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 HESS J1640-465

We present a Chandra X-ray observation of the very high energy $\gamma$-ray source HESS$$J1640-465. We identify a point source surrounded by a diffuse emission that fills the extended object previously detected by XMM Newton at the centroid of the HESS source, within the shell of the radio supernova remnant (SNR) G338.3-0.0. The morphology of the diffuse emission strongly resembles that of a pulsar wind nebula (PWN) and extends asymmetrically to the South-West of a point-source presented as a potential pulsar. The spectrum of the putative pulsar and compact nebula are well-characterized by an absorbed power-law model which, for a reasonable $N_{\rm H}$ value of $14\times 10^{22} \rm cm^{-2}$, exhibit an index of 1.1 and 2.5 respectively, typical of Vela-like PWNe. We demonstrate that, given the H$$I absorption features observed along the line of sight, the SNR and the H$$II 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 $L_{\rm PSR} \sim 1.3 \times 10^{33} d_{10 \rm kpc}^{2} \rm erg.s^{-1}$ and $L_{\rm PWN} \sim 3.9 \times 10^{33} d_{10}^{2} \rm erg.s^{-1}$ ($d_{10} = d / 10{\rm kpc}$). Both the flux ratio of $L_{\rm PWN}/L_{\rm PSR} \sim 3.4$ and the total luminosity of this system predict a pulsar spin-down power around $\dot{E} \sim 4 \times 10^{36} \rm erg s^{-1}$. 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 ($\sim 30$ 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, $N_f \leq 3$. In Newtonian hydrodynamics, e.g., there are indeed usually three free parameters, $N_f = 3$. If, however, the dynamical equations contain a universal dimensional constant, such as the speed of light in vacuum $c$ or the gravitational constant $G$, 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 $N_f = 2$. This is the case, for example, in magneto-hydrodynamics (MHD) or special relativistic hydrodynamics, where $c$ 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 $G$ appears in the equations. More generally, when there are $N_{udc}$ independent (in terms of their units) universal dimensional constants, then the number of free parameters is $N_f = max(0,3-N_{udc})$. When both gravity and relativity are included, there is only one free parameter ($N_f = 1$, as both $G$ and $c$ appear in the equations so that $N_{udc} = 2$), 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 ($\sim$ 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