications for the input of energy and hot gas into the inter-galactic medium (IGM) since it is considerably less directional than that from jets.Comment: to appear in ApJ Letters (4 pages

Enhanced MHD transport in astrophysical accretion flows: turbulence, winds and jets

Astrophysical accretion is arguably the most prevalent physical process in the Universe; it occurs during the birth and death of individual stars and plays a pivotal role in the evolution of entire galaxies. Accretion onto a black hole, in particular, is also the most efficient mechanism known in nature, converting up to 40% of accreting rest mass energy into spectacular forms such as high-energy (X-ray and gamma-ray) emission and relativistic jets. Whilst magnetic fields are thought to be ultimately responsible for these phenomena, our understanding of the microphysics of MHD turbulence in accretion flows as well as large-scale MHD outflows remains far from complete. We present a new theoretical model for astrophysical disk accretion which considers enhanced vertical transport of momentum and energy by MHD winds and jets, as well as transport resulting from MHD turbulence. We also describe new global, 3D simulations that we are currently developing to investigate the extent to which non-ideal MHD effects may explain how small-scale, turbulent fields (generated by the magnetorotational instability -- MRI) might evolve into large-scale, ordered fields that produce a magnetized corona and/or jets where the highest energy phenomena necessarily originate.Comment: 8 pages, 2 figures. Minor revision, published version: Proc 14th International Congress on Plasma Physics, Fukuoka, Japan, Sep 200

Black hole masses and accretion states in ULXs

We summarize indirect empirical arguments used for estimating black hole (BH) masses in ultraluminous X-ray sources (ULXs). The interpretation of the X-ray data is still too model-dependent to provide tight constraints, but masses <~ 100 Msun seem the most likely. It is getting clearer that ULXs do not show the same evolutionary sequence between canonical spectral states as stellar-mass BHs, nor the same timescale for state transitions. Most ULX spectra are consistent either with a power-law-dominated state (apparently identical to the canonical low/hard state), or with a very high state (or slim-disk state). Despite often showing luminosity variability, there is little evidence of ULXs settling into a canonical high/soft state, dominated by a standard disk (disk-blackbody spectrum). It is possible that the mass accretion rate (but not necessarily the luminosity) is always higher than Eddington; but there may be additional physical differences between stellar-mass BHs and ULXs, which disfavour transitions to the standard-disk, radio-quiet state in the latter class. We speculate that the hard state in ULXs is associated with jet or magnetic processes rather than an ADAF, can persist up to accretion rates ~ Eddington, and can lead directly to the very high state.Comment: 8 pages; to appear in the proceedings of the conference "Observational Evidence of Black Holes", Kolkata, February 200