Radiation-Driven Warping: The Origin of Warps and Precession in Accretion Disks

A geometrically thin, optically thick, warped accretion disk with a central source of luminosity is subject to non-axisymmetric forces due to radiation pressure; the resulting torque acts to modify the warp. In a recent paper, \cite{pri96} used a local analysis to show that initially planar accretion disks are unstable to warping driven by radiation torque. Here we extend this work with a global analysis of the stable and unstable modes. We confirm Pringle's conclusion that thin centrally-illuminated accretion disks are generically unstable to warping via this mechanism; we discuss the time-evolution and likely steady-state of such systems and show specifically that this mechanism can explain the warping of the disk of water masers in NGC 4258 and the 164-day precession period of the accretion disk in SS 433. Radiation-driven warping and precession provides a robust mechanism for producing warped, precessing accretion disks in active galactic nuclei and X-ray binary systems.Comment: 16 pages, latex, 3 figure

Magnetospherically-trapped dust and a possible model for the unusual transits at WD 1145+017

The rapidly evolving dust and gas extinction observed towards WD 1145+017 has opened a real-time window onto the mechanisms for destruction-accretion of planetary bodies onto white dwarf stars, and has served to underline the importance of considering the dynamics of dust particles around such objects. Here it is argued that the interaction between (charged) dust grains and the stellar magnetic field is an important ingredient in understanding the physical distribution of infrared emitting particles in the vicinity of such white dwarfs. These ideas are used to suggest a possible model for WD 1145+017 in which the unusual transit shapes are caused by opaque clouds of dust trapped in the stellar magnetosphere. The model can account for the observed transit periodicities if the stellar rotation is near 4.5 h, as the clouds of trapped dust are then located near or within the co-rotation radius. The model requires the surface magnetic field to be at least around some tens of kG. In contrast to the eccentric orbits expected for large planetesimals undergoing tidal disintegration, the orbits of magnetospherically-trapped dust clouds are essentially circular, consistent with the observations.Comment: 5 pages, accepted to MNRAS Letter

An instability mechanism for particulate pipe flow

We present linear stability analysis for a simple model of particle-laden pipe flow. The model consists of a continuum approximation for the particles two-way coupled to the fluid velocity field via Stokes drag (Saffman 1962). We extend previous analysis in a channel (Klinkenberg et al. 2011) to allow for the initial distribution of particles to be inhomogeneous and in particular consider the effect of allowing the particles to be preferentially located around one radius in accordance with experimental observations. This simple modification of the problem is enough to alter the stability properties of the flow, and in particular can lead to a linear instability at experimentally realistic parameters. The results are compared to the experimental work of Matas et al. (2004a) and are shown to be consistent with the reported flow regimes.Comment: 15 pages, 11 figure

Magnetospherically-trapped dust and a possible model for the unusual transits at WD 1145+017

The rapidly evolving dust and gas extinction observed towards WD 1145+017 has opened a real-time window on to the mechanisms for destruction-accretion of planetary bodies on to white dwarf stars, and has served to underline the importance of considering the dynamics of dust particles around such objects. Here it is argued that the interaction between (charged) dust grains and the stellar magnetic field is an important ingredient in understanding the physical distribution of infrared emitting particles in the vicinity of such white dwarfs. These ideas are used to suggest a possible model for WD 1145+017 in which the unusual transit shapes are caused by opaque clouds of dust trapped in the stellar magnetosphere. The model can account for the observed transit periodicities if the stellar rotation is near 4.5 h, as the clouds of trapped dust are then located near or within the co-rotation radius. The model requires the surface magnetic field to be at least around some tens of kG. In contrast to the eccentric orbits expected for large planetesimals undergoing tidal disintegration, the orbits of magnetospherically-trapped dust clouds are essentially circular, consistent with the observations

The Stability of Magnetized Rotating Plasmas with Superthermal Fields

During the last decade it has become evident that the magnetorotational instability is at the heart of the enhanced angular momentum transport in weakly magnetized accretion disks around neutron stars and black holes. In this paper, we investigate the local linear stability of differentially rotating, magnetized flows and the evolution of the magnetorotational instability beyond the weak-field limit. We show that, when superthermal toroidal fields are considered, the effects of both compressibility and magnetic tension forces, which are related to the curvature of toroidal field lines, should be taken fully into account. We demonstrate that the presence of a strong toroidal component in the magnetic field plays a non-trivial role. When strong fields are considered, the strength of the toroidal magnetic field not only modifies the growth rates of the unstable modes but also determines which modes are subject to instabilities. We find that, for rotating configurations with Keplerian laws, the magnetorotational instability is stabilized at low wavenumbers for toroidal Alfven speeds exceeding the geometric mean of the sound speed and the rotational speed. We discuss the significance of our findings for the stability of cold, magnetically dominated, rotating fluids and argue that, for these systems, the curvature of toroidal field lines cannot be neglected even when short wavelength perturbations are considered. We also comment on the implications of our results for the validity of shearing box simulations in which superthermal toroidal fields are generated.Comment: 24 pages, 12 figures. Accepted for publication in ApJ. Sections 2 and 5 substantially expanded, added Appendix A and 3 figures with respect to previous version. Animations are available at http://www.physics.arizona.edu/~mpessah/research

Low Luminosity States of the Black Hole Candidate GX~339--4. II. Timing Analysis

Here we present timing analysis of a set of eight Rossi X-ray Timing Explorer (RXTE) observations of the black hole candidate GX 339-4 that were taken during its hard/low state. On long time scales, the RXTE All Sky Monitor data reveal evidence of a 240 day periodicity, comparable to timescales expected from warped, precessing accretion disks. On short timescales all observations save one show evidence of a persistent f approximately equal to 0.3 Hz QPO. The broad band (10^{-3}-10^2 Hz) power appears to be dominated by two independent processes that can be modeled as very broad Lorentzians with Q approximately less than 1. The coherence function between soft and hard photon variability shows that if these are truly independent processes, then they are individually coherent, but they are incoherent with one another. This is evidenced by the fact that the coherence function between the hard and soft variability is near unity between 0.005-10 Hz but shows evidence of a dip at f approximately equal to 1 Hz. This is the region of overlap between the broad Lorentzian fits to the PSD. Similar to Cyg X-1, the coherence also drops dramatically at frequencies approximately greater than 10 Hz. Also similar to Cyg X-1, the hard photon variability is seen to lag the soft photon variability with the lag time increasing with decreasing Fourier frequency. The magnitude of this time lag appears to be positively correlated with the flux of GX 339-4. We discuss all of these observations in light of current theoretical models of both black hole spectra and temporal variability.Comment: To Appear in the AStrophysical Journa

Inclination Effects and Beaming in Black Hole X-ray Binaries

We investigate the dependence of observational properties of black hole X-ray binaries on the inclination angle i of their orbits. We find the following: (1) Transient black hole binaries show no trend in their quiescent X-ray luminosities as a function of i, suggesting that the radiation is not significantly beamed. This is consistent with emission from an accretion disk. If the X-rays are from a jet, then the Lorentz factor gamma of the jet is less than 1.24 at the 90% confidence level. (2) The X-ray binary 4U1543-47 with i of order 21 degrees has a surprisingly strong fluorescent iron line in the high soft state. Quantifying an earlier argument by Park et al. (2004), we conclude that if the continuum X-ray emission in this source is from a jet, then gamma < 1.04. (3) None of the known binaries has cos i 75 degrees. This fact, plus the lack of eclipses among the 20 black hole binaries in our sample, strongly suggests at the 99.5% confidence level that systems with large inclination angles are hidden from view. The obscuration could be the result of disk flaring, as suggested by Milgrom (1978) for neutron star X-ray binaries. (4) Transient black hole binaries with i ~ 70-75 degrees have significantly more complex X-ray light curves than systems with i < 65 degrees. This may be the result of variable obscuration and/or variable height above the disk of the radiating gas.Comment: 26 pages, to appear in The Astrophysical Journal, vol. 624, May 1, 200