123 research outputs found
The SDSS Galaxy Angular Two-Point Correlation Function
We present the galaxy two-point angular correlation function for galaxies
selected from the seventh data release of the Sloan Digital Sky Survey. The
galaxy sample was selected with -band apparent magnitudes between 17 and 21;
and we measure the correlation function for the full sample as well as for the
four magnitude ranges: 17-18, 18-19, 19-20, and 20-21. We update the flag
criteria to select a clean galaxy catalog and detail specific tests that we
perform to characterize systematic effects, including the effects of seeing,
Galactic extinction, and the overall survey uniformity. Notably, we find that
optimally we can use observed regions with seeing < 1\farcs5, and -band
extinction < 0.13 magnitudes, smaller than previously published results.
Furthermore, we confirm that the uniformity of the SDSS photometry is minimally
affected by the stripe geometry. We find that, overall, the two-point angular
correlation function can be described by a power law, with , over the range
0\fdg005--10\degr. We also find similar relationships for the four
magnitude subsamples, but the amplitude within the same angular interval for
the four subsamples is found to decrease with fainter magnitudes, in agreement
with previous results. We find that the systematic signals are well below the
galaxy angular correlation function for angles less than approximately
5\degr, which limits the modeling of galaxy angular correlations on larger
scales. Finally, we present our custom, highly parallelized two-point
correlation code that we used in this analysis.Comment: 22 pages, 17 figures, accepted by MNRA
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
GRMHD simulations of accretion onto Sgr A*: How important are radiative losses?
We present general relativistic magnetohydrodynamic (GRMHD) numerical
simulations of the accretion flow around the supermassive black hole in the
Galactic centre, Sagittarius A* (Sgr A*). The simulations include for the first
time radiative cooling processes (synchrotron, bremsstrahlung, and inverse
Compton) self-consistently in the dynamics, allowing us to test the common
simplification of ignoring all cooling losses in the modeling of Sgr A*. We
confirm that for Sgr A*, neglecting the cooling losses is a reasonable
approximation if the Galactic centre is accreting below ~10^{-8} Msun/yr i.e.
Mdot < 10^{-7} Mdot_Edd. But above this limit, we show that radiative losses
should be taken into account as significant differences appear in the dynamics
and the resulting spectra when comparing simulations with and without cooling.
This limit implies that most nearby low-luminosity active galactic nuclei are
in the regime where cooling should be taken into account.
We further make a parameter study of axisymmetric gas accretion around the
supermassive black hole at the Galactic centre. This approach allows us to
investigate the physics of gas accretion in general, while confronting our
results with the well studied and observed source, Sgr A*, as a test case. We
confirm that the nature of the accretion flow and outflow is strongly dependent
on the initial geometry of the magnetic field. For example, we find it
difficult, even with very high spins, to generate powerful outflows from discs
threaded with multiple, separate poloidal field loops.Comment: Resubmitted to MNRAS, including modifications in response to referee
report. 13 pages, 15 figure
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
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
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
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