7 research outputs found
Multi-Wavelength Variability of the Synchrotron Self-Compton Model for Blazar Emission
Motivated by recent reports of strongly correlated radio and X-ray
variability in 3C279 (Grandi, etal 1995), we have computed the relative
amplitudes of variations in the synchrotron flux at and the self-Compton
X-ray flux at 1 keV () for a homogeneous sphere of relativistic
electrons orbiting in a tangled magnetic field. Relative to synchrotron
self-Compton scattering without induced Compton scattering, stimulated
scattering reduces the amplitude of by as much as an order of
magnitude when \tau_T \gtwid 1. When varies in a fixed magnetic
field, increases monotonically from 0.01 at , the
self-absorption turnover frequency, to at . The relative
amplitudes of the correlated fluctuations in the radio-mm and X-ray fluxes from
3C279 are consistent with the synchrotron self-Compton model if varies
in a fixed magnetic field and induced Compton scattering is the dominant source
of radio opacity. The variation amplitudes are are too small to be produced by
the passage of a shock through the synchrotron emission region unless the
magnetic field is perpendicular to the shock front.Comment: 21 pages, 4 fig
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 Vertical Structure and Ultraviolet Spectrum of Accretion Disks Heated by Internal Dissipation in Active Galactic Nuclei
We present an improved calculation of the vertical structure and ultraviolet
spectrum of a dissipative accretion disk in an AGN. We calculate model spectra
in which the viscous stress is proportional to the total pressure, the gas
pressure only and the geometric mean of the radiation and gas pressures (cf.
Laor & Netzer 1989: LN89). As a result of a more complete treatment of
absorptive opacity, we find greater overall spectral curvature than did LN89,
as well as larger amplitudes in both the Lyman and HeII photoionization edges.
The local black body approximation is not a good description of the near UV
spectrum. With relativistic corrections (appropriate to non-rotating black
holes) included, we find that the near UV spectrum hardens with increasing
m-dot / m_8 (m-dot is the accretion rate in Eddington units, m_8 the black hole
mass in units of 10^8 M_Sun). The near UV spectrum is consistent with
observations if m-dot/ m_8 \sim 10^{-3}, but disks this cold would have large,
and unobserved, absorption features at the Lyman edge. The edge amplitude is
reduced when m-dot/m_8 is larger, but then the near-UV slope is too hard to
match observations. We conclude that models in which conventional disks orbit
non-rotating black holes do not adequately explain UV continuum production in
AGN.Comment: AAS LaTe