Arieh Konigl

Disk-driven hydromagnetic winds as a key ingredient of active galactic nuclei unification schemes

Centrifugally driven winds from the surfaces of magnetized accretion disks have been recognized as an attractive mechanism of removing the angular momentum of the accreted matter and of producing the bipolar outflows and jets that are often associated with compact astronomical objects. As previously suggested in the context of young stellar objects, such winds have unique observational manifestations stemming from their highly stratified density and velocity structure and from their exposure to the strong continuum radiation field of the compact object. We have applied this scenario to active galactic nuclei (AGNs) and investigated the properties of hydromagnetic outflows that originate within approximately 10(M(sub 8)) pc of the central 10(exp 8)(M(sub 8)) solar mass black hole. On the basis of our results, we propose that hydromagnetic disk-driven winds may underlie the classification of broad-line and narrow-line AGNs (e.g., the Seyfert 1/Seyfert 2 dichotomy) as well as the apparent dearth of luminous Seyfert 2 galaxies. More generally, we demonstrate that such winds could strongly influence the spectral characteristics of Seyfert galaxies, QSOs, and BL Lac objects (BLOs). In our picture, the torus is identified with the outer regions of the wind where dust uplifted from the disk surfaces by gas-grain collisions is embedded in the outflow. Using an efficient radiative transfer code, we show that the infrared emission of Seyfert galaxies and QSOs can be attributed to the reprocessing of the UV/soft X-ray AGN continuum by the dust in the wind and the disk. We demonstrate that the radiation pressure force flattens the dust distribution in objects with comparatively high (but possibly sub-Eddington) bolometric luminosities, and we propose this as one likely reason for the apparent paucity of narrow-line objects among certain high-luminosity AGNs. Using the XSTAR photoionization code, we show that the inner regions of the wind could naturally account for the warm (greater than or approximately equal to 10(exp 5) K) and hot (greater than or approximately equal to 10(exp 6) K) gas components that have been inferred to exist on scales less than or approximately equal to 10(exp 2) pc in several Seyfert galaxies. We suggest that the partially ionized gas in the inner regions of the wind, rather than the dusty, neutral outflow that originates further out in the disk, could account for the bulk of the X-ray absorption in Seyferts observed at relatively small angles to their symmetry axes. Finally, we discuss the application of this model to the interpretation of the approximately 0.6 keV X-ray absorption feature reported in several BLOs.

Magnetic acceleration of ultrarelativistic jets in gamma-ray burst sources

We present numerical simulations of axi symmetric, magnetically driven outflows that reproduce the inferred properties of ultrarelativistic gamma-ray burst (GRB) jets. These results extend our previous simulations of outflows accelerated to moderately relativistic speeds, which are applicable to jets of active galactic nuclei. In contrast to several recent investigations, which have employed the magnetodynamics approximation, our numerical scheme solves the full set of equations of special relativistic, ideal magnetohydrodynamics, which enables us to explicitly calculate the jet velocity and magnetic-to-kinetic energy conversion efficiency - key parameters of interest for astrophysical applications. We confirm that the magnetic acceleration scheme remains robust into the ultrarelativistic regime, as previously indicated by semi-analytic self-similar solutions. We find that all current-carrying outflows exhibit self-collimation and consequent acceleration near the rotation axis, but that unconfined outflows lose causal connectivity across the jet and therefore do not collimate or accelerate efficiently in their outer regions. We show that magnetically accelerated jets confined by an external pressure that varies as z -α (0 1), and we obtain analytic expressions for the one-to-one correspondence between the pressure distribution and the asymptotic jet shape. We demonstrate that the acceleration efficiency of jets with paraboloidal streamlines is ≥50 per cent, with the numerical value being higher the lower the initial magnetization. We derive asymptotic analytic expressions for the acceleration of initially cold outflows along paraboloidal streamlines and verify that they provide good descriptions of the simulated flows. Our modelled jets (corresponding to 3/2 < a < 3) attain Lorentz factors Γ ≥ 10 2 on scales ∼ 10 10 -10 12 cm, consistent with the possibility that long/soft GRB jets are accelerated within envelopes of collapsing massive stars, and r ≥ 30 on scales ∼9 x 10 8 -3 x 10 10 cm, consistent with the possibility that short/hard GRB jets are accelerated on scales where they can be confined by moderately relativistic winds from accretion discs. We also find that Γθ v ∼ 1 for outflows that undergo an efficient magnetic-to-kinetic energy conversion, where θ v is the opening half-angle of the poloidal streamlines. This relation implies that the γ-ray emitting components of GRB outflows accelerated in this way are very narrow, with θ v ≤ 1° in regions where Γ ≥ 100, and that the afterglow light curves of these components would either exhibit a very early jet break or show no jet break at all.

Relativistic Magnetohydrodynamics with Application to Gamma-Ray Burst Outflows. I. Theory and Semianalytic Trans-Alfvénic Solutions

We present a general formulation of special relativistic magnetohydrodynamics and derive exact radially self-similar solutions for axisymmetric outflows from strongly magnetized, rotating compact objects. We generalize previous work by including thermal effects and analyze in detail the various forces that guide, accelerate, and collimate the flow. We demonstrate that, under the assumptions of a quasi-steady poloidal magnetic field and of a highly relativistic poloidal velocity, the equations become effectively time independent and the motion can be described as a frozen pulse. We concentrate on trans-Alfv?nic solutions and consider outflows that are super-Alfv?nic throughout in the companion paper. Our results are applicable to relativistic jets in gamma-ray burst (GRB) sources, active galactic nuclei, and microquasars, but our discussion focuses on GRBs. We envision the outflows in this case to initially consist of a hot and optically thick mixture of baryons, electron-positron pairs, and photons. We show that the flow is at first accelerated thermally but that the bulk of the acceleration is magnetic, with the asymptotic Lorentz factor corresponding to a rough equipartition between the Poynting and kinetic energy fluxes (i.e., ~50% of the injected total energy is converted into baryonic kinetic energy). The electromagnetic forces also strongly collimate the flow, giving rise to an asymptotically cylindrical structure.

Magnetic Driving of Relativistic Outflows in Active Galactic Nuclei. I. Interpretation of Parsec-Scale Accelerations

There is growing evidence that relativistic jets in active galactic nuclei undergo extended (parsec-scale) acceleration. We argue that, contrary to some suggestions in the literature, this acceleration cannot be purely hydrodynamic. Using exact semianalytic solutions of the relativistic MHD equations, we demonstrate that the parsec-scale acceleration to relativistic speeds inferred in sources such as the radio galaxy NGC 6251 and the quasar 3C 345 can be attributed to magnetic driving. Additional observational implications of this model will be explored in future papers in this series.

Self-similar models of magnetized accretion disks

Steady state models of magnetized accretion disks that power centrifugally driven outflows are presented. Self-similar solutions for Keplerian disks that are dominated by ambipolar diffusion are constructed and the analogous solutions for resistive disks are presented. In each case, it is shown that one can obtain steady state field configurations that match onto magnetically dominated, asymptotically super-Alfvenic wind solutions of the type considered by Blandford and Payne (1982). The application of these results to molecular disks associated with bipolar outflow sources is considered. 61 refs.

Implications of the early X‐ray afterglow light curves of Swift gamma‐ray bursts

According to current models, gamma-ray bursts (GRBs) are produced when the energy carried by a relativistic outflow is dissipated and converted into radiation. The efficiency of this process, ∈ γ , is one of the critical factors in any GRB model. The X-ray afterglow light curves of Swift GRBs show an early stage of flattish decay. This has been interpreted as reflecting energy injection. When combined with previous estimates, which have concluded that the kinetic energy of the late (?10 h) afterglow is comparable to the energy emitted in γ-rays, this interpretation implies very high values of ∈ γ , corresponding to?90 per cent of the initial energy being converted into γ-rays. Such a high efficiency is hard to reconcile with most models, including in particular the popular internal-shocks model. We re-analyse the derivation of the kinetic energy from the afterglow X-ray flux and re-examine the resulting estimates of the efficiency. We confirm that, if the flattish decay arises from energy injection and the pre-Swift broad-band estimates of the kinetic energy are correct, then ∈ γ ? 0.9. We discuss various issues related to this result, including an alternative interpretation of the light curve in terms of a two-component outflow model, which we apply to the X-ray observations of GRB 050315. We point out, however, that another interpretation of the flattish decay - a variable X-ray afterglow efficiency (e.g. due to a time dependence of afterglow shock microphysical parameters) - is possible. We also show that direct estimates of the kinetic energy from the late X-ray afterglow flux are sensitive to the assumed values of the shock microphysical parameters and suggest that broad-band afterglow fits might have underestimated the kinetic energy (e.g. by overestimating the fraction of electrons that are accelerated to relativistic energies). Either one of these possibilities implies a lower γ-ray efficiency, and their joint effect could conceivably reduce the estimate of the typical e y to a value in the range ∼0.1-0.5.

The radiative deceleration of ultrarelativistic jets in active galactic nuclei

A detailed study of the dynamical interaction between a highly relativistic jet and the thermal radiation field from an AGN accretion disk is reported, and the Comptonized spectrum arising from this interaction is self-consistently determined. A simple model that captures the essential radiative and geometrical features of realistic disk configurations is presented, and the disk radiation field is calculated. The results confirm Phinneys (1987) suggestion that the thermal radiation field produced by accretion in an AGN could be very effective in decelerating ultrarelativistic jets that are accreted by electromagnetic or hydromagnetic forces closer to the central black hole. Terminal Lorentz factors are consistent with the values inferred in superluminal radio sources are readily produced in this model for plausible disk and jet parameters without additional acceleration in the interaction zone. A new interpretation of the hard X-ray component detected in BL Lac spectra is proposed. 55 refs.

Gamma-Ray Variability of the BL Lacertae Object Markarian 421

We report on the γ-ray variability of Mrk 421 at Eγ > 300 GeV during the 1995 season, and concentrate on the results of an intense multiwavelength observing campaign in the period April 20 to May 5, which included >100 MeV γ-ray, X-ray, extreme-ultraviolet, optical, and radio observations, some of which show evidence for correlated behavior. Rapid variations in the TeV γ-ray light curve with doubling and decay times of 1 day require a compact emission region and significant Doppler boosting. The TeV data reveal that the γ-ray emission is best characterized by a succession of rapid flares with a relatively low baseline level of steady emission.

Magnetically Linked Star-Disk Systems. I. Force-free Magnetospheres and Effects of Disk Resistivity

We consider disk accretion onto a magnetic star under the assumption that the stellar magnetic field permeates the disk and that the magnetosphere that lies between the disk and the star is force free. Using simplified axisymmetric models (both semianalytic and numerical), we study the time evolution of the magnetic field configuration induced by the relative rotation between the disk and the star. We show that, if both the star and the magnetosphere can be approximated as being perfectly conducting, then the behavior of the twisted field lines depends on the magnitude of the surface conductivity of the disk. For any given relative azimuthal speed ∆vφ between the disk and the star (measured at the disk surface), there is a maximum surface conductivity Σmax ∼ c/|∆vφ| such that, if the actual surface conductivity is smaller than Σmax, then a steady-state field configuration can be established, whereas for larger values no steady state is possible and the field lines inflate and open up when a critical twist angle (which for an initially dipolar field is ∼ 2 rad) is attained. We argue that thin astrophysical disks are likely to have surface conductivities that exceed the local Σmax except in regions where |δvφ| is ∗Also at the Enrico Fermi Institute, University of Chicago.We consider the interaction between a magnetic star and its circumstellar disk under the assumption that the stellar magnetic field permeates the disk and that the systems magnetosphere is force-free. Using simplified axisymmetric models (both semianalytic and numerical), we study the time evolution of the magnetic field configuration induced by the relative rotation between the disk and the star. We show that if both the star and the magnetosphere are perfectly conducting, then there is a maximum disk surface conductivity Σmax for which a steady state field configuration can be established. For larger values of conductivity, no steady state is possible, and the field lines inflate and effectively open up when a critical twist angle (which for an initially dipolar field is on the order of a few radian) is attained. We argue that for thin astrophysical disks, surface conductivities are likely to exceed the local Σmax except in the immediate vicinity of the corotation radius in a Keplerian disk. If the disk conductivity is high enough, then the radial magnetic field at the disk surface will become large and induce radial migration of the field lines across the disk. We find, however, that the radial diffusion in the disk is generally much slower than the field-line expansion in the magnetosphere, which suggests that the opening of the magnetosphere is achieved before the diffusive outward expulsion of the field lines from the disk can occur. The effects of magnetospheric inertial effects and of field-line reconnection are considered in the companion paper.

RELATIVISTIC MAGNETOHYDRODYNAMICS WITH APPLICATION TO GAMMA-RAY BURST OUTFLOWS. II. SEMIANALYTIC SUPER-ALFVENIC SOLUTIONS

We present exact radially self-similar solutions of special relativistic magnetohydrodynamics representing hot super-Alfv?nic outflows from strongly magnetized, rotating compact objects. We argue that such outflows can plausibly arise in gamma-ray burst (GRB) sources and demonstrate that, just as in the case of the trans-Alfv?nic flows considered in the companion paper, they can attain Lorentz factors that correspond to a rough equipartition between the Poynting and kinetic energy fluxes and become cylindrically collimated on scales compatible with GRB observations. As in the trans-Alfv?nic case, the initial acceleration is thermal, but, in contrast to the solutions presented in the companion paper, part of the enthalpy flux is transformed into Poynting flux during this phase. The subsequent, magnetically dominated acceleration can be significantly less rapid than in trans-Alfv?nic flows.