A. R. King

A Possible Relativistic Jetted Outburst from a Massive Black Hole Fed by a Tidally Disrupted Star

A recent bright emission observed by the Swift satellite is due to the sudden accretion of a star onto a massive black hole. Gas accretion onto some massive black holes (MBHs) at the centers of galaxies actively powers luminous emission, but most MBHs are considered dormant. Occasionally, a star passing too near an MBH is torn apart by gravitational forces, leading to a bright tidal disruption flare (TDF). Although the high-energy transient Sw 1644+57 initially displayed none of the theoretically anticipated (nor previously observed) TDF characteristics, we show that observations suggest a sudden accretion event onto a central MBH of mass about 106 to 107 solar masses. There is evidence for a mildly relativistic outflow, jet collimation, and a spectrum characterized by synchrotron and inverse Compton processes; this leads to a natural analogy of Sw 1644+57 to a temporary smaller-scale blazar.

A high-velocity ionized outflow and XUV photosphere in the narrow emission line quasar PG1211+143

The definitive version is available from www.blackwell-synergy.com. Erratum published in Monthly Notices of the Royal Astronomical Society, 2005, 356, p.1599

The evolution of black hole mass and spin in active galactic nuclei

Observations show that the central black hole in galaxies has a mass M of only ∼10 -3 of the stellar bulge mass. Thus, whatever process grows the black hole also promotes star formation with far higher efficiency. We interpret this in terms of the generic tendency of active galactic nucleus (AGN) accretion discs to become self-gravitating outside some small radius R sg ∼ 0.01-0.1 pc from the black hole. We argue that mergers consist of sequences of such episodes, each limited by self-gravity to a mass ΔM episode ∼ 10 -3 M, with angular momentum characteristic of the small part of the accretion flow which formed it. In this picture, a major merger with ΔM merger ∼ M gives rise to a long series of low-mass accretion disc episodes, all chaotically oriented with respect to one another. Thus, the angular momentum vector oscillates randomly during the accretion process, on mass-scales ∼ 10 3 times smaller than the total mass accreted in a major merger event. We show that for essentially all AGN parameters, the disc produced by any accretion episode of this type has lower angular momentum than the hole, allowing stable co- and counter-alignment of the discs through the Lense-Thirring effect. A sequence of randomly oriented accretion episodes as envisaged above then produces accretion discs stably co- or counter- aligned with the black hole spin with almost equal frequency. Accretion from these discs very rapidly adjusts the holes spin parameter to average values a ∼ 0.1-0.3 (the precise range depending slightly on the disc vertical viscosity coefficient α 2 ) from any initial conditions, but with significant fluctuations (Aa ∼ ±0.2) about these. We conclude that (i) supermassive black holes (SMBH) should on average spin moderately, with the mean value a decreasing slowly as the mass increases; (ii) SMBH coalescences leave little long-term effect on a; (iii) SMBH coalescence products in general have modest recoil velocities, so that there is little likelihood of their being ejected from the host galaxy; (iv) black holes can grow even from stellar masses to ∼5 x 10 9 M ⊙ at high redshift z ∼ 6; and (v) jets produced in successive accretion episodes can have similar directions, but after several episodes the jet direction deviates significantly. Rare examples of massive holes with larger spin parameters could result from prograde coalescences with SMBHs of similar mass, and are most likely to be found in giant ellipticals. We compare these results with observation.

Growing Supermassive Black Holes by Chaotic Accretion

We study the contamination of the B–mode of the Cosmic Microwave Background Polarization (CMBP) by Galactic synchrotron in the lowest emission regions of the sky. The 22.8-GHz polarization map of the 3-years WMAP data release is used to identify and analyse such regions. Two areas are selected with signal-to-noise ratio S/N < 2 and S/N < 3, covering ∼ 16% and ∼ 26% fraction of the sky, respectively. The polarization power spectra of these two areas are dominated by the sky signal on large angular scales (multipoles l < 15), while the noise prevails on degree scales. Angular extrapolations show that the synchrotron emission competes with the CMBP B–mode signal for tensor-to-scalar perturbation power ratio T/S = 10–10 at 70-GHz in the 16% lowest emission sky (S/N < 2 area). These values worsen by a factor ∼ 5 in the S/N < 3 region. The novelty is that our estimates regard the whole lowest emission regions and outline a contamination better than that of the whole high Galactic latitude sky found by the WMAP team (T/S > 0.3). Such regions allow T/S ∼ 10 to be measured directly which approximately corresponds to the limit imposed by using a sky coverage of 15%. This opens interesting perspectives to investigate the inflationary model space in lowest emission regions.

Aligning spinning black holes and accretion discs

We consider the alignment torque between a spinning black hole and an accretion disc whose angular momenta are misaligned. This situation must hold initially in almost all gas accretion events on to supermassive black holes, and may occur in binaries where the black hole receives a natal supernova kick. We show that the torque always acts to align the hole’s spin with the total angular momentum without changing its magnitude. The torque acts dissipatively on the disc, reducing its angular momentum, and aligning it with the hole if and only if the angle θ between the angular momenta J d of the disc and J h of the hole satisfy the inequality cos θ> −J d/2J h .I fthis condition fails, which requires both θ> π/2 and J d < 2J h, the disc counteraligns. Ke yw ords: accretion, accretion discs ‐ black hole physics.

Fuelling active galactic nuclei

We suggest that most nearby active galactic nuclei are fed by a series of small-scale, randomly oriented accretion events. Outside a certain radius these events promote rapid star formation, while within it they fuel the supermassive black hole. We show that the events have a characteristic time-evolution. This picture agrees with several observational facts. The expected luminosity function is broadly in agreement with that observed for moderate-mass black holes. The spin of the black hole is low, and aligns with the inner disc in each individual feeding event. This implies radio jets aligned with the axis of the obscuring torus, and uncorrelated with the large-scale structure of the host galaxy. The ring of young stars observed about the Galactic Centre are close to where our picture predicts that star formation should occur.

Evidence of a high-velocity ionized outflow in a second narrow-line quasar PG 0844+349

Following the discovery of X-ray absorption in a high-velocity outflow from the bright quasar PG 1211 + 143 we have searched for similar features in XMM–Newton archival data of a second (high accretion rate) quasar PG 0844+349. Evidence is found for several faint absorption lines in both the EPIC and RGS spectra, whose most likely identification with resonance transitions in H-like Fe, S and Ne implies an origin in highly ionized matter with an outflow velocity of order ∼0.2c. The line equivalent widths require a line-of-sight column density of NH∼ 4 × 1023 cm−2, at an ionization parameter of log ξ∼ 3.7. Assuming a radial outflow being driven by radiation pressure from the inner accretion disc, as suggested previously for PG 1211 + 143, the flow in PG 0844+349 is also likely to be optically thick, in this case within ∼25 Schwarzschild radii. Our analysis suggests that a high-velocity, highly ionized outflow is likely to be a significant component in the mass and energy budgets of active galactic nuclei accreting at or above the Eddington rate.

Black hole mergers: can gas discs solve the ‘final parsec’ problem?

We compute the effect of an orbiting gas disc in promoting the coalescence of a central supermassive black hole binary. Unlike earlier studies, we consider a finite mass of gas with explicit time dependence: we do not assume that the gas necessarily adopts a steady state or a spatially constant accretion rate, i.e. that the merging black hole was somehow inserted into a pre-existing accretion disc. We consider the tidal torque of the binary on the disc, and the binarys gravitational radiation. We study the effects of star formation in the gas disc in a simple energy feedback framework. The disc spectrum differs in detail from that found before. In particular, tidal torques from the secondary black hole heat the edges of the gap, creating bright rims around the secondary. These rims do not in practice have uniform brightness either in azimuth or time, but can on average account for as much as 50 per cent of the integrated light from the disc. This may lead to detectable high-photon-energy variability on the relatively long orbital time-scale of the secondary black hole, and thus offer a prospective signature of a coalescing black hole binary. We also find that the disc can drive the binary to merger on a reasonable time-scale only if its mass is at least comparable with that of the secondary black hole, and if the initial binary separation is relatively small, i.e. a0≲ 0.05 pc. Star formation complicates the merger further by removing mass from the disc. In the feedback model we consider, this sets an effective limit to the disc mass. As a result, binary merging is unlikely unless the black hole mass ratio is ≲0.001. Gas discs thus appear not to be an effective solution to the ‘last parsec’ problem for a significant class of mergers.

Powerful Outflows and Feedback from Active Galactic Nuclei

Active Galactic Nuclei (AGN) represent the growth phases of the supermassive black holes in the center of almost every galaxy. Powerful, highly ionized winds, with velocities

Masses, beaming and Eddington ratios in ultraluminous X-ray sources

\sim 0.1- 0.2c