1,297 research outputs found
Scalability of Hydrodynamic Simulations
Many hydrodynamic processes can be studied in a way that is scalable over a
vastly relevant physical parameter space. We systematically examine this
scalability, which has so far only briefly discussed in astrophysical
literature. We show how the scalability is limited by various constraints
imposed by physical processes and initial conditions. Using supernova remnants
in different environments and evolutionary phases as application examples, we
demonstrate the use of the scaling as a powerful tool to explore the
interdependence among relevant parameters, based on a minimum set of
simulations. In particular, we devise a scaling scheme that can be used to
adaptively generate numerous seed remnants and plant them into 3D hydrodynamic
simulations of the supernova-dominated interstellar medium.Comment: 12 pages, 1 figure, submitted to MNRAS; comments are welcom
Ultrasonic flaw sizing—An overview
The time-of-flight diffraction (TOFD) technique is one of the most common sizing methods in practical use by industry today. This method was developed over 40 years ago and is based on the technology and state of knowledge present at that time. A combination of phased arrays and equivalent flaw sizing methods are proposed as the foundation for a new generation of sizing methods that go beyond TOFD sizing
High Resolution X-ray Observations of the Pulsar Wind Nebula Associated with the Gamma-ray Source HESSJ1640-465
We present a Chandra X-ray observation of the very high energy -ray
source HESSJ1640-465. We identify a point source surrounded by a diffuse
emission that fills the extended object previously detected by XMM Newton at
the centroid of the HESS source, within the shell of the radio supernova
remnant (SNR) G338.3-0.0. The morphology of the diffuse emission strongly
resembles that of a pulsar wind nebula (PWN) and extends asymmetrically to the
South-West of a point-source presented as a potential pulsar. The spectrum of
the putative pulsar and compact nebula are well-characterized by an absorbed
power-law model which, for a reasonable value of , exhibit an index of 1.1 and 2.5 respectively, typical of
Vela-like PWNe. We demonstrate that, given the HI absorption features
observed along the line of sight, the SNR and the HII surrounding region are
probably connected and lie between 8 kpc and 13 kpc. The resulting age of the
system is between 10 and 30 kyr. For a 10 kpc distance (also consistent with
the X-ray absorption) the 2-10 keV X-ray luminosities of the putative pulsar
and nebula are and
(). Both the flux ratio of and the total luminosity of this system predict a pulsar spin-down
power around . We finally
consider several reasons for the asymmetries observed in the PWN morphology and
discuss the potential association with the HESS source in term of a
time-dependent one-zone leptonic model.Comment: 35 pages, 9 figure
Cosmic rays in the surroundings of SNR G35.6-0.4
HESS J1858+020 is a TeV gamma-ray source that was reported not to have any
clear cataloged counterpart at any wavelength. However, it has been recently
proposed that this source is indirectly associated with the radio source,
re-identified as a supernova remnant (SNR), G35.6-0.4. The latter is found to
be middle-aged ( kyr) and to have nearby molecular clouds (MCs). HESS
J1858+020 was proposed to be the result of the interaction of protons
accelerated in the SNR shell with target ions residing in the clouds. The Fermi
Large Area Telescope (LAT) First Source Catalog does not list any source
coincident with the position of HESS J1858+020, but some lie close. Here, we
analyse more than 2 years of data obtained with the Fermi-LAT for the region of
interest, and consider whether it is indeed possible that the closest LAT
source, 1FGL J1857.1+0212c, is related to HESS J1858+020. We conclude it is
not, and we impose upper limits on the GeV emission originating from HESS
J1858+020. Using a simplified 3D model for the cosmic-ray propagation out from
the shell of the SNR, we consider whether the interaction between SNR G35.6-0.4
and the MCs nearby could give rise to the TeV emission of HESS J1858+020
without producing a GeV counterpart. If so, the pair of SNR/TeV source with no
GeV detection would be reminiscent of other similarly-aged SNRs, such as some
of the TeV hotspots near W28, for which cosmic-ray diffusion may be used to
explain their multi-frequency phenomenology. However, for HESS J1858+020, we
found that although the phase space in principle allows for such GeV--TeV
non-correlation to appear, usual and/or observationally constrained values of
the parameters (e.g., diffusion coefficients and cloud-SNR likely distances)
would disfavor it.Comment: In press in MNRA
Scaling relations between numerical simulations and physical systems they represent
The dynamical equations describing the evolution of a physical system
generally have a freedom in the choice of units, where different choices
correspond to different physical systems that are described by the same
equations. Since there are three basic physical units, of mass, length and
time, there are up to three free parameters in such a rescaling of the units,
. In Newtonian hydrodynamics, e.g., there are indeed usually three
free parameters, . If, however, the dynamical equations contain a
universal dimensional constant, such as the speed of light in vacuum or the
gravitational constant , then the requirement that its value remains the
same imposes a constraint on the rescaling, which reduces its number of free
parameters by one, to . This is the case, for example, in
magneto-hydrodynamics (MHD) or special relativistic hydrodynamics, where
appears in the dynamical equations and forces the length and time units to
scale by the same factor, or in Newtonian gravity where the gravitational
constant appears in the equations. More generally, when there are
independent (in terms of their units) universal dimensional constants, then the
number of free parameters is . When both gravity and
relativity are included, there is only one free parameter (, as both
and appear in the equations so that ), and the units of
mass, length and time must all scale by the same factor. The explicit
rescalings for different types of systems are discussed and summarized here.
Such rescalings of the units also hold for discrete particles, e.g. in N-body
or particle in cell simulations. They are very useful when numerically
investigating a large parameter space or when attempting to fit particular
experimental results, by significantly reducing the required number of
simulations.Comment: 6 pages, 2 tables, accepted to MNRAS (expanded discussion of the
general context in the introduction
Type-Ia Supernova-driven Galactic Bulge Wind
Stellar feedback in galactic bulges plays an essential role in shaping the
evolution of galaxies. To quantify this role and facilitate comparisons with
X-ray observations, we conduct 3D hydrodynamical simulations with the adaptive
mesh refinement code, FLASH, to investigate the physical properties of hot gas
inside a galactic bulge, similar to that of our Galaxy or M31. We assume that
the dynamical and thermal properties of the hot gas are dominated by mechanical
energy input from SNe, primarily Type Ia, and mass injection from evolved stars
as well as iron enrichment from SNe. We study the bulge-wide outflow as well as
the SN heating on scales down to ~4 pc. An embedding scheme that is devised to
plant individual SNR seeds, allows to examine, for the first time, the effect
of sporadic SNe on the density, temperature, and iron ejecta distribution of
the hot gas as well as the resultant X-ray morphology and spectrum. We find
that the SNe produce a bulge wind with highly filamentary density structures
and patchy ejecta. Compared with a 1D spherical wind model, the non-uniformity
of simulated gas density, temperature, and metallicity substantially alters the
spectral shape and increases the diffuse X-ray luminosity. The differential
emission measure as a function of temperature of the simulated gas exhibits a
log-normal distribution, with a peak value much lower than that of the
corresponding 1D model. The bulk of the X-ray emission comes from the
relatively low temperature and low abundance gas shells associated with SN
blastwaves. SN ejecta are not well mixed with the ambient medium, at least in
the bulge region. These results, at least partly, account for the apparent lack
of evidence for iron enrichment in the soft X-ray-emitting gas in galactic
bulges and intermediate-mass elliptical galaxies.[...]Comment: 37 pages, 19 figures, submitted to MNRAS; comments are welcom
Radio and X-ray Observations of the Type Ic SN 2007gr Reveal an Ordinary, Non-relativistic Explosion
We present extensive radio and X-ray observations of the nearby Type Ic SN
2007gr in NGC 1058 obtained with the Very Large Array and the Chandra X-ray
Observatory and spanning 5 to 150 days after explosion. Through our detailed
modeling of these data, we estimate the properties of the blastwave and the
circumstellar environment. We find evidence for a freely-expanding and
non-relativistic explosion with an average blastwave velocity, v~0.2c, and a
total internal energy for the radio emitting material of E ~ 2 x 10^46 erg
assuming equipartition of energy between electrons and magnetic fields
(epsilon_e=epsilon_B=0.1). The temporal and spectral evolution of the radio
emission points to a stellar wind-blown environment shaped by a steady
progenitor mass loss rate of Mdot ~ 6 x 10^-7 solar masses per year (wind
velocity, v_w=10^3 km/s). These parameters are fully consistent with those
inferred for other SNe Ibc and are in line with the expectations for an
ordinary, homologous SN explosion. Our results are at odds with those of Paragi
et al. (2010) who recently reported evidence for a relativistic blastwave in SN
2007gr based on their claim that the radio emission was resolved away in a low
signal-to-noise Very Long Baseline Interferometry (VLBI) observation. Here we
show that the exotic physical scenarios required to explain the claimed
relativistic velocity -- extreme departures from equipartition and/or a highly
collimated outflow -- are excluded by our detailed Very Large Array radio
observations. Moreover, we present an independent analysis of the VLBI data and
propose that a modest loss of phase coherence provides a more natural
explanation for the apparent flux density loss which is evident on both short
and long baselines. We conclude that SN 2007gr is an ordinary Type Ibc
supernova.Comment: 14 pages, 6 figures, submitted to Ap
Modeling Gamma-Ray Burst X-Ray Flares within the Internal Shock Model
X-ray afterglow light curves have been collected for over 400 Swift gamma-ray
bursts with nearly half of them having X-ray flares superimposed on the regular
afterglow decay. Evidence suggests that gamma-ray prompt emission and X-ray
flares share a common origin and that at least some flares can only be
explained by long-lasting central engine activity. We have developed a shell
model code to address the question of how X-ray flares are produced within the
framework of the internal shock model. The shell model creates randomized GRB
explosions from a central engine with multiple shells and follows those shells
as they collide, merge and spread, producing prompt emission and X-ray flares.
We pay special attention to the time history of central engine activity,
internal shocks, and observed flares, but do not calculate the shock dynamics
and radiation processes in detail. Using the empirical E_p - E_iso (Amati)
relation with an assumed Band function spectrum for each collision and an
empirical flare temporal profile, we calculate the gamma-ray (Swift/BAT band)
and X-ray (Swift/XRT band) lightcurves for arbitrary central engine activity
and compare the model results with the observational data. We show that the
observed X-ray flare phenomenology can be explained within the internal shock
model. The number, width and occurring time of flares are then used to diagnose
the central engine activity, putting constraints on the energy, ejection time,
width and number of ejected shells. We find that the observed X-ray flare time
history generally reflects the time history of the central engine, which
reactivates multiple times after the prompt emission phase with progressively
reduced energy...Comment: 32 pages, 11 figures. ApJ, in pres
A past capture event at Sagittarius A* inferred from the fluorescent X-ray emission of Sagittarius B clouds
The fluorescent X-ray emission from neutral iron in the molecular clouds (Sgr
B) indicates that the clouds are being irradiated by an external X-ray source.
The source is probably associated with the Galactic central black hole (Sgr
A*), which triggered a bright outburst one hundred years ago. We suggest that
such an outburst could be due to a partial capture of a star by Sgr A*, during
which a jet was generated. By constraining the observed flux and the time
variability ( 10 years) of the Sgr B's fluorescent emission, we find that
the shock produced by the interaction of the jet with the dense interstellar
medium represents a plausible candidate for the X-ray source emission.Comment: 7 pages, 1 figure, accepted for publication in MNRA
Investigating the influence of magnetic fields upon structure formation with AMIGA - a C code for cosmological magnetohydrodynamics
Despite greatly improved observational methods, the presence of magnetic
fields at cosmological scales and their role in the process of large-scale
structure formation still remains unclear. In this paper we want to address the
question how the presence of a hypothetical primordial magnetic field on large
scales influences the cosmic structure formation in numerical simulations. As a
tool for carrying out such simulations, we present our new numerical code
AMIGA. It combines an N-body code with an Eulerian grid-based solver for the
full set of MHD equations in order to conduct simulations of dark matter,
baryons and magnetic fields in a self-consistent way in a fully cosmological
setting. Our numerical scheme includes effective methodes to ensure proper
capturing of shocks and highly supersonic flows and a divergence-free magnetic
field. The high accuracy of the code is demonstrated by a number of numerical
tests. We then present a series of cosmological MHD simulations and confirm
that, in order to have a significant effect on the distribution of matter on
large scales, the primordial magnetic field strength would have to be
significantly higher than the current observational and theoretical
constraints.Comment: accepted by MNRAS, 24 pages, 14 figure
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