Search

AbstractWe present a unified model for the structure and appearance of accretion powered sources across their entire luminosity range from galactic X-ray binaries to luminous quasars, with emphasis on AGN and their phenomenology. Central to this model is the notion of MHD winds launched from the accretion disks that power these objects. These winds provide the matter that manifests as blueshifted absorption features s in the UV and X-ray spectra of a large fraction of these sources; furthermore, their density distribution in the poloidal plane determines the “appearance” (i.e. the column and velocity structure of these absorption features) as a function of the observer inclination angle. This work focuses on just the broadest characteristics of these objects; nonetheless, it provides scaling laws that allow one to reproduce within this model the properties of objects spanning a very wide luminosity range and viewed at different inclination angles, and trace them to a common underlying dynamical structure. Its general conclusion is that the AG phenomenology can be accounted for in terms of three parameters: The wind mass flux in units of the Eddington value,ṁ, the observer’s inclination angle θ And the logarithmic slope between the O/UV and X-ray fluxes αox· However, because of a significant correlation between αox and UV luminosity, we conclude that the AGN structure depends on only two parameters. Interestingly, the correlations implied by this model appear to extend to and consistent with the, characteristics of galactic X-ray sources, suggesting the presence of a truly unified underlying structure for accretion powered sources.

We present the two-dimensional ionization structure of self-similar magnetohydrodynamic winds off accretion disks around and irradiated by a central X-ray point source. On the basis of earlier observational clues and theoretical arguments, we focus our attention on a subset of these winds, namely those with radial density dependence n(r) [?] 1/r (r is the spherical radial coordinate). We employ the photoionization code XSTAR to compute the ionic abundances of a large number of ions of different elements and then compile their line-of-sight (LOS) absorption columns. We focus our attention on the distribution of the column density of the various ions as a function of the ionization parameter x (or equivalently r) and the angle th. Particular attention is paid to the absorption measure distribution (AMD), namely their hydrogen-equivalent column per logarithmic x interval, dN H/dlog x, which provides a measure of the winds' radial density profiles. For the chosen density profile n(r) [?] 1/r, the AMD is found to be independent of x, in good agreement with its behavior inferred from the X-ray spectra of several active galactic nuclei (AGNs). For the specific wind structure and X-ray spectrum, we also compute detailed absorption line profiles for a number of ions to obtain their LOS velocities, v [?] 100-300 km s-1 (at log x [?] 2-3) for Fe XVII and v [?] 1000-4000 km s-1 (at log x [?] 4-5) for Fe XXV, in good agreement with the observation. Our models describe the X-ray absorption properties of these winds with only two parameters, namely the mass-accretion rate $\dot{m}$ and the LOS angle th. The probability of obscuration of the X-ray ionizing source in these winds decreases with increasing $\dot{m}$ and increases steeply with the LOS inclination angle th. As such, we concur with previous authors that these wind configurations, viewed globally, incorporate all the requisite properties of the parsec scale "torii" invoked in AGN unification schemes. We indicate that a combination of the AMD and absorption line profile observations can uniquely determine these model parameters and their bearing on AGN population demographics.

We describe the results of numerical ``2.5-dimensional" MHD simulations of an initially unmagnetized disk orbiting a central point mass and responding to the continual generation of poloidal magnetic field by a secular source emulating the Poynting-Robertson (PR) drag on electrons in the vicinity of a luminous stellar or compact accreting object. The secular PR term has dual purpose both as the magnetic field source and the trigger of magnetorotational instability (MRI). The disk and surrounding hotter atmosphere fluids have finite resistivity, allowing the magnetic field to diffuse out of its generation sites, while at the same time the disk differential rotation twists the poloidal field and induces a substantial toroidal-field component. For moderate disk resistivity (diffusion timescales up to ~16 local dynamical times) and after ~100 orbits, the MRI allows the fluid of the disk inner edge along with its magnetic flux to flow toward the central point mass where a new, magnetized, nuclear disk forms. The toroidal field in this nuclear disk is amplified by differential rotation and, when near equipartition, unwinds vertically, producing episodic jetlike outflows. For diffusion times longer than the flow time, the poloidal field in the inner region cannot diffuse out; it grows linearly in time, as the outer sections of new poloidal loops are drawn outward by the MRI while their inner sections continue to accumulate onto the compact inner disk. However, for low resistivity (diffusion timescales larger than ~16 local dynamical times), the inflowing matter does not form a nuclear disk or jets and the linear growth of the poloidal magnetic field is interrupted after ~20 orbits because of magnetic reconnection and asymmetric outflows.

We reinvestigate the generation and accumulation of magnetic flux in optically thin accretion flows around active gravitating objects. The source of the magnetic field is the azimuthal electric current associated with the Poynting-Robertson drag on the electrons of the accreting plasma. This current generates magnetic field loops that open up because of the differential rotation of the flow. We show through simple numerical simulations that what regulates the generation and accumulation of magnetic flux near the center is the value of the plasma conductivity. Although the conductivity is usually considered to be effectively infinite for the fully ionized plasmas expected near the inner edge of accretion disks, the turbulence of those plasmas may actually render them much less conducting due to the presence of anomalous resistivity. We have discovered that if the resistivity is sufficiently high throughout the turbulent disk while it is suppressed inside its inner edge, an interesting steady state process is established: accretion carries and accumulates magnetic flux of one polarity inside the inner edge of the disk, whereas magnetic diffusion releases magnetic flux of the opposite polarity to large distances. In this scenario, magnetic flux of one polarity grows and accumulates at a steady rate in the region inside the inner edge and up to the point of equipartition when it becomes dynamically important. We argue that this inward growth and outward expulsion of oppositely directed magnetic fields that we propose could account for the ~30 minute cyclic variability observed in the Galactic microquasar GRS 1915+105.

Using the exact solution of the axisymmetric pulsar magnetosphere derived in a previous publication and the conservation laws of the associated MHD flow, we show that the Lorentz factor of the outflowing plasma increases linearly with distance from the light cylinder. Therefore, the ratio of the Poynting to particle energy flux, generically referred to as s, decreases inversely proportional to distance from a large value (typically [?]104) near the light cylinder to s [?] 1 at a transition distance Rtrans. Beyond this distance, the inertial effects of the outflowing plasma become important, and the magnetic field geometry must deviate from the almost monopolar form it attains between Rlc and Rtrans. We anticipate that this is achieved by collimation of the poloidal field lines toward the rotation axis, ensuring that the magnetic field pressure in the equatorial region will fall off faster than 1/R2 (R being the cylindrical radius). This leads both to a value s = ss [?] 1 at the nebular reverse shock at distance Rs (Rs [?] Rtrans) and to a component of the flow perpendicular to the equatorial component, as required by observation. The presence of the strong shock at R = Rs allows for the efficient conversion of kinetic energy into radiation. We speculate that the Crab pulsar is unique in requiring ss [?] 3 x 10-3 because of its small translational velocity, which allows for the shock distance Rs to grow to values [?]Rtrans.

We present, for the first time, the structure of the axisymmetric force-free magnetosphere of an aligned rotating magnetic dipole, in the case in which there exists a sufficiently large charge density (whose origin we do not question) to satisfy the ideal MHD condition, E * B=0, everywhere. The unique distribution of electric current along the open magnetic field lines that is required for the solution to be continuous and smooth is obtained numerically. With the geometry of the field lines thus determined, we compute the dynamics of the associated MHD wind. The main result is that the relativistic outflow contained in the magnetosphere is not accelerated to the extremely relativistic energies required for the flow to generate gamma rays. We expect that our solution will be useful as the starting point for detailed studies of pulsar magnetospheres under more general conditions, namely, when the force-free and/or the ideal MHD condition, E * B=0, are not valid in the entire magnetosphere. Based on our solution, we consider that the most likely positions of such an occurrence are the polar cap, the crossings of the zero space charge surface by open field lines, and the return current boundary, but not the light cylinder.

We show that the Poynting-Robertson drag effect in an optically thin advection-dominated accretion flow around active gravitating objects generates strong azimuthal electric currents that give rise to astrophysically significant magnetic fields. Although the mechanism is most effective in accreting compact objects, it also seems very promising as a way to account for the generation of stellar dipolar fields during the late protostellar collapse phase, when the star approaches the main sequence.

We obtain self-similar solutions that describe the gravitational collapse of nonrotating, isothermal, magnetic molecular cloud cores. We use simplifying assumptions but explicitly include the induction equation, and the semianalytic solutions we derive are the first to account for the effects of ambipolar diffusion and its critical dependence on the magnetic tension force following the formation of a central point mass. Our results demonstrate that, after the protostar first forms, ambipolar diffusion causes the magnetic flux to decouple in a growing region around the center. The decoupled field lines remain approximately stationary and drive a hydromagnetic C-shock that moves outward at a fraction of the speed of sound (typically a few tenths of a kilometer per second), reaching a distance of a few thousand AU at the end of the main accretion phase for a solar-mass star. We also show that, in the absence of field diffusivity, a contracting core will not give rise to a shock if, as is likely to be the case, the inflow speed near the origin is nonzero at the time of point-mass formation. Although the evolution of realistic molecular cloud cores will not be exactly self similar, our results reproduce the main qualitative features found in detailed core-collapse simulations.

We obtain self-similar solutions that describe the gravitational collapse of nonrotating, isothermal, magnetic molecular cloud cores. We use simplifying assumptions but explicitly include the induction equation, and the semianalytic solutions we derive are the first to account for the effects of ambipolar diffusion and its critical dependence on the magnetic tension force following the formation of a central point mass. Our results demonstrate that, after the protostar first forms, ambipolar diffusion causes the magnetic flux to decouple in a growing region around the center. The decoupled field lines remain approximately stationary and drive a hydromagnetic C-shock that moves outward at a fraction of the speed of sound (typically a few tenths of a kilometer per second), reaching a distance of a few thousand AU at the end of the main accretion phase for a solar-mass star. We also show that, in the absence of field diffusivity, a contracting core will not give rise to a shock if, as is likely to be the case, the inflow speed near the origin is nonzero at the time of point-mass formation. Although the evolution of realistic molecular cloud cores will not be exactly self similar, our results reproduce the main qualitative features found in detailed core-collapse simulations.