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Maxim V Barkov - One of the best experts on this subject based on the ideXlab platform.
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3d dynamics and morphology of bow shock pulsar wind nebulae
Monthly Notices of the Royal Astronomical Society, 2019Co-Authors: Maxim V Barkov, Maxim Lyutikov, D KhangulyanAbstract:Bow-shock pulsar wind nebulae (PWNe) show a variety of morphological shapes. We attribute this diversity to the geometrical factors: relative orientations of the pulsar rotation axis, proper velocity, and the line of sight (magnetic inclination angle may also have a certain influence on the morphology). We identify three basic types of bow-shock nebulae: (i) a "Rifle Bullet" (pulsar spin axis and proper velocity are aligned); (ii) a "Frisbee" (pulsar spin axis and proper velocity are orthogonal with the spin axis lying in the plane of the sky), and (iii) a Cart Wheel" (like frisbee but the spin axis is perpendicular to the plane of the sky). Using 3D RMHD simulations, as well as analytical calculations, we reproduce the key morphological features of the bow-shock PWNe, as well as variations, are seen across different systems. magnetic stresses within the shocked pulsar wind affect the overall structure strongly, producing "whiskers", "tails", "filled-in" and "mushroom" shapes, as well as non-symmetric morphologies. On the other hand, the interstellar medium inhomogeneities and the anisotropy of the energy flux in the pulsar wind have only a mild impact of the PWN morphology. In a few cases, when we clearly identify specific morphological structures, our results do not favor alignment of the pulsar spin axis and proper velocity. Our calculations of the underlying emission processes explain the low synchrotron X-ray efficiency (in terms of the spin-down luminosity) and imply an energetical subdominant contribution of the inverse Compton process.
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simulations of stellar pulsar wind interaction along one full orbit
arXiv: High Energy Astrophysical Phenomena, 2012Co-Authors: V Boschramon, Maxim V Barkov, D Khangulyan, M PeruchoAbstract:The winds from a non-accreting pulsar and a massive star in a binary system collide forming a bow-shaped shock structure. The Coriolis force induced by orbital motion deflects the shocked flows, strongly affecting their dynamics. We study the evolution of the shocked stellar and pulsar winds on scales in which the orbital motion is important. Potential sites of non-thermal activity are investigated. Relativistic hydrodynamical simulations in two dimensions, performed with the code PLUTO and using the adaptive mesh refinement technique, are used to model interacting stellar and pulsar winds on scales ~80 times the distance between the stars. The hydrodynamical results suggest the suitable locations of sites for particle acceleration and non-thermal emission. In addition to the shock formed towards the star, the shocked and unshocked components of the pulsar wind flowing away from the star terminate by means of additional strong shocks produced by the orbital motion. Strong instabilities lead to the development of turbulence and an effective two-wind mixing in both the leading and trailing sides of the interaction structure, which starts to merge with itself after one orbit. The adopted moderate pulsar-wind Lorentz factor already provides a good qualitative description of the phenomena involved in high-mass binaries with pulsars, and can capture important physical effects that would not appear in non-relativistic treatments. Simulations show that shocks, instabilities, and mass-loading yield efficient mass, momentum, and energy exchanges between the pulsar and the stellar winds. This renders a rapid increase in the entropy of the shocked structure, which will likely be disrupted on scales beyond the simulated ones. Several sites of particle acceleration and low- and high-energy emission can be identified. Doppler boosting will have significant and complex effects on radiation.
D Khangulyan - One of the best experts on this subject based on the ideXlab platform.
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3d dynamics and morphology of bow shock pulsar wind nebulae
Monthly Notices of the Royal Astronomical Society, 2019Co-Authors: Maxim V Barkov, Maxim Lyutikov, D KhangulyanAbstract:Bow-shock pulsar wind nebulae (PWNe) show a variety of morphological shapes. We attribute this diversity to the geometrical factors: relative orientations of the pulsar rotation axis, proper velocity, and the line of sight (magnetic inclination angle may also have a certain influence on the morphology). We identify three basic types of bow-shock nebulae: (i) a "Rifle Bullet" (pulsar spin axis and proper velocity are aligned); (ii) a "Frisbee" (pulsar spin axis and proper velocity are orthogonal with the spin axis lying in the plane of the sky), and (iii) a Cart Wheel" (like frisbee but the spin axis is perpendicular to the plane of the sky). Using 3D RMHD simulations, as well as analytical calculations, we reproduce the key morphological features of the bow-shock PWNe, as well as variations, are seen across different systems. magnetic stresses within the shocked pulsar wind affect the overall structure strongly, producing "whiskers", "tails", "filled-in" and "mushroom" shapes, as well as non-symmetric morphologies. On the other hand, the interstellar medium inhomogeneities and the anisotropy of the energy flux in the pulsar wind have only a mild impact of the PWN morphology. In a few cases, when we clearly identify specific morphological structures, our results do not favor alignment of the pulsar spin axis and proper velocity. Our calculations of the underlying emission processes explain the low synchrotron X-ray efficiency (in terms of the spin-down luminosity) and imply an energetical subdominant contribution of the inverse Compton process.
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simulations of stellar pulsar wind interaction along one full orbit
arXiv: High Energy Astrophysical Phenomena, 2012Co-Authors: V Boschramon, Maxim V Barkov, D Khangulyan, M PeruchoAbstract:The winds from a non-accreting pulsar and a massive star in a binary system collide forming a bow-shaped shock structure. The Coriolis force induced by orbital motion deflects the shocked flows, strongly affecting their dynamics. We study the evolution of the shocked stellar and pulsar winds on scales in which the orbital motion is important. Potential sites of non-thermal activity are investigated. Relativistic hydrodynamical simulations in two dimensions, performed with the code PLUTO and using the adaptive mesh refinement technique, are used to model interacting stellar and pulsar winds on scales ~80 times the distance between the stars. The hydrodynamical results suggest the suitable locations of sites for particle acceleration and non-thermal emission. In addition to the shock formed towards the star, the shocked and unshocked components of the pulsar wind flowing away from the star terminate by means of additional strong shocks produced by the orbital motion. Strong instabilities lead to the development of turbulence and an effective two-wind mixing in both the leading and trailing sides of the interaction structure, which starts to merge with itself after one orbit. The adopted moderate pulsar-wind Lorentz factor already provides a good qualitative description of the phenomena involved in high-mass binaries with pulsars, and can capture important physical effects that would not appear in non-relativistic treatments. Simulations show that shocks, instabilities, and mass-loading yield efficient mass, momentum, and energy exchanges between the pulsar and the stellar winds. This renders a rapid increase in the entropy of the shocked structure, which will likely be disrupted on scales beyond the simulated ones. Several sites of particle acceleration and low- and high-energy emission can be identified. Doppler boosting will have significant and complex effects on radiation.
Justin C. Kasper - One of the best experts on this subject based on the ideXlab platform.
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Temperature Anisotropy in a Shocked Plasma: Mirror-Mode Instabilities in the Heliosheath
The Astrophysical Journal, 2007Co-Authors: Ying Liu, John D. Richardson, John W. Belcher, Justin C. KasperAbstract:We show that temperature anisotropies induced at a shock can account for interplanetary and planetary bow shock observations. Shocked plasma with enhanced plasma β is preferentially unstable to the mirror-mode instability downstream of a quasi-perpendicular shock and to the fire-hose instability downstream of a quasi-parallel shock, consistent with magnetic fluctuations observed downstream of a large variety of shocks. Our theoretical analysis of the solar wind termination shock suggests that the magnetic holes observed by Voyager 1 in the heliosheath are produced by the mirror-mode instability. The results are also of astrophysical interest, providing an energy source for plasma heating.
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temperature anisotropy in a shocked plasma mirror mode instabilities in the heliosheath
arXiv: Astrophysics, 2007Co-Authors: John D. Richardson, Ying Liu, John W. Belcher, Justin C. KasperAbstract:We show that temperature anisotropies induced at a shock can account for interplanetary and planetary bow shock observations. Shocked plasma with enhanced plasma beta is preferentially unstable to the mirror mode instability downstream of a quasi-perpendicular shock and to the firehose instability downstream of a quasi-parallel shock, consistent with magnetic fluctuations observed downstream of a large variety of shocks. Our theoretical analysis of the solar wind termination shock suggests that the magnetic holes observed by Voyager 1 in the heliosheath are produced by the mirror mode instability. The results are also of astrophysical interest, providing an energy source for plasma heating.
Gaosheng Zhang - One of the best experts on this subject based on the ideXlab platform.
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optimization of initial substrate and ph levels for germination of sporing hydrogen producing anaerobes in cow dung compost
Bioresource Technology, 2004Co-Authors: Yaoting Fan, Jiunnjyi Lay, Hongwei Hou, Gaosheng ZhangAbstract:Biohydrogen production by anaerobic microbes enriched from heat-shocked cow dung compost was studied by using an artificial medium containing sucrose. Initial pH and substrate levels were selected as target factors in this study. Our experimental results demonstrated that optimal substrate concentration and pH for the composts generating hydrogen gas were 4.0+/-0.5 g sucrose/l and 5.4+/-0.2, respectively. Supplementary experiments confirmed that chemical oxygen demand reduction efficiency (69%) obtained from the conditions of sucrose=4.0 g/l and pH=5.5 was significantly greater than that (37%) from sucrose=5.0 g/l and pH=5.0. Experimental results of metabolites analysis led us to the conclusion that Clostridium sp. predominated in the anaerobic composts, suggesting that inocula used to seed the batch experiment can be obtained from a common natural source.
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optimization of initial substrate and ph levels for germination of sporing hydrogen producing anaerobes in cow dung compost
Bioresource Technology, 2004Co-Authors: Chenlin Li, Gaosheng ZhangAbstract:AbstractBiohydrogenproductionbyanaerobicmicrobesenrichedfromheat-shockedcowdungcompostwasstudiedbyusinganartificialmedium containing sucrose. Initial pH and substrate levels were selected as target factors in this study. Our experimental resultsdemonstratedthatoptimalsubstrateconcentrationandpHforthecompostsgeneratinghydrogengaswere4.0±0.5gsucrose/land5.4±0.2, respectively. Supplementary experiments confirmed that chemical oxygen demand reduction efficiency (69%) obtainedfromtheconditionsofsucrose¼4.0g/landpH¼5.5wassignificantlygreaterthanthat(37%)fromsucrose¼5.0g/landpH¼5.0.ExperimentalresultsofmetabolitesanalysisledustotheconclusionthatClostridiumsp.predominatedintheanaerobiccomposts,suggesting that inocula used to seed the batch experiment can be obtained from a common natural source. 2003 Elsevier Ltd. All rights reserved. Keywords:Cow dungcompost; Hydrogen; Sucrose;Anaerobic;Clostridia; COD 1. IntroductionHydrogenisanexcellentalternativeenergycandidateforthefuture.Thisisclearlythecasesincehydrogencanbe conveniently stored as a metal hydride and water isthe only by-product resulting from burning pure hy-drogen (Billings, 1991). Concerning global environ-mental impacts, such as greenhouse effect and resourcerecovery, microbial hydrogen production from renew-able biomass reduces dependence on fossil fuel, de-creasescarbondioxideemission,andrecoversbioenergy(Borkris, 1973).The microbial conversion of agricultural and indus-trial wastes and residues into hydrogen is attracting in-creasinginterest.Recently,someinvestigatorshaveusedanaerobic microorganisms, taken from anaerobic di-gested sludge, to generate hydrogen from starch in achemostatreactor(Afschar andSchaller,1991).Severalinvestigations also have been carried out using purecultures of anaerobic bacteria, such as Clostridium sp.,to study the conversion ofcarbohydrates (e.g., glucose)to hydrogen gas (Zeikus, 1980; Miyake et al., 1984;Taguchietal.,1992).However,onlytracesofhydrogenare usually evolved with continuous flow digesters duetotheubiquitousnatureofhydrogenconsumers(Kidbyand Nedwell, 1991). The clostridial bacteria could beenriched from soil, cracked cereals and comparablesourcesusingaheat-shocked treatment toexclude non-spore-formers (Biebl, 1999). Preliminary experimentshavesuccessfullyusedaheat-shockednaturalanaerobicmicroorganism to convert a simulated organic waste-waterand some agricultural solid wastes into hydrogengas (Lay et al., 1999; Fan et al., 2002, 2003).The objective of this study was to investigate theeffects of the environmental factors of initial pH andsubstrate concentration on clostridia-rich natural an-aerobegerminationforhydrogenproduction.Forthesepurposes, the following experiments were performedwith heat-shocked treatment on small batch reactorsusing sucrose and an undefined bacterial consortiumderived from anaerobic cow dung compost as inocula.
D Kraussvarban - One of the best experts on this subject based on the ideXlab platform.
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wave and ion evolution downstream of quasi perpendicular bow shocks
Journal of Geophysical Research, 1995Co-Authors: M E Mckean, N Omidi, D KraussvarbanAbstract:Distribution functions of ions heated in quasi-perpendicular bow shocks have a large perpendicular temperature anisotropy that provides free energy for the growth of Alfven ion cyclotron (AIC) waves and mirror waves. Both types of waves have been observed in the Earth's magnetosheath downstream of quasi-perpendicular shocks. The question of whether these waves are produced at the shock and convected downstream or whether they are produced locally in the magnetosheath has not yet been answered. If the latter were true, then under most magnetosheath conditions AIC waves should dominate the wave activity, yet frequently mirror waves either dominate or are competitive with the AIC mode. We address this question by using two-dimensional hybrid simulations to give a self-consistent description of the evolution of the wave spectra downstream of quasi-perpendicular shocks. Both mirror and AIC waves are identified in the simulated magnetosheath. They are generated at or near the shock front and convected away from it by the sheath plasma. Near the shock, the waves have a broad spectrum, but downstream of the shock, shorter-wavelength modes are heavily damped and only longer-wavelength modes persist. The characteristics of these surviving modes can be predicted with reasonable accuracy by linear kinetic theory appropriate for downstream conditions. Throughout the downstream region, the power in compressive magnetic oscillations is of the same order as the power in transverse oscillations. We also follow the evolution of the ion distribution function. The shocked ions that provide the free energy for wave growth have a two-component distribution function: a core population of directly transmitted ions and a smaller halo of initially reflected ions that contains the bulk of the free energy. The halo is initially gyrophase-bunched and extremely anisotropic. Within a relatively short distance downstream of the shock (of the order of 10 ion inertial lengths), wave-particle interactions remove these features from the halo and reduce the anisotropy of the distribution to near-threshhold levels for the mirror and AIC instabilities. A similar evolution has been observed for ions at the Earth's bow shock.
