138 research outputs found
Should One Use the Ray-by-Ray Approximation in Core-Collapse Supernova Simulations?
We perform the first self-consistent, time-dependent, multi-group
calculations in two dimensions (2D) to address the consequences of using the
ray-by-ray+ transport simplification in core-collapse supernova simulations.
Such a dimensional reduction is employed by many researchers to facilitate
their resource-intensive calculations. Our new code (F{\sc{ornax}}) implements
multi-D transport, and can, by zeroing out transverse flux terms, emulate the
ray-by-ray+ scheme. Using the same microphysics, initial models, resolution,
and code, we compare the results of simulating 12-, 15-, 20-, and
25-M progenitor models using these two transport methods. Our
findings call into question the wisdom of the pervasive use of the ray-by-ray+
approach. Employing it leads to maximum post-bounce/pre-explosion shock radii
that are almost universally larger by tens of kilometers than those derived
using the more accurate scheme, typically leaving the post-bounce matter less
bound and artificially more "explodable." In fact, for our 25-M
progenitor, the ray-by-ray+ model explodes, while the corresponding multi-D
transport model does not. Therefore, in two dimensions the combination of
ray-by-ray+ with the axial sloshing hydrodynamics that is a feature of 2D
supernova dynamics can result in quantitatively, and perhaps qualitatively,
incorrect results.Comment: Updated and revised text; 13 pages; 13 figures; Accepted to Ap.
Pair Production in Low Luminosity Galactic Nuclei
Electron-positron pairs may be produced near accreting black holes by a
variety of physical processes, and the resulting pair plasma may be accelerated
and collimated into a relativistic jet. Here we use a self-consistent dynamical
and radiative model to investigate pair production by \gamma\gamma collisions
in weakly radiative accretion flows around a black hole of mass M and accretion
rate \dot{M}. Our flow model is drawn from general relativistic
magnetohydrodynamic simulations, and our radiation field is computed by a Monte
Carlo transport scheme assuming the electron distribution function is thermal.
We argue that the pair production rate scales as r^{-6} M^{-1} \dot{M}^{6}. We
confirm this numerically and calibrate the scaling relation. This relation is
self-consistent in a wedge in M, \dot{M} parameter space. If \dot{M} is too low
the implied pair density over the poles of the black hole is below the
Goldreich-Julian density and \gamma\gamma pair production is relatively
unimportant; if \dot{M} is too high the models are radiatively efficient. We
also argue that for a power-law spectrum the pair production rate should scale
with the observables L_X \equiv X-ray luminosity and M as L_X^2 M^{-4}. We
confirm this numerically and argue that this relation likely holds even for
radiatively efficient flows. The pair production rates are sensitive to black
hole spin and to the ion-electron temperature ratio which are fixed in this
exploratory calculation. We finish with a brief discussion of the implications
for Sgr A* and M87.Comment: 21 pages, 10 figures, 1 table. Accepted for publication in Ap
Pair production in low luminosity galactic nuclei
ABSTRACT We compute the distribution of pair production by γγ collisions in weakly radiative accretion flows around a black hole of mass M and accretion rateṀ . We use a flow model drawn from general relativistic magnetohydrodynamic simulations and a Monte Carlo radiation field that assumes the electron distribution function is thermal. We find that
Parthenon -- a performance portable block-structured adaptive mesh refinement framework
On the path to exascale the landscape of computer device architectures and
corresponding programming models has become much more diverse. While various
low-level performance portable programming models are available, support at the
application level lacks behind. To address this issue, we present the
performance portable block-structured adaptive mesh refinement (AMR) framework
Parthenon, derived from the well-tested and widely used Athena++ astrophysical
magnetohydrodynamics code, but generalized to serve as the foundation for a
variety of downstream multi-physics codes. Parthenon adopts the Kokkos
programming model, and provides various levels of abstractions from
multi-dimensional variables, to packages defining and separating components, to
launching of parallel compute kernels. Parthenon allocates all data in device
memory to reduce data movement, supports the logical packing of variables and
mesh blocks to reduce kernel launch overhead, and employs one-sided,
asynchronous MPI calls to reduce communication overhead in multi-node
simulations. Using a hydrodynamics miniapp, we demonstrate weak and strong
scaling on various architectures including AMD and NVIDIA GPUs, Intel and AMD
x86 CPUs, IBM Power9 CPUs, as well as Fujitsu A64FX CPUs. At the largest scale
on Frontier (the first TOP500 exascale machine), the miniapp reaches a total of
zone-cycles/s on 9,216 nodes (73,728 logical GPUs) at ~92%
weak scaling parallel efficiency (starting from a single node). In combination
with being an open, collaborative project, this makes Parthenon an ideal
framework to target exascale simulations in which the downstream developers can
focus on their specific application rather than on the complexity of handling
massively-parallel, device-accelerated AMR.Comment: 17 pages, 11 figures, accepted for publication in IJHPCA, Codes
available at https://github.com/parthenon-hpc-la
Crucial Physical Dependencies of the Core-Collapse Supernova Mechanism
We explore with self-consistent 2D F{\sc{ornax}} simulations the dependence
of the outcome of collapse on many-body corrections to neutrino-nucleon cross
sections, the nucleon-nucleon bremsstrahlung rate, electron capture on heavy
nuclei, pre-collapse seed perturbations, and inelastic neutrino-electron and
neutrino-nucleon scattering. Importantly, proximity to criticality amplifies
the role of even small changes in the neutrino-matter couplings, and such
changes can together add to produce outsized effects. When close to the
critical condition the cumulative result of a few small effects (including
seeds) that individually have only modest consequence can convert an anemic
into a robust explosion, or even a dud into a blast. Such sensitivity is not
seen in one dimension and may explain the apparent heterogeneity in the
outcomes of detailed simulations performed internationally. A natural
conclusion is that the different groups collectively are closer to a realistic
understanding of the mechanism of core-collapse supernovae than might have
seemed apparent.Comment: 25 pages; 10 figure
Fast variability from black-hole binaries
Currently available information on fast variability of the X-ray emission
from accreting collapsed objects constitutes a complex phenomenology which is
difficult to interpret. We review the current observational standpoint for
black-hole binaries and survey models that have been proposed to interpret it.
Despite the complex structure of the accretion flow, key observational
diagnostics have been identified which can provide direct access to the
dynamics of matter motions in the close vicinity of black holes and thus to the
some of fundamental properties of curved spacetimes, where strong-field general
relativistic effects can be observed.Comment: 20 pages, 11 figures. Accepted for publication in Space Science
Reviews. Also to appear in hard cover in the Space Sciences Series of ISSI
"The Physics of Accretion onto Black Holes" (Springer Publisher
SPH Simulations of Negative (Nodal) Superhumps: A Parametric Study
Negative superhumps in cataclysmic variable systems result when the accretion
disc is tilted with respect to the orbital plane. The line of nodes of the
tilted disc precesses slowly in the retrograde direction, resulting in a
photometric signal with a period slightly less than the orbital period. We use
the method of smoothed particle hydrodynamics to simulate a series of models of
differing mass ratio and effective viscosity to determine the retrograde
precession period and superhump period deficit as a function of
system mass ratio . We tabulate our results and present fits to both
and versus , as well as compare the
numerical results with those compiled from the literature of negative superhump
observations. One surprising is that while we find negative superhumps most
clearly in simulations with an accretion stream present, we also find evidence
for negative superhumps in simulations in which we shut off the mass transfer
stream completely, indicating that the origin of the photometric signal is more
complicated than previously believed.Comment: 14 pages, 15 figures. Accepted for publication in MNRA
GYOTO: a new general relativistic ray-tracing code
GYOTO, a general relativistic ray-tracing code, is presented. It aims at
computing images of astronomical bodies in the vicinity of compact objects, as
well as trajectories of massive bodies in relativistic environments. This code
is capable of integrating the null and timelike geodesic equations not only in
the Kerr metric, but also in any metric computed numerically within the 3+1
formalism of general relativity. Simulated images and spectra have been
computed for a variety of astronomical targets, such as a moving star or a
toroidal accretion structure. The underlying code is open source and freely
available. It is user-friendly, quickly handled and very modular so that
extensions are easy to integrate. Custom analytical metrics and astronomical
targets can be implemented in C++ plug-in extensions independent from the main
code.Comment: 20 pages, 11 figure
Pharmacotherapy-Based Problems in the Management of Diabetes Mellitus: Needs Much More to be Done!
A total of 856 diabetic patients were evaluated for pharmacotherapy-based problems like for possible drug interactions, adverse drug reactions, and other mismatches, if any. Poor correlation between the advised insulin therapy and patients’ fasting blood glucose levels (12%, n=103) was observed. To most of the patients (41.66%, n= 357), insulin therapy was advised in combination with glucocorticoides, thiazides diuretics, and propranolol. Prescribing beta blocker (propranolol) with insulin is contraindicated. The higher incidence of diabetic foot patients was in the mean age of 57±3.4 years that was controlled with combination therapy of insulin and oral antidiabetics (63.0%, n=516). 11.1% of the treated patients could not take the prescribed therapy due to poor acceptance of insulin therapy due to its syringe needle prick. 41.66% risks of potential drug interactions, 7.93% adverse drug reactions, and 6.6% mismatches were recorded, as per the international approved algorithm, for managing a diabetes mellitus that reflects poor health care system. All these events necessitate for coordinating with other health professionals to make the therapy safer in the better interest of the patients. It is concluded that in practice prescribing pattern carries more risks for patients. It is imperative to improve the practice of pharmacotherapeutics rather than to practice in routine
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