12,523 research outputs found
A numerical simulation of the evolution and fate of a FRI jet. The case of 3C 31
The evolution of FRI jets has been long studied in the framework of the
FRI-FRII dichotomy. In this paper, we test the present theoretical and
observational models via a relativistic numerical simulation of the jets in the
radio galaxy 3C 31. We use the parameters derived from the modelling presented
by \cite{lb02a,lb02b} as input parameters for the simulation of the evolution
of the source, thus assuming that they have not varied over the lifetime of the
source. We simulate about 10 % of the total lifetime of the jets in 3C 31.
Realistic density and pressure gradients for the atmosphere are used. The
simulation includes an equation of state for a two-component relativistic gas
that allows a separate treatment of leptonic and baryonic matter. We compare
our results with the modelling of the observational data of the source. Our
results show that the bow shock evolves self-similarly at a quasi-constant
speed, with slight deceleration by the end of the simulation, in agreement with
recent X-ray observations that show the presence of bow shocks in FRI sources.
The jet expands until it becomes underpressured with respect to the ambient
medium, and then recollimates. Subsequent oscillations around pressure
equilibrium and generation of standing shocks lead to the mass loading and
disruption of the jet flow. We derive an estimate for the minimum age of the
source of , which may imply continuous activity of 3C 31
since the triggering of its activity. The simulation shows that weak CSS
sources may be the young counterparts of FRIs. We conclude that the observed
properties of the jets in 3C 31 are basically recovered by the standing shock
scenario.Comment: Accepted for publication in MNRAS. For better quality figures, please
check http://www.mpifr-bonn.mpg.de/staff/mperucho/Research.htm
Limit to the radio emission from a putative central compact source in SN1993J
SN1993J in M81 is the best studied young radio-luminous supernova in the
Northern Hemisphere. We recently reported results from the analysis of a
complete set of VLBI observations of this supernova at 1.7, 2.3, 5.0, and 8.4
GHz, covering a time baseline of more than one decade. Those reported results
were focused on the kinematics of the expanding shock, the particulars of its
evolving non-thermal emission, the density profile of the circumstellar medium,
and the evolving free-free opacity by the supernova ejecta. In the present
paper, we complete our analysis by performing a search for any possible signal
from a compact source (i.e., a stellar-mass black hole or a young pulsar
nebula) at the center of the expanding shell. We have performed a stacking of
all our VLBI images at each frequency, after subtraction of our best-fit shell
model at each epoch, and measured the peak intensity in the stacked residual
image. Given the large amount of available global VLBI observations, the
stacking of all the residual images allows us to put upper limits to the
eventual emission of a putative compact central source at the level of
Jy at 5 GHz (or, more conservatively, Jy, if we
make a further correction for the ejecta opacity) and somewhat larger at other
wavelengths.Comment: 4 pages, 3 figures. Accepted for publication in A&
Simulations of the relativistic parsec-scale jet in 3C273
We present a hydrodynamical 3D simulation of the relativistic jet in 3C273,
in comparison to previous linear perturbation analysis of Kelvin-Helmholtz
instability developing in the jet. Our aim is to assess advantages and
limitations of both analytical and numerical approaches and to identify spatial
and temporal scales on which the linear regime of Kelvin-Helmholtz instability
can be applied in studies of morphology and kinematics of parsec-scale jets.Comment: 4 pages, 3 figures; to be published in Proceedings of the workshop
"Multiband Approach to AGN", held on Sep.30-Oct.2 in Bonn. Publication:
Memorie della Societa Astronomica Italiana, v. 26, No.1 (2005). Reduced
figure resolution! Version with original figures is availavble at
http://www.mpifr-bonn.mpg.de/bonn04/proceedings/perucho.pd
Clusters and cycles in the cosmic ray age distributions of meteorites
Statistically significant clusters in the cosmic ray exposure age distributions of some groups of iron and stone meteorites were observed, suggesting epochs of enhanced collision and breakups. Fourier analyses of the age distributions of chondrites reveal no significant periods, nor does the same analysis when applied to iron meteorite clusters
RAM: A Relativistic Adaptive Mesh Refinement Hydrodynamics Code
We have developed a new computer code, RAM, to solve the conservative
equations of special relativistic hydrodynamics (SRHD) using adaptive mesh
refinement (AMR) on parallel computers. We have implemented a
characteristic-wise, finite difference, weighted essentially non-oscillatory
(WENO) scheme using the full characteristic decomposition of the SRHD equations
to achieve fifth-order accuracy in space. For time integration we use the
method of lines with a third-order total variation diminishing (TVD)
Runge-Kutta scheme. We have also implemented fourth and fifth order Runge-Kutta
time integration schemes for comparison. The implementation of AMR and
parallelization is based on the FLASH code. RAM is modular and includes the
capability to easily swap hydrodynamics solvers, reconstruction methods and
physics modules. In addition to WENO we have implemented a finite volume module
with the piecewise parabolic method (PPM) for reconstruction and the modified
Marquina approximate Riemann solver to work with TVD Runge-Kutta time
integration. We examine the difficulty of accurately simulating shear flows in
numerical relativistic hydrodynamics codes. We show that under-resolved
simulations of simple test problems with transverse velocity components produce
incorrect results and demonstrate the ability of RAM to correctly solve these
problems. RAM has been tested in one, two and three dimensions and in
Cartesian, cylindrical and spherical coordinates. We have demonstrated
fifth-order accuracy for WENO in one and two dimensions and performed detailed
comparison with other schemes for which we show significantly lower convergence
rates. Extensive testing is presented demonstrating the ability of RAM to
address challenging open questions in relativistic astrophysics.Comment: ApJS in press, 21 pages including 18 figures (6 color figures
Orbital X-Ray Variability of the Microquasar LS 5039
The properties of the orbit and the donor star in the high mass X-ray binary
microquasar LS 5039 indicate that accretion processes should mainly occur via a
radiatively driven wind. In such a scenario, significant X-ray variability
would be expected due to the eccentricity of the orbit. The source has been
observed at X-rays by several missions, although with a poor coverage that
prevents to reach any conclusion about orbital variability. Therefore, we
conducted RossiXTE observations of the microquasar system LS 5039 covering a
full orbital period of 4 days. Individual observations are well fitted with an
absorbed power-law plus a Gaussian at 6.7 keV, to account for iron line
emission that is probably a diffuse background feature. In addition, we have
taken into account that the continuum is also affected by significant diffuse
background contamination. Our results show moderate power-law flux variations
on timescales of days, as well as the presence of miniflares on shorter
timescales. The new orbital ephemeris of the system recently obtained by
Casares et al. have allowed us to show, for the first time, that an increase of
emission is seen close to the periastron passage, as expected in an accretion
scenario. Moreover, the detected orbital variability is a factor of ~4 smaller
than the one expected by using a simple wind accretion model, and we suggest
that an accretion disk around the compact object could be responsible for this
discrepancy. On the other hand, significant changes in the photon index are
also observed clearly anti-correlated with the flux variations. We interpret
the overall X-ray spectral characteristics of LS 5039 in the context of X-ray
radiation produced by inverse Compton and/or synchrotron processes in the jet
of this microquasar.Comment: published in Astrophysical Journal, submission format (real number of
pages: 7, 4 figures
Lectures on dynamical models for quantum measurements
In textbooks, ideal quantum measurements are described in terms of the tested
system only by the collapse postulate and Born's rule. This level of
description offers a rather flexible position for the interpretation of quantum
mechanics. Here we analyse an ideal measurement as a process of interaction
between the tested system S and an apparatus A, so as to derive the properties
postulated in textbooks. We thus consider within standard quantum mechanics the
measurement of a quantum spin component by an apparatus A, being a
magnet coupled to a bath. We first consider the evolution of the density
operator of S+A describing a large set of runs of the measurement process. The
approach describes the disappearance of the off-diagonal terms ("truncation")
of the density matrix as a physical effect due to A, while the registration of
the outcome has classical features due to the large size of the pointer
variable, the magnetisation. A quantum ambiguity implies that the density
matrix at the final time can be decomposed on many bases, not only the one of
the measurement. This quantum oddity prevents to connect individual outcomes to
measurements, a difficulty known as the "measurement problem". It is shown that
it is circumvented by the apparatus as well, since the evolution in a small
time interval erases all decompositions, except the one on the measurement
basis. Once one can derive the outcome of individual events from quantum
theory, the so-called "collapse of the wave function" or the "reduction of the
state" appears as the result of a selection of runs among the original large
set. Hence nothing more than standard quantum mechanics is needed to explain
features of measurements. The employed statistical formulation is advocated for
the teaching of quantum theory.Comment: 43 pages, 5 figures. Lectures given in the "Advanced School on
Quantum Foundations and Open Quantum Systems", Joao Pessoa, Brazil, summer
2012. To appear in the proceedings and in IJMP
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