13 research outputs found
Radiative Transfer in Obliquely Illuminated Accretion Disks
The illumination of an accretion disk around a black hole or neutron star by
the central compact object or the disk itself often determines its spectrum,
stability, and dynamics. The transport of radiation within the disk is in
general a multi-dimensional, non-axisymmetric problem, which is challenging to
solve. Here, I present a method of decomposing the radiative transfer equation
that describes absorption, emission, and Compton scattering in an obliquely
illuminated disk into a set of four one-dimensional transfer equations. I show
that the exact calculation of the ionization balance and radiation heating of
the accretion disk requires the solution of only one of the one-dimensional
equations, which can be solved using existing numerical methods. I present a
variant of the Feautrier method for solving the full set of equations, which
accounts for the fact that the scattering kernels in the individual transfer
equations are not forward-backward symmetric. I then apply this method in
calculating the albedo of a cold, geometrically thin accretion disk.Comment: 16 pages, 3 figures; to appear in The Astrophysical Journa
Simulating Radiating and Magnetized Flows in Multi-Dimensions with ZEUS-MP
This paper describes ZEUS-MP, a multi-physics, massively parallel, message-
passing implementation of the ZEUS code. ZEUS-MP differs significantly from the
ZEUS-2D code, the ZEUS-3D code, and an early "version 1" of ZEUS-MP distributed
publicly in 1999. ZEUS-MP offers an MHD algorithm better suited for
multidimensional flows than the ZEUS-2D module by virtue of modifications to
the Method of Characteristics scheme first suggested by Hawley and Stone
(1995), and is shown to compare quite favorably to the TVD scheme described by
Ryu et. al (1998). ZEUS-MP is the first publicly-available ZEUS code to allow
the advection of multiple chemical (or nuclear) species. Radiation hydrodynamic
simulations are enabled via an implicit flux-limited radiation diffusion (FLD)
module. The hydrodynamic, MHD, and FLD modules may be used in one, two, or
three space dimensions. Self gravity may be included either through the
assumption of a GM/r potential or a solution of Poisson's equation using one of
three linear solver packages (conjugate-gradient, multigrid, and FFT) provided
for that purpose. Point-mass potentials are also supported. Because ZEUS-MP is
designed for simulations on parallel computing platforms, considerable
attention is paid to the parallel performance characteristics of each module.
Strong-scaling tests involving pure hydrodynamics (with and without
self-gravity), MHD, and RHD are performed in which large problems (256^3 zones)
are distributed among as many as 1024 processors of an IBM SP3. Parallel
efficiency is a strong function of the amount of communication required between
processors in a given algorithm, but all modules are shown to scale well on up
to 1024 processors for the chosen fixed problem size.Comment: Accepted for publication in the ApJ Supplement. 42 pages with 29
inlined figures; uses emulateapj.sty. Discussions in sections 2 - 4 improved
per referee comments; several figures modified to illustrate grid resolution.
ZEUS-MP source code and documentation available from the Laboratory for
Computational Astrophysics at http://lca.ucsd.edu/codes/currentcodes/zeusmp2
The Effects of Magnetic Fields and Inhomogeneities on Accretion Disk Spectra and Polarization
We present the results of one and three-dimensional radiative transfer
calculations of polarized spectra emerging from snapshots of radiation
magnetohydrodynamical simulations of the local vertical structure of black hole
accretion disks. The simulations cover a wide range of physical regimes
relevant for the high/soft state of black hole X-ray binaries. We constrain the
uncertainties in theoretical spectral color correction factors due to the
presence of magnetic support of the disk surface layers and strong density
inhomogeneities. For the radiation dominated simulation, magnetic support
increases the color correction factor by about ten percent, but this is largely
compensated by a ten percent softening due to inhomogeneities. We also compute
the effects of inhomogeneities and Faraday rotation on the resulting
polarization. Magnetic fields in the simulations are just strong enough to
produce significant Faraday depolarization near the spectral peak of the
radiation field. X-ray polarimetry may therefore be a valuable diagnostic of
accretion disk magnetic fields, being able to directly test simulations of
magnetorotational turbulence.Comment: 18 pages, accepted for publication in Ap
Heating of gas inside radio sources to mildly relativistic temperatures via induced Compton scattering
Measured values of the brightness temperature of low-frequency synchrotron
radiation emitted by powerful extragalactic sources reach 10^11--10^12 K. If
some amount of nonrelativistic ionized gas is present within such sources, it
should be heated as a result of induced Compton scattering of the radiation. If
this heating is counteracted by cooling due to inverse Compton scattering of
the same radio radiation, then the plasma can be heated up to mildly
relativistic temperatures kT~10--100 keV. The stationary electron velocity
distribution can be either relativistic Maxwellian or quasi-Maxwellian (with
the high-velocity tail suppressed), depending on the efficiency of Coulomb
collisions and other relaxation processes. We derive several easy-to-use
approximate expressions for the induced Compton heating rate of mildly
relativistic electrons in an isotropic radiation field, as well as for the
stationary distribution function and temperature of electrons. We also give
analytic expressions for the kernel of the integral kinetic equation (one as a
function of the scattering angle and another for the case of an isotropic
radiation field), which describes the redistribution of photons in frequency
caused by induced Compton scattering in thermal plasma. These expressions can
be used in the parameter range hnu<< kT<~ 0.1mc^2 (the formulae earlier
published in Sazonov, Sunyaev, 2000 are less accurate).Comment: 22 pages, 7 figures, submitted to Astronomy Letter
Supermassive Black Holes in Galactic Nuclei: Past, Present and Future Research
This review discusses the current status of supermassive black hole research,
as seen from a purely observational standpoint. Since the early '90s, rapid
technological advances, most notably the launch of the Hubble Space Telescope,
the commissioning of the VLBA and improvements in near-infrared speckle imaging
techniques, have not only given us incontrovertible proof of the existence of
supermassive black holes, but have unveiled fundamental connections between the
mass of the central singularity and the global properties of the host galaxy.
It is thanks to these observations that we are now, for the first time, in a
position to understand the origin, evolution and cosmic relevance of these
fascinating objects.Comment: Invited Review, 114 pages. Because of space requirements, this
version contains low resolution figures. The full resolution version can be
downloaded from http://www.physics.rutgers.edu/~lff/publications.htm