Julian H. Krolik

Molecular tori in Seyfert galaxies - Feeding the monster and hiding it

The principal properties of the tori of gas which surround Seyfert nuclei are discussed. The internal state of the clouds and their size distribution function are examined, and it is shown that the Jeans mass scale results in clouds which are individually sufficiently opaque to block out the nucleus, and that the balance of processes which controls their size distribution function also forces the covering factor to be of the order of or greater than unity. Where the gravitational potential is dominated by stars, cloud-cloud collisions keep the molecular clouds close to the equatorial plane. Stirring by stellar processes is never strong enough to compete with collisional losses. The position of the inner edge of the torus is determined by a balance between the inward flow of clouds and the rate at which the nuclear continuum can evaporate them. 43 references.

Magnetically Driven Accretion Flows in the Kerr Metric. I. Models and Overall Structure

This is the first in a series of papers that investigate the properties of accretion flows in the Kerr metric through three-dimensional general relativistic magnetohydrodynamic simulations of tori with a nearly Keplerian initial angular velocity profile. We study four models with increasing black hole spin, from a/M = 0 to 0.998, for which the structural parameters of the initial tori are maintained nearly constant. The subsequent accretion flows arise self-consistently from stresses and turbulence created by the magnetorotational instability. We investigate the overall evolution and the late-time global structure in the resulting nonradiative accretion flows, including the magnetic fields within the disks, the properties of the flow in the plunging region, and the flux of conserved quantities into the black hole. Independent of black hole spin, the global structure is described in terms of five regions: the main disk body, the coronal envelope, the inner disk (consisting of an inner torus and plunging region), an evacuated axial funnel, and a biconical outflow confined to the corona-funnel boundary. We find evidence for lower accretion rates, stronger funnel-wall outflows, and increased stress in the near-hole region with increasing black hole spin.

Infrared Spectra of Obscuring Dust Tori around Active Galactic Nuclei. II. Comparison with Observations

In a previous paper we computed the IR properties predicted for the dusty obscuring tori believed to surround many AGNs. Here we make a detailed comparison of the predictions with published observations. We find good agreement between the model predictions and the observed continuum of NGC 1068, the 3.5-10.5 μm colors of type 2 Seyfert galaxies, the observed range in behavior of the 10 μm silicate feature and 8-13 μm spectral index in both types of Seyfert galaxies, and several examples of anisotropy in the IR emission from AGNs. To explain the IR continua of type 1 Seyfert galaxies and quasars, we suggest that a small amount of dust may exist well inside the inner edge of the torus proper and that the host galaxy often dominates in the far-infrared

Magnetically Driven Jets in the Kerr Metric

We compute a series of three-dimensional general relativistic magnetohydrodynamic simulations of accretion flows in the Kerr metric and investigate the properties of the resulting unbound outflows. The strength of the outflows increases sharply with increasing black hole rotation rate. Several generic features are observed. The mass in the outflow in concentrated in a hollow cone whose opening angle is largely determined by the effective potential for matter with specific angular momentum comparable to that of the innnermost stable circular orbit. The pressure in the accretion disks corona provides the dominant force accelerating the matter outward. The principle element shaping the outflow is therefore the centrifugal barrier preventing accreting matter from coming close to the rotation axis. The funnel inside the centrifugal barrier contains very little matter and is dominated by electromagnetic fields that rotate at a rate tied closely to the rotation of the black hole, even when the black hole spins in a sense opposite to the rotation of the accretion flow. These fields carry an outward-going Poynting flux whose immediate energy source is the rotating spacetime of the Kerr black hole. When the spin parameter a/M of the black hole exceeds 0.9, the energy carried to infinity by these outflows can be comparable to the nominal radiative efficiency predicted in the Novikov-Thorne model. Similarly, the expelled angular momentum can be comparable to that accreted by the black hole. Both the electromagnetic and the matter outflows contribute significantly to the energy and angular momentum of the outflow.

CANDIDATE TYPE II QUASARS FROM THE SLOAN DIGITAL SKY SURVEY. I. SELECTION AND OPTICAL PROPERTIES OF A SAMPLE AT 0.3 < Z < 0.83

Type II quasars are the long-sought luminous analogs of type 2 (narrow emission line) Seyfert galaxies, suggested by unification models of active galactic nuclei (AGNs) and postulated to account for an appreciable fraction of the cosmic hard X-ray background. We present a sample of 291 type II AGNs at redshifts 0.3 < Z < 0.83 from the spectroscopic data of the Sloan Digital Sky Survey. These objects have narrow (FWHM < 2000 km s-1), high equivalent width emission lines with high-ionization line ratios. We describe the selection procedure and discuss the optical properties of the sample. About 50% of the objects have [O III] λ5008 line luminosities in the range 3 × 108–1010 L⊙, comparable to those of luminous (-27 < MB < -23) quasars; this, along with other evidence, suggests that the objects in the luminous subsample are type II quasars.

Ultraviolet variability of NGC 5548 - Dynamics of the continuum production region and geometry of the broad-line region

Data from the 1989-1990 IUE monitoring of the Seyfert galaxy NGC 5548 are used here to analyze the continuum variability properties of the galaxy and to derive the structure or its emission-line region. The mean shape of the UV continuum is well fit by an accretion disk model with a given black hole mass and an additional component required to reproduce the observed soft X-ray flux. The continuum fluctuation power spectrum is very steep, with most of the variance coming from about 1 yr time scales. The entire optical/UV continuum rises and falls almost simultaneously, so that the logarithmic slope of the power spectrum is nearly the same for all bands, but the flux at higher photon frequencies varies with larger amplitude. The emission-line material around the nucleus may best be described by a highly ionized inner zone of high and nearly constant pressure that stretches about 4-14 light-days from the center and an outer, more weakly ionized zone of considerably lower ionization at least 20-30 light-days out. 44 refs.

Magnetic Stress at the Marginally Stable Orbit: Altered Disk Structure, Radiation, and Black Hole Spin Evolution

Magnetic connections to the plunging region can exert stresses on the inner edge of an accretion disk around a black hole. We recompute the relativistic corrections to the thin-disk dynamics equations when these stresses take the form of a time-steady torque on the inner edge of the disk. The additional dissipation associated with these stresses is concentrated relatively close outside the marginally stable orbit, scaling as r-7/2 at large radius. As a result of these additional stresses, spin-up of the central black hole is retarded; the maximum spin-equilibrium accretion efficiency is 36% and occurs at a/M = 0.94; the disk spectrum is extended toward higher frequencies; line profiles (such as Fe Kα) are broadened if the line emissivity scales with local flux; limb brightening, especially at the higher frequencies, is enhanced; and the returning radiation fraction is substantially increased, up to 58%. This last effect creates possible explanations for both synchronized continuum fluctuations in active galactic nuclei and polarization rises shortward of the Lyman edge in quasars. We show that no matter what additional stresses occur, when a/M < 0.36, the second law of black hole dynamics sets an absolute upper bound on the accretion efficiency.

Warm Absorbers in Active Galactic Nuclei: A Multitemperature Wind

Although soft X-ray absorption features in AGNs were discovered almost ten years ago, the nature and location of the gas creating them has remained controversial. Guided by the results of recent high-resolution X-ray spectroscopy, we argue that these features are created in a multitemperature wind whose source of matter is photoionized evaporation from the inner edge of the obscuring torus often found surrounding an AGN. Photoionized evaporation in the presence of a copious mass source locks the ratio of ionizing intensity to pressure to a critical value. However, a broad range of temperatures can coexist in equilibrium for this value of the ratio of ionizing intensity to pressure. Consequently, the flow is expected to be strongly inhomogeneous in temperature. The inferred distance of this material from the source of ionizing radiation depends on how much matter exists at the highest obtainable temperature. This distance can be measured by monitoring how ionic column densities respond to changes in the ionizing continuum on timescales of days to years.

Unusual broad absorption line quasars from the Sloan Digital Sky Survey

The Sloan Digital Sky Survey has confirmed the existence of populations of broad absorption line (BAL) quasars with various unusual properties. We present and discuss twenty-three such objects and consider the implications of their wide range of properties for models of BAL outflows and quasars in general. We have discovered one BAL quasar with a record number of absorption lines. Two other similarly complex objects with many narrow troughs show broad Mgii absorption extending longward of their systemic host galaxy redshifts. This can be explained as absorption of an extended continuum source by the rotation-dominated base of a disk wind. Five other objects have absorption which removes an unprecedented ∼90% of all flux shortward of Mgii. The absorption in one of them has varied across the ultraviolet with an amplitude and rate of change as great as ever seen. This same object may also show broad Hβ absorption. Numerous reddened BAL quasars have been found, including at least one reddened mini-BAL quasar with very strong Feii emission. The five reddest objects have continuum reddenings of E(B − V ) ≃ 0.5, and in two of them we find strong evidence that the reddening curve is even steeper than that of the SMC. We have found at least one object with absorption from Feiii but not Feii. This may be due to a high column density of moderately high-ionization gas, but the Feiii level populations must also be affected by some sort of resonance. Finally, we have found two luminous, probably reddened high-redshift objects which may be BAL quasars whose troughs partially cover different regions of the continuum source as a function of velocity.

Magnetically driven accretion in the kerr metric. III. Unbound outflows

We have carried out fully relativistic numerical simulations of accretion disks in the Kerr metric. In this paper we focus on the unbound outflows that emerge self-consistently from the accretion flow. These outflows are found in the axial funnel region and consist of two components: a hot, fast, tenuous outflow in the axial funnel proper and a colder, slower, denser jet along the funnel wall. The funnel-wall jet is excluded from the axial funnel by elevated angular momentum and is also pressure-confined by a magnetized corona. Inside the funnel, a large-scale poloidal magnetic field spontaneously arises from the coupled dynamics of accretion and outflow, although there was no large-scale field in the initial state. Black hole rotation is not required to produce these unbound outflows, but their strength is enhanced by black hole spin. When the black hole spins rapidly, the energy ejected can be tens of percent of the accreted rest mass. At low spin, kinetic energy and enthalpy of the matter dominate the outflow energetics; at high spin, the balance shifts to Poynting flux. We compare the outflows observed in our simulations with those seen in other simulations.