513 research outputs found
The Exact Solution of the Riemann Problem in Relativistic MHD
We discuss the procedure for the exact solution of the Riemann problem in
special relativistic magnetohydrodynamics (MHD). We consider both initial
states leading to a set of only three waves analogous to the ones in
relativistic hydrodynamics, as well as generic initial states leading to the
full set of seven MHD waves. Because of its generality, the solution presented
here could serve as an important test for those numerical codes solving the MHD
equations in relativistic regimes.Comment: 36 pages, 13 figures. Minor changes to match published versio
Prompt Electromagnetic Transients from Binary Black Hole Mergers
Binary black hole (BBH) mergers provide a prime source for current and future
interferometric GW observatories. Massive BBH mergers may often take place in
plasma-rich environments, leading to the exciting possibility of a concurrent
electromagnetic (EM) signal observable by traditional astronomical facilities.
However, many critical questions about the generation of such counterparts
remain unanswered. We explore mechanisms that may drive EM counterparts with
magnetohydrodynamic simulations treating a range of scenarios involving
equal-mass black-hole binaries immersed in an initially homogeneous fluid with
uniform, orbitally aligned magnetic fields. We find that the time development
of Poynting luminosity, which may drive jet-like emissions, is relatively
insensitive to aspects of the initial configuration. In particular, over a
significant range of initial values, the central magnetic field strength is
effectively regulated by the gas flow to yield a Poynting luminosity of
, with BBH mass
scaled to and ambient density . We also calculate the
direct plasma synchrotron emissions processed through geodesic ray-tracing.
Despite lensing effects and dynamics, we find the observed synchrotron flux
varies little leading up to merger.Comment: 22 pages, 21 figures; additional reference + clarifying text added to
match published versio
Faranoff-Riley type I jet deceleration at density discontinuities "Relativistic hydrodynamics with realistic equation of state"
The deceleration mechanisms for relativistic jets in active galactic nuclei
remain an open question, and in this paper we propose a model which could
explain sudden jet deceleration, invoking density discontinuities. This is
particularly motivated by recent indications from HYMORS. Exploiting high
resolution, numerical simulations, we demonstrate that for both high and low
energy jets, always at high Lorentz factor, a transition to a higher density
environment can cause a significant fraction of the directed jet energy to be
lost on reflection. This can explain how one-sided jet deceleration and a
transition to FR I type can occur in HYMORS, which start as FR II (and remain
so on the other side). For that purpose, we implemented in the relativistic
hydrodynamic grid-adaptive AMRVAC code, the Synge-type equation of state
introduced in the general polytropic case by Meliani et al. (2004). We present
results for 10 model computations, varying the inlet Lorentz factor from 10 to
20, including uniform or decreasing density profiles, and allowing for
cylindrical versus conical jet models. As long as the jet propagates through
uniform media, we find that the density contrast sets most of the propagation
characteristics, fully consistent with previous modeling efforts. When the jet
runs into a denser medium, we find a clear distinction in the decelaration of
high energy jets depending on the encountered density jump. For fairly high
density contrast, the jet becomes destabilised and compressed, decelerates
strongly (up to subrelativistic speeds) and can form knots. We point out
differences that are found between cylindrical and conical jet models, together
with dynamical details like the Richtmyer-Meshkov instabilities developing at
the original contact interface.Comment: accepted in A&
Analytic modelling of tidal effects in the relativistic inspiral of binary neutron stars
To detect the gravitational-wave (GW) signal from binary neutron stars and extract information about the equation of state of matter at nuclear density, it is necessary to match the signal with a bank of accurate templates. We present the two longest (to date) general-relativistic simulations of equal-mass binary neutron stars with different compactnesses, C=0.12 and C=0.14, and compare them with a tidal extension of the effective-one-body (EOB)model. The typical numerical phasing errors over the GW cycles are rad. By calibrating only one parameter (representing a higher-order amplification of tidal effects), the EOB model can reproduce, within the numerical error, the two numerical waveforms essentially up to the merger. By contrast, the third post-Newtonian Taylor-T4 approximant with leading-order tidal corrections dephases with respect to the numerical waveforms by several radians
Magnetohydrodynamic Effects in Propagating Relativistic Ejecta: Reverse Shock and Magnetic Acceleration
We solve the Riemann problem for the deceleration of arbitrarily magnetized relativistic ejecta injected into a static unmagnetized medium. We find that for the same initial Lorentz factor, the reverse shock becomes progressively weaker with increasing magnetization s (the Poynting-to-kinetic energy flux ratio), and the shock becomes a rarefaction wave when s exceeds a critical value, sc, defined by the balance between the magnetic pressure in the ejecta and the thermal pressure in the forward shock. In the rarefaction wave regime, we find that the rarefied region is accelerated to a Lorentz factor that is significantly larger than the initial value. This acceleration mechanism is due to the strong magnetic pressure in the ejecta
Implementing a new recovery scheme for primitive variables in the general relativistic magnetohydrodynamic code Spritz
General relativistic magnetohydrodynamic (GRMHD) simulations represent a fundamental tool to probe various underlying mechanisms at play during binary neutron star (BNS) and neutron star (NS) - black hole (BH) mergers. Contemporary flux-conservative GRMHD codes numerically evolve a set of conservative equations based on `conserved' variables which then need to be converted back into the fundamental (`primitive') variables. The corresponding conservative-to-primitive variable recovery procedure, based on root-finding algorithms, constitutes one of the core elements of such GRMHD codes. Recently, a new robust, accurate and efficient recovery scheme called RePrimAnd was introduced, which has demonstrated the ability to always converge to a unique solution. The scheme provides fine-grained error policies to handle invalid states caused by evolution errors, and also provides analytical bounds for the error of all primitive variables. In this work, we describe the technical aspects of implementing the RePrimAnd scheme into the GRMHD code Spritz. To check our implementation as well as to assess the various features of the scheme, we perform a number of GRMHD tests in three dimensions. Our tests, which include critical cases such as a NS collapse to a BH as well as the early evolution (~50 ms) of a Fishbone-Moncrief BH-accrection disk system, show that RePrimAnd is able to support magnetized, low density environments with magnetic-to-fluid pressure ratios as high as 10^4, in situations where the previously used recovery scheme fails
Accurate evolutions of inspiralling neutron-star binaries: assessment of the truncation error
We have recently presented an investigation in full general relativity of the
dynamics and gravitational-wave emission from binary neutron stars which
inspiral and merge, producing a black hole surrounded by a torus (see
arXiv:0804.0594). We here discuss in more detail the convergence properties of
the results presented in arXiv:0804.0594 and, in particular, the deterioration
of the convergence rate at the merger and during the survival of the merged
object, when strong shocks are formed and turbulence develops. We also show
that physically reasonable and numerically convergent results obtained at
low-resolution suffer however from large truncation errors and hence are of
little physical use. We summarize our findings in an "error budget", which
includes the different sources of possible inaccuracies we have investigated
and provides a first quantitative assessment of the precision in the modelling
of compact fluid binaries.Comment: 13 pages, 5 figures. Minor changes to match published version. Added
figure 5 right pane
A multidimensional grid-adaptive relativistic magnetofluid code
A robust second order, shock-capturing numerical scheme for multi-dimensional
special relativistic magnetohydrodynamics on computational domains with
adaptive mesh refinement is presented. The base solver is a total variation
diminishing Lax-Friedrichs scheme in a finite volume setting and is combined
with a diffusive approach for controlling magnetic monopole errors. The
consistency between the primitive and conservative variables is ensured at all
limited reconstructions and the spatial part of the four velocity is used as a
primitive variable. Demonstrative relativistic examples are shown to validate
the implementation. We recover known exact solutions to relativistic MHD
Riemann problems, and simulate the shock-dominated long term evolution of
Lorentz factor 7 vortical flows distorting magnetic island chains.Comment: accepted for publication in Computer Physics Communication
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