73 research outputs found
Runko: Modern multi-physics toolbox for simulating plasma
Runko is a new open-source plasma simulation framework implemented in C++ and
Python. It is designed to function as an easy-to-extend general toolbox for
simulating astrophysical plasmas with different theoretical and numerical
models. Computationally intensive low-level "kernels" are written in modern
C++14 taking advantage of polymorphic classes, multiple inheritance, and
template metaprogramming. High-level functionality is operated with Python3
scripts. This hybrid program design ensures fast code and ease of use. The
framework has a modular object-oriented design that allow the user to easily
add new numerical algorithms to the system. The code can be run on various
computing platforms ranging from laptops (shared-memory systems) to massively
parallel supercomputer architectures (distributed-memory systems). The
framework also supports heterogeneous multi-physics simulations in which
different physical solvers can be combined and run simultaneously. Here we
report on the first results from the framework's relativistic particle-in-cell
(PIC) module. Using the PIC module, we simulate decaying relativistic kinetic
turbulence in suddenly stirred magnetically-dominated pair plasma. We show that
the resulting particle distribution can be separated into a thermal part that
forms the turbulent cascade and into a separate decoupled non-thermal particle
population that acts as an energy sink for the system.Comment: 17 pages, 6 figures. Comments welcome! Code available from
https://github.com/natj/runk
Models of neutron star atmospheres enriched with nuclear burning ashes
Low-mass X-ray binaries hosting neutron stars (NS) exhibit thermonuclear
(type-I) X-ray bursts, which are powered by unstable nuclear burning of helium
and/or hydrogen into heavier elements deep in the NS "ocean". In some cases the
burning ashes may rise from the burning depths up to the NS photosphere by
convection, leading to the appearance of the metal absorption edges in the
spectra, which then force the emergent X-ray burst spectra to shift toward
lower energies. These effects may have a substantial impact on the color
correction factor and the dilution factor , the parameters of the
diluted blackbody model that is commonly used
to describe the emergent spectra from NSs. The aim of this paper is to quantify
how much the metal enrichment can change these factors. We have developed a new
NS atmosphere modeling code, which has a few important improvements compared to
our previous code required by inclusion of the metals. The opacities and the
internal partition functions (used in the ionization fraction calculations) are
now taken into account for all atomic species. In addition, the code is now
parallelized to counter the increased computational load. We compute a detailed
grid of atmosphere models with different exotic chemical compositions that
mimic the presence of the burning ashes. From the emerging model spectra we
compute the color correction factors and the dilution factors that
can then be compared to the observations. We find that the metals may change
by up to about 40%, which is enough to explain the scatter seen in the
blackbody radius measurements. The presented models open up the possibility for
determining NS mass and radii more accurately, and may also act as a tool to
probe the nuclear burning mechanisms of X-ray bursts.Comment: 14 pages, 7 figures, to be published in A&
Bayesian parameter constraints for neutron star masses and radii using X-ray timing observations of accretion-powered millisecond pulsars
We present a Bayesian method to constrain the masses and radii of neutron
stars (NSs) using the information encoded in the X-ray pulse profiles of
accreting millisecond pulsars. We model the shape of the pulses using "oblate
Schwarzschild" approximation, which takes into account the deformed shape of
the star together with the special and general relativistic corrections to the
photon trajectories and angles. The spectrum of the radiation is obtained from
an empirical model of Comptonization in a hot slab in which a fraction of seed
blackbody photons is scattered into a power-law component. By using an
affine-invariant Markov chain Monte Carlo ensemble sampling method, we obtain
posterior probability distributions for the different model parameters,
especially for the mass and the radius. To test the robustness of our method,
we first analyzed self-generated synthetic data with known model parameters.
Similar analysis was then applied for the observations of SAX J1808.4-3658 by
the Rossi X-ray Timing Explorer (RXTE). The results show that our method can
reproduce the model parameters of the synthetic data, and that accurate
constraints for the radius can be obtained using the RXTE pulse profile
observations if the mass is a priori known. For a mass in the range 1.5-1.8
Msun, the radius of the NS in SAX J1808.4-3658 is constrained between 9 and 13
km. If the mass is accurately known, the radius can be determined with an
accuracy of 5% (68% credibility). For example, for the mass of 1.7 Msun the
equatorial radius is Req = 11.9+0.5 -0.4 km. Finally, we show that further
improvements can be obtained when the X-ray polarization data from the Imaging
X-ray Polarimeter Explorer will become available.Comment: 16 pages, 9 figures, published in A&
Repeated Cyclogenesis on Hot-Exoplanet Atmospheres with Deep Heating
We study the effect of deep heating (stellar irradiation deposited at the
~10^5 Pa level, as well as in the shallow region at the ~10^3 level) in
hot-exoplanet atmospheres. Unlike with shallow heating (only), the atmosphere
with deep heating exhibits a single equilibrium state, characterized by
repeated generation of giant cyclonic vortices that move away westward from the
point of emergence. The generation is accompanied by a burst of heightened
turbulence activity, leading to the production of small-scale structures and
large-scale mixing of temperature on a timescale of ~3 planetary rotations.Comment: 5 pages, 4 figures, 1 appendi
Neutron star mass and radius measurements from atmospheric model fits to X-ray burst cooling tail spectra
Observations of thermonuclear X-ray bursts from accreting neutron stars (NSs)
in low-mass X-ray binary systems can be used to constrain NS masses and radii.
Most previous work of this type has set these constraints using Planck function
fits as a proxy: both the models and the data are fit with diluted blackbody
functions to yield normalizations and temperatures which are then compared
against each other. Here, for the first time, we fit atmosphere models of X-ray
bursting NSs directly to the observed spectra. We present a hierarchical
Bayesian fitting framework that uses state-of-the-art X-ray bursting NS
atmosphere models with realistic opacities and relativistic exact Compton
scattering kernels as a model for the surface emission. We test our approach
against synthetic data, and find that for data that are well-described by our
model we can obtain robust radius, mass, distance, and composition
measurements. We then apply our technique to Rossi X-ray Timing Explorer
observations of five hard-state X-ray bursts from 4U 1702-429. Our joint fit to
all five bursts shows that the theoretical atmosphere models describe the data
well but there are still some unmodeled features in the spectrum corresponding
to a relative error of 1-5% of the energy flux. After marginalizing over this
intrinsic scatter, we find that at 68% credibility the circumferential radius
of the NS in 4U 1702-429 is R = 12.4+-0.4 km, the gravitational mass is
M=1.9+-0.3 Msun, the distance is 5.1 < D/kpc < 6.2, and the hydrogen mass
fraction is X < 0.09.Comment: 15 pages, 11 figures, submitted to A&
Radiation from rapidly rotating oblate neutron stars
A theoretical framework for emission originating from rapidly rotating oblate compact objects is described in detail. Using a Hamilton-Jacobi formalism, we show that special relativistic rotational effects such as aberration of angles, Doppler boosting, and time dilatation naturally emerge from the general relativistic treatment of rotating compact objects. We use the Butterworth-Ipser metric expanded up to the second order in rotation and hence include effects of light bending, frame-dragging, and quadrupole deviations on our geodesic calculations. We also give detailed descriptions of the numerical algorithms used and provide an open-source implementation of the numerical framework called BENDER. As an application, we study spectral line profiles (i.e., smearing kernels) from rapidly rotating oblate neutron stars. We find that in this metric description, the second-order quadrupole effects are not strong enough to produce narrow observable features in the spectral energy distribution for almost any physically realistic parameter combination, and hence, actually detecting them is unlikely. The full width at tenth-maximum and full width at half-maximum of the rotation smearing kernels are also reported for all viewing angles. These can then be used to quantitatively estimate the effects of rotational smearing on the observed spectra. We also calculate accurate pulse profiles and observer skymaps of emission from hot spots on rapidly rotating accreting millisecond pulsars. These allow us to quantify the strength of the pulse fractions one expects to observe from typical fast-spinning millisecond pulsars
Pulsar Wind-heated Accretion Disk and the Origin of Modes in Transitional Millisecond Pulsar PSR J1023+0038
Transitional millisecond pulsars provide a unique set of observational data for understanding accretion at low rates onto magnetized neutron stars. In particular, PSR.J1023+0038 exhibits a remarkable bimodality of the X-ray luminosity (low and high modes), pulsations extending from the X-ray to the optical band, GeV emission, and occasional X-ray flares. We discuss a scenario for the pulsar interaction with the accretion disk capable of explaining the observed behavior. We suggest that during the high mode the disk is truncated outside the light cylinder, allowing the pulsar wind to develop near the equatorial plane and strike the disk. The dissipative wind-disk collision energizes the disk particles and generates synchrotron emission, which peaks in the X-ray band and extends down to the optical band. The emission is modulated by the pulsar wind rotation, resulting in a pulse profile with two peaks 180 degrees apart. This picture explains the high mode luminosity, spectrum, and pulse profile (X-ray and optical) of PSR.J1023+0038. It may also explain the X-ray flares as events of sudden increase in the effective disk cross section intercepting the wind. In contrast to previously proposed models, we suggest that the disk penetrates the light cylinder only during the low X-ray mode. This penetration suppresses the dissipation caused by the pulsar wind-disk collision, and the system enters the propeller regime. The small duty cycle of the propeller explains the low spin-down rate of the pulsar
New applications for the Boris Spectral Deferred Correction algorithm for plasma simulations
The paper investigates two new use cases for the Boris Spectral Deferred Corrections (Boris-SDC) time integrator for plasma simulations. First, we show that using Boris-SDC as a particle pusher in an electrostatic particle-in-cell (PIC) code can, at least in the linear regime, improve simulation accuracy compared with the standard second order Boris method. In some instances, the higher order of Boris-SDC even allows a much larger time step, leading to modest computational gains. Second, we propose a modification of Boris-SDC for the relativistic regime. Based on an implementation of Boris-SDC in the RUNKO PIC code, we demonstrate for a relativistic Penning trap that Boris-SDC retains its high order of convergence for velocities ranging from 0.5c to >0.99c
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