Alan P. Lightman

Effects of cold matter in active galactic nuclei: a broad hump in the X-ray spectra

Recent observations and interpretations of the strong UV emission from active galactic nuclei (AGNs) suggest that relatively cold, thermal matter coexists with the hot, X-ray-emitting matter near the centers of these objects. A fraction of the X-rays will be reprocessed by the cold material, and the composite X-ray spectrum should help diagnose the conditions of this material and its energy source. In a variety of situations, reprocessing of the X-rays should lead to a composite X-ray spectrum with a broad hump between about 10 keV and about 300 keV. The lower limit of this energy range is determined by atomic absorption and the upper limit by electron scattering in the cold material. Where available, observed spectra are consistent with such a broad hump; however, the predicted amplitude of the hump is about 0.1-0.5, and observations with smaller error bars are clearly needed. 30 references.

Pair production and Compton scattering in compact sources and comparison to observations of active galactic nuclei

The steady-state particle and photon distributions are determined under the assumption of continuous injection of relativistic electrons and low-energy photons throughout a spherical volume. Consideration is given to the problem of electron-positron pair cooling down to subrelativistic energies before thermalizing and annihilating. An earlier investigation of this problem is improved upon and results are compared to the data from AGNs. 51 references.

The X-ray structure of Centaurus A

The Einstein X-ray observatory imaging detectors have found X-ray emission associated with several components of the nearby radio galaxy Cen A = NGC 5128: (1) the compact nucleus; (2) an X-ray jet pointed toward the NE radio lobes; (3) the middle NE radio lobe; (4) the disk or dust lane; and (5) diffuse emission extending several arcmin around the nucleus. The intensity of the nucleus changed by a factor of seven over six months. The X-ray jet is considered in terms of thermal, inverse Compton, and synchrotron models. The emission of the NE radio lobe is greater than that expected from inverse Compton or synchrotron processes. Two ridges of emission are found along each edge of the dust lane, within several arcmin of the nucleus. The diffuse X-ray component has a luminosity which is too high to be due to bulge population X-ray sources, but which may be produced by main sequence stars under appropriate circumstances.

Magnetic viscosity in relativistic accretion disks

We formulate a detailed, self-consistent model of magnetic viscosity in an accretion disk around a rotating, Kerr black hole, in order to help clarify the nature of viscosity in the theory of accretion disks. The magnetic field is amplified by shear and dissipated by reconnection; in steady state a stable balance occurs, leading to a chaotic field configuration consisting of magnetic cells that reconnect with each other about once per orbit. We give detailed models of stationary disks based on the viscosity of this field. The observable properties of these models are similar to those of previous models based on less detailed models of viscosity. We include the relativistic equations governing the time-evolution of an accretion disk around a Kerr black hole, and some criteria for the existence of a radiation-pressure-dominated region or an optically thin region in a stationary disk.

Black holes in X-ray binaries: Marginal existence and rotation reversals of accretion disks

We adopt the hypothesis that black holes in X-ray binaries are accreting gas from an accretion disk fed by a stellar wind emanating from the primary. The formation of such a disk is found to be marginal in the case of a steady, spherically symmetric stellar wind and may be controlled by random fluctuations in the wind parameters. Such fluctuations may cause reversals in the rotation direction of the disk. Reversals in a disk around a rapidly rotating black hole lead to two distinct intensity states and may be associated with the two states observed for Cyg X-1. (AIP)

Compton reflection of gamma rays by cold electrons

Using a Monte Carlo method an approximation was derived to the Green function for the Compton reflection of relativistic photon h nu greater than about m(e) c-sq, by cold electrons, extending previous treatments of this problem for nonrelativistic photons. In Compton reflection, an external flux of photons impinges upon a semiinfinite slab of electrons, gives a portion of its energy to the electrons (or receives energy if the electrons are hotter), and reemerges with an altered spectrum. This Green function may be used to easily calculate the emergent spectrum and albedo for any incident spectrum. Such reprocessing of gamma-rays may be astrophysically important when a source of gamma-rays coexists with a cool, nonrelativistic plasma, such as for nonthermal coronas above accretion disks, high energy emission near the surface of neutron stars, or a chaotic, two-phase medium near the centers of active galaxies. 26 references.

Hot accretion disks with electron-positron pairs

The hot thermal accretion disks of the 1970s are studied and consideration is given to the effects of electron-positron pairs, which were originally neglected. It is found that disks cooled by internally produced photons have a critical accretion rate above which equilibrium is not possible in a radial annulus centered around r = 10 GM/c-squared, where M is the mass of the central object. This confirms and extends previous work by Kusunose and Takahara. Above the critical rate, pairs are created more rapidly than they can be destroyed. Below the critical rate, there are two solutions to the disk structure, one with a high pair density and one with a low pair density. Depending on the strength of the viscosity, the critical accretion rate corresponds to a critical luminosity of about 3-10 percent of the Eddington limit. 32 refs.

Cygnus x-1: a two-temperature accretion disk model which explains the observed hard X-ray spectrum

We present a model for Cygnus X-1, involving an accretion disk around a black hole, which fits the observed X-ray spectrum from 8 to 500 keV. The hard component of the spectrum (E>8 keV) is produced by an optically thin inner portion of the disk in which the electrons are 10

The distribution of stars around a massive black hole

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X-ray flares in NGC 4151: A thermal model and constraints on a central black hole

K and the ions are 3-- 300 times hotter.