663 research outputs found
Optimising Boltzmann codes for the Planck era
High precision measurements of the Cosmic Microwave Background (CMB)
anisotropies, as can be expected from the Planck satellite, will require
high-accuracy theoretical predictions as well. One possible source of
theoretical uncertainty is the numerical error in the output of the Boltzmann
codes used to calculate angular power spectra. In this work, we carry out an
extensive study of the numerical accuracy of the public Boltzmann code CAMB,
and identify a set of parameters which determine the error of its output. We
show that at the current default settings, the cosmological parameters
extracted from data of future experiments like Planck can be biased by several
tenths of a standard deviation for the six parameters of the standard
Lambda-CDM model, and potentially more seriously for extended models. We
perform an optimisation procedure that leads the code to achieve sufficient
precision while at the same time keeping the computation time within reasonable
limits. Our conclusion is that the contribution of numerical errors to the
theoretical uncertainty of model predictions is well under control -- the main
challenges for more accurate calculations of CMB spectra will be of an
astrophysical nature instead.Comment: 13 pages, 4 figure
Magnetohydrodynamic jets from different magnetic field configurations
Using axisymmetric MHD simulations we investigate how the overall jet
formation is affected by a variation in the disk magnetic flux profile and/or
the existence of a central stellar magnetosphere. Our simulations evolve from
an initial, hydrostatic equilibrium state in a force-free magnetic field
configuration. We find a unique relation between the collimation degree and the
disk wind magnetization power law exponent. The collimation degree decreases
for steeper disk magnetic field profiles. Highly collimated outflows resulting
from a flat profile tend to be unsteady. We further consider a magnetic field
superposed of a stellar dipole and a disk field in parallel or anti-parallel
alignment. Both stellar and disk wind may evolve in a pair of outflows,
however, a reasonably strong disk wind component is essential for jet
collimation. Strong flares may lead to a sudden change in mass flux by a factor
two. We hypothesize that such flares may eventually trigger jet knots.Comment: 5 pages, 4 figures; proceedings from conference: Protostellar Jets in
Context, held in Rhodes, July 7-12, 200
Pico: Parameters for the Impatient Cosmologist
We present a fast, accurate, robust and flexible method of accelerating
parameter estimation. This algorithm, called Pico, can compute the CMB power
spectrum and matter transfer function as well as any computationally expensive
likelihoods in a few milliseconds. By removing these bottlenecks from parameter
estimation codes, Pico decreases their computational time by 1 or 2 orders of
magnitude. Pico has several important properties. First, it is extremely fast
and accurate over a large volume of parameter space. Furthermore, its accuracy
can continue to be improved by using a larger training set. This method is
generalizable to an arbitrary number of cosmological parameters and to any
range of l-values in multipole space. Pico is approximately 3000 times faster
than CAMB for flat models, and approximately 2000 times faster then the WMAP 3
year likelihood code. In this paper, we demonstrate that using Pico to compute
power spectra and likelihoods produces parameter posteriors that are very
similar to those using CAMB and the official WMAP3 code, but in only a fraction
of the time. Pico and an interface to CosmoMC are made publicly available at
http://www.astro.uiuc.edu/~bwandelt/pico/.Comment: 9 pages, 10 figures, submitted to ApJ, LaTeX with emulateap
Accretion-Powered Stellar Winds II: Numerical Solutions for Stellar Wind Torques
[Abridged] In order to explain the slow rotation observed in a large fraction
of accreting pre-main-sequence stars (CTTSs), we explore the role of stellar
winds in torquing down the stars. For this mechanism to be effective, the
stellar winds need to have relatively high outflow rates, and thus would likely
be powered by the accretion process itself. Here, we use numerical
magnetohydrodynamical simulations to compute detailed 2-dimensional
(axisymmetric) stellar wind solutions, in order to determine the spin down
torque on the star. We explore a range of parameters relevant for CTTSs,
including variations in the stellar mass, radius, spin rate, surface magnetic
field strength, the mass loss rate, and wind acceleration rate. We also
consider both dipole and quadrupole magnetic field geometries.
Our simulations indicate that the stellar wind torque is of sufficient
magnitude to be important for spinning down a ``typical'' CTTS, for a mass loss
rate of yr. The winds are wide-angle,
self-collimated flows, as expected of magnetic rotator winds with moderately
fast rotation. The cases with quadrupolar field produce a much weaker torque
than for a dipole with the same surface field strength, demonstrating that
magnetic geometry plays a fundamental role in determining the torque. Cases
with varying wind acceleration rate show much smaller variations in the torque
suggesting that the details of the wind driving are less important. We use our
computed results to fit a semi-analytic formula for the effective Alfv\'en
radius in the wind, as well as the torque. This allows for considerable
predictive power, and is an improvement over existing approximations.Comment: Accepted for publication in Ap
Particle acceleration close to the supermassive black hole horizon: the case of M87
The radio galaxy M87 has recently been found to be a rapidly variable TeV
emitting source. We analyze the implications of the observed TeV
characteristics and show that it proves challenging to account for them within
conventional acceleration and emission models. We discuss a new pulsar-type
scenario for the origin of variable, very high energy (VHE) emission close to
the central supermassive black hole and show that magneto-centrifugally
accelerated electrons could efficiently Compton upscatter sub-mm ADAF disk
photons to the TeV regime, leading to VHE characteristics close to the observed
ones. This suggests, conversely, that VHE observations of highly under-luminous
AGNs could provide an important diagnostic tool for probing the conditions
prevalent in the inner accretion disk of these sources.Comment: 5 pages, one figure (typos corrected); based on presentation at "High
Energy Phenomena in Relativistic Outflows", Dublin, Sept. 2007; accepted for
publication in International Journal of Modern Physics
Ultra-Relativistic Magneto-Hydro-Dynamic Jets in the context of Gamma Ray Bursts
We present a detailed numerical study of the dynamics and evolution of
ultrarelativistic magnetohydrodynamic jets in the black hole-disk system under
extreme magnetization conditions. We find that Lorentz factors of up to 3000
are achieved and derived a modifiedMichel scaling (Gamma ~ sigma) which allows
for a wide variation in the flow Lorentz factor. Pending contamination induced
by mass-entrainment, the linear Michel scaling links modulations in the
ultrarelativistic wind to variations in mass accretion in the disk for a given
magnetization. The jet is asymptotically dominated by the toroidal magnetic
field allowing for efficient collimation. We discuss our solutions (jets) in
the context of Gamma ray bursts and describe the relevant features such as the
high variability in the Lorentz factor and how high collimation angles (~ 0-5
degrees), or cylindrical jets, can be achieved. We isolate a jet instability
mechanism we refer to as the "bottle-neck" instability which essentially relies
on a high magnetization and a recollimation of the magnetic flux surfaces. The
instability occurs at large radii where any dissipation of the magnetic energy
into radiation would in principle result in an optically thin emission.Comment: 31 pages, 6 figures. Submitted to ApJ. Higher Quality figures at
http://www.capca.ucalgary.ca/paper
Dynamics and Structure of Three-Dimensional Trans-Alfvenic Jets. II. The Effect of Density and Winds
Two three-dimensional magnetohydrodynamical simulations of strongly
magnetized conical jets, one with a poloidal and one with a helical magnetic
field, have been performed. In the poloidal simulation a significant sheath
(wind) of magnetized moving material developed and partially stabilized the jet
to helical twisting. The fundamental pinch mode was not similarly affected and
emission knots developed in the poloidal simulation. Thus, astrophysical jets
surrounded by outflowing winds could develop knotty structures along a straight
jet triggered by pinching. Where helical twisting dominated the dynamics,
magnetic field orientation along the line-of-sight could be organized by the
toroidal flow field accompanying helical twisting. On astrophysical jets such
structure could lead to a reversal of the direction of Faraday rotation in
adjacent zones along a jet. Theoretical analysis showed that the different
dynamical behavior of the two simulations could be entirely understood as a
result of dependence on the velocity shear between jet and wind which must
exceed a surface Alfven speed before the jet becomes unstable to helical and
higher order modes of jet distortion.Comment: 25 pages, 15 figures, in press Astrophysical Journal (September
Rico: An Accurate Cosmological Recombination Code
We present Rico, a code designed to compute the ionization fraction of the
Universe during the epoch of hydrogen and helium recombination with an
unprecedented combination of speed and accuracy. This is accomplished by
training the machine learning code Pico on the calculations of a multi-level
cosmological recombination code which self-consistently includes several
physical processes that were neglected previously. After training, Rico is used
to fit the free electron fraction as a function of the cosmological parameters.
While, for example at low redshifts (z<~900), much of the net change in the
ionization fraction can be captured by lowering the hydrogen fudge factor in
Recfast by about 3%, Rico provides a means of effectively using the accurate
ionization history of the full recombination code in the standard cosmological
parameter estimation framework without the need to add new or refined fudge
factors or functions to a simple recombination model. Within the new approach
presented here it is easy to update Rico whenever a more accurate full
recombination code becomes available. Once trained, Rico computes the
cosmological ionization history with negligible fitting error in ~10
milliseconds, a speed-up of at least 10^6 over the full recombination code that
was used here. Also Rico is able to reproduce the ionization history of the
full code to a level well below 0.1%, thereby ensuring that the theoretical
power spectra of CMB fluctuations can be computed to sufficient accuracy and
speed for analysis from upcoming CMB experiments like Planck. Furthermore it
will enable cross-checking different recombination codes across cosmological
parameter space, a comparison that will be very important in order to assure
the accurate interpretation of future cosmic microwave background data.Comment: 14 pages, 11 figures, submitted to PR
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