127 research outputs found
Improving the Efficiency and Effectiveness of Agri-environmental Policies for the Chesapeake Bay
Water Quality, Nonpoint Source Pollution, Policy Instrument, Chesapeake Bay, Environmental Economics and Policy, Resource /Energy Economics and Policy, Q25, Q58,
A Second Order Godunov Method for Multidimensional Relativistic Magnetohydrodynamics
We describe a new Godunov algorithm for relativistic magnetohydrodynamics
(RMHD) that combines a simple, unsplit second order accurate integrator with
the constrained transport (CT) method for enforcing the solenoidal constraint
on the magnetic field. A variety of approximate Riemann solvers are implemented
to compute the fluxes of the conserved variables. The methods are tested with a
comprehensive suite of multidimensional problems. These tests have helped us
develop a hierarchy of correction steps that are applied when the integration
algorithm predicts unphysical states due to errors in the fluxes, or errors in
the inversion between conserved and primitive variables. Although used
exceedingly rarely, these corrections dramatically improve the stability of the
algorithm. We present preliminary results from the application of these
algorithms to two problems in RMHD: the propagation of supersonic magnetized
jets, and the amplification of magnetic field by turbulence driven by the
relativistic Kelvin-Helmholtz instability (KHI). Both of these applications
reveal important differences between the results computed with Riemann solvers
that adopt different approximations for the fluxes. For example, we show that
use of Riemann solvers which include both contact and rotational
discontinuities can increase the strength of the magnetic field within the
cocoon by a factor of ten in simulations of RMHD jets, and can increase the
spectral resolution of three-dimensional RMHD turbulence driven by the KHI by a
factor of 2. This increase in accuracy far outweighs the associated increase in
computational cost. Our RMHD scheme is publicly available as part of the Athena
code.Comment: 75 pages, 28 figures, accepted for publication in ApJS. Version with
high resolution figures available from
http://jila.colorado.edu/~krb3u/Athena_SR/rmhd_method_paper.pd
Counts and Colors of Faint Galaxies in the U and R Bands
Ground-based counts and colors of faint galaxies in the U and R bands in one
field at high Galactic latitude are presented. Integrated over flux, a total of
1.2x10^5 sources per square degree are found to U=25.5 mag and 6.3x10^5 sources
per square degree to R=27 mag, with d log N/dm ~ 0.5 in the U band and d log
N/dm ~ 0.3 in the R band. Consistent with these number-magnitude curves,
sources become bluer with increasing magnitude to median U-R=0.6 mag at 24<U<25
mag and U-R=1.2 mag at 25 < R < 26 mag. Because the Lyman break redshifts into
the U band at z~3, at least 1.2x10^5 sources per square degree must be at
redshifts z<3. Measurable U-band fluxes of 73 percent of the 6.3x10^5 sources
per square degree suggest that the majority of these also lie at z < 3. These
results require an enormous space density of objects in any cosmological model.Comment: 17 pages, MNRAS in pres
Asymmetric Supernovae from Magneto-Centrifugal Jets
Strong toroidal magnetic fields generated in stellar collapse can generate
magneto-centrifugal jets in analogy to those found in simulations of black hole
accretion and explain why all core collapse supernovae are found to be
substantially asymmetric and predominantly bi-polar. We describe two phases:
the initial LeBlanc-Wilson jet and a subsequent protopulsar or toroidal jet
that propagates at about the core escape velocity. The jets will produce bow
shocks that tend to expel matter, including iron and silicon, into equatorial
tori, accounting for observations of the element distribution in Cas A. A
magnetic ``switch'' mechanism may apply in instances of low density and large
magnetic field with subsequent increase in the speed and collimation of the
toroidal jet, depositing relatively little momentum. The result could be enough
infall to form a black hole with a third, highly relativistic jet that could
catch up to the protopulsar jet after it has emerged from the star. The
interaction of these two jets could generate internal shocks and explain the
presence of iron lines in the afterglow. Recent estimates that typical
gamma-ray burst energy is about 3x10^50 erg imply either a very low efficiency
for conversion of rotation into jets, or a rather rapid turnoff of the jet
process even though the black hole still rotates rapidly. Magnetars and
``hypernovae'' might arise in an intermediate parameter regime of energetic
jets that yield larger magnetic fields and provide more energy than the routine
case, but that are not so tightly collimated that they yield failed supernova.
(slightly abridged)Comment: AASTeX, 29 pages, 2 postscript figures, accepted by ApJ, November 20,
200
Hydrodynamical Non-radiative Accretion Flows in Two-Dimensions
Two-dimensional (axially symmetric) numerical hydrodynamical calculations of
accretion flows which cannot cool through emission of radiation are presented.
The calculations begin from an equilibrium configuration consisting of a thick
torus with constant specific angular momentum. Accretion is induced by the
addition of a small anomalous azimuthal shear stress which is characterized by
a function \nu. We study the flows generated as the amplitude and form of \nu
are varied. A spherical polar grid which spans more than two orders of
magnitude in radius is used to resolve the flow over a wide range of spatial
scales. We find that convection in the inner regions produces significant
outward mass motions that carry away both the energy liberated by, and a large
fraction of the mass participating in, the accretion flow. Although the
instantaneous structure of the flow is complex and dominated by convective
eddies, long time averages of the dynamical variables show remarkable
correspondence to certain steady-state solutions. Near the equatorial plane,
the radial profiles of the time-averaged variables are power-laws with an index
that depends on the radial scaling of the shear stress. We find that regardless
of the adiabatic index of the gas, or the form or magnitude of the shear
stress, the mass inflow rate is a strongly increasing function of radius, and
is everywhere nearly exactly balanced by mass outflow. The net mass accretion
rate through the disc is only a fraction of the rate at which mass is supplied
to the inflow at large radii, and is given by the local, viscous accretion rate
associated with the flow properties near the central object.Comment: 33 pages, 12 figures, accepted by MNRA
How to Tell a Jet from a Balloon: A Proposed Test for Beaming in Gamma Ray Bursts
If gamma ray bursts are highly collimated, the energy requirements of each
event may be reduced by several (~ 4-6) orders of magnitude, and the event rate
increased correspondingly. Extreme conditions in gamma ray bursters lead to
highly relativistic motions (bulk Lorentz factors Gamma > 100). This results in
strong forward beaming of the emitted radiation in the observer's rest frame.
Thus, all information on gamma ray bursts comes from those ejecta emitted in a
narrow cone (opening angle 1/Gamma) pointing towards the observer. We are at
present ignorant of whether there are ejecta outside that cone or not.
The recent detection of longer wavelength transients following gamma ray
bursts allows an empirical test of whether gamma ray bursts are collimated jets
or spherical fireballs. The bulk Lorentz factor of the burst ejecta will
decrease with time after the event, as the ejecta sweep up the surrounding
medium. Thus, radiation from the ejecta is beamed into an ever increasing solid
angle as the burst remnant evolves. It follows that if gamma ray bursts are
highly collimated, many more optical and radio transients should be observed
without associated gamma rays than with them. Published supernova searches may
contain enough data to test the most extreme models of gamma ray beaming. We
close with a brief discussion of other possible consequences of beaming,
including its effect on the evolution of burst remnants.Comment: Original replaced with accepted refereed manuscript. 11 pages, uses
AASTeX 4.0 LaTeX macros. To be published in The Astrophysical Journal
Letters, vol. 487, p. L1 (20 September 1997
The External Shear Acting on Gravitational Lens B 1422+231
In a number of multiply imaged quasar systems, a significant contribution to
the lensing potential is provided by groups and clusters of galaxies associated
with the primary lens. As part of an ongoing effort to gather observational
data on these systems, we present spectroscopy and near-infrared and optical
photometry of galaxies in the field of the quadruple lens system B 1422+231.
The spectra show that the primary lens and five nearby galaxies belong to a
compact group at z = 0.338. The median projected radius of this group is 35
h^{-1} kpc and its velocity dispersion is 550 km/s. A straightforward
application of the virial theorem yields a group mass of 1.4 x 10^{13} h^{-1}
M(sun), which provides sufficient external shear to produce the observed image
configuration. This data rules out a class of models and improves the system's
prospects for a measurement of the Hubble constant.Comment: 16 pages including 3 tables, 2 eps figures and 2 jpeg images.
Submitted to the Astronomical Journa
Stellar-Mass Black Holes in the Solar Neighborhood
We search for nearby, isolated, accreting, ``stellar-mass'' (3 to
) black holes. Models suggest a synchrotron spectrum in visible
wavelengths and some emission in X-ray wavelengths. Of 3.7 million objects in
the Sloan Digital Sky Survey Early Data Release, about 150,000 objects have
colors and properties consistent with such a spectrum, and 87 of these objects
are X-ray sources from the ROSAT All Sky Survey. Thirty-two of these have been
confirmed not to be black-holes using optical spectra. We give the positions
and colors of these 55 black-hole candidates, and quantitatively rank them on
their likelihood to be black holes. We discuss uncertainties the expected
number of sources, and the contribution of blackholes to local dark matter.Comment: Replaced with version accepted by ApJ. 40 pages, 8 figure
XMM-EPIC observation of MCG-6-30-15: Direct evidence for the extraction of energy from aspinning black hole?
We present XMM-Newton European Photon Imaging Camera (EPIC) observations of
the bright Seyfert 1 galaxy MCG-6-30-15, focusing on the broad Fe K
line at ~6keV and the associated reflection continuum, which is believed to
originate from the inner accretion disk. We find these reflection features to
be extremely broad and red-shifted, indicating its origin from the very most
central regions of the accretion disk. It seems likely that we have caught this
source in the ``deep minimum'' state first observed by Iwasawa et al. (1996).
The implied central concentration of X-ray illumination is difficult to
understand in any pure accretion disk model. We suggest that we are witnessing
the extraction and dissipation of rotational energy from a spinning black hole
by magnetic fields connecting the black hole or plunging region to the disk.Comment: 6 pages and one postscript figure. Accepted for publication in MNRAS
letter
AGN effect on cooling flow dynamics
We analyzed the feedback of AGN jets on cooling flow clusters using
three-dimensional AMR hydrodynamic simulations. We studied the interaction of
the jet with the intracluster medium and creation of low X-ray emission
cavities (Bubbles) in cluster plasma. The distribution of energy input by the
jet into the system was quantified in its different forms, i.e. internal,
kinetic and potential. We find that the energy associated with the bubbles, (pV
+ gamma pV/(gamma-1)), accounts for less than 10 percent of the jet energy.Comment: "Accepted for publication in Astrophysics & Space Science
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