Electron-ion coupling upstream of relativistic collisionless shocks

It is argued and demonstrated by particle-in-cell simulations that the synchrotron maser instability could develop at the front of a relativistic, magnetized shock. The instability generates strong low-frequency electromagnetic waves propagating both upstream and downstream of the shock. Upstream of the shock, these waves make electrons lag behind ions so that a longitudinal electric field arises and the electrons are accelerated up to the ion kinetic energy. Then thermalization at the shock front results in a plasma with equal temperatures of electrons and ions. Downstream of the shock, the amplitude of the maser-generated wave may exceed the strength of the shock-compressed background magnetic field. In this case the shock-accelerated particles radiate via nonlinear Compton scattering rather than via a synchrotron mechanism. The spectrum of the radiation differs, in the low-frequency band, from that of the synchrotron radiation, providing possible observational tests of the model.Comment: 22 pages, 10 figures. To appear in ApJ vol. 65

Electrodynamics of Magnetars: Implications for the Persistent X-ray Emission and Spindown of the Soft Gamma Repeaters and Anomalous X-ray Pulsars

(ABBREVIATED) We consider the structure of neutron star magnetospheres threaded by large-scale electrical currents, and the effect of resonant Compton scattering by the charge carriers (both electrons and ions) on the emergent X-ray spectra and pulse profiles. In the magnetar model for the SGRs and AXPs, these currents are maintained by magnetic stresses acting deep inside the star. We construct self-similar, force-free equilibria of the current-carrying magnetosphere with a power-law dependence of magnetic field on radius, B ~ r^(-2-p), and show that a large-scale twist softens the radial dependence to p < 1. The spindown torque acting on the star is thereby increased in comparison with a vacuum dipole. We comment on the strength of the surface magnetic field in the SGR and AXP sources, and the implications of this model for the narrow measured distribution of spin periods. A magnetosphere with a strong twist, B_\phi/B_\theta = O(1) at the equator, has an optical depth ~ 1 to resonant cyclotron scattering, independent of frequency (radius), surface magnetic field strength, or charge/mass ratio of the scattering charge. When electrons and ions supply the current, the stellar surface is also heated by the impacting charges at a rate comparable to the observed X-ray output of the SGR and AXP sources, if B_{dipole} ~ 10^{14} G. Redistribution of the emerging X-ray flux at the ion and electron cyclotron resonances will significantly modify the emerging pulse profile and, through the Doppler effect, generate a non-thermal tail to the X-ray spectrum. The sudden change in the pulse profile of SGR 1900+14 after the 27 August 1998 giant flare is related to an enhanced optical depth to electron cyclotron scattering, resulting from a sudden twist imparted to the external magnetic field.Comment: 31 January 2002, minor revisions, new section 5.4.

A right-handed isotropic medium with a negative refractive index

The sign of the refractive index of any medium is soley determined by the requirement that the propagation of an electromagnetic wave obeys Einstein causality. Our analysis shows that this requirement predicts that the real part of the refractive index may be negative in an isotropic medium even if the electric permittivity and the magnetic permeability are both positive. Such a system may be a route to negative index media at optical frequencies. We also demonstrate that the refractive index may be positive in left-handed media that contain two molecular species where one is in its excited state.Comment: 4.1 pages, 4 figures, submitted to Physical Review Letter

Advanced research of the impact of rotor bars anisotropic conductivity on starting torque of an induction motor with a die-cast copper cage

The paper deals with an optimal design of an induction motor with a die-cast copper cage rotor characterized by an anisotropic layer in the upper part of the rotor bars. 1D FEM optimization of an explosion-proof induction motor with a die-cast copper rotor with overall parameters of a commercial induction motor with a die-cast aluminum rotor (180 kW, 1140 V, 1500 rpm) is made with the help of numeric computations. In the paper, the design features and experimental results are given. It is shown that induction motors with a special "starting" layer placed in the upper part the diecast copper rotor bar have maximum value of starting torque for relative conductivity factors ranging from 0.3 to 0.5. The basic performance data obtained in both bench and service tests of a prototype 210kW induction motor are in close agreement with analytical data.Изложены результаты исследований влияния анизотропии проводимости стержней ротора на пусковой момент асинхронного двигателя, стендовых и эксплуатационных испытаний взрывозащищенного двигателя типа 2ЭКВ3,5-210 мощностью 210 кВт с литой анизотропной медной обмоткой ротора

Collective processes in relativistic plasma and their implications for gamma-ray burst afterglows

We consider the effects of collective plasma processes on synchrotron emission from highly relativistic electrons. We find, in agreement with Sazonov (1970), that strong effects are possible also in the absence of a non-relativistic plasma component, due to the relativistic electrons (and protons) themselves. In contrast with Sazonov, who infers strong effects only in cases where the ratio of plasma frequency to cyclotron frequency is much larger than the square of the characteristic electron Lorentz factor, nu_p/nu_B >> gamma^2, we find strong effects also for 1 << nu_p/nu_B << gamma^2. The modification of the spectrum is prominent at frequencies nu < nu_{R*} = nu_p min[gamma, (nu_p/nu_B)^(1/2)], where nu_{R*} generalizes the Razin-Tsytovich frequency, nu_R = gamma nu_p, to the regime nu_p/nu_B << gamma^2. Applying our results to gamma-ray burst (GRB) plasmas, we predict a strong modification of the radio spectrum on minute time scale following the GRB, at the onset of fireball interaction with its surrounding medium, in cases where the ratio of the energy carried by the relativistic electrons to the energy carried by the magnetic field exceeds ~ 10^5. Plausible electron distribution functions may lead to negative synchrotron reabsorption, i.e to coherent radio emission, which is characterized by a low degree of circular polarization. Detection of these effects would constrain the fraction of energy in the magnetic field, which is currently poorly determined by observations, and, moreover, would provide a novel handle on the properties of the environment into which the fireball expands.Comment: 28 pages, 1 figure, submitted to Ap

Resonant Cyclotron Radiation Transfer Model Fits to Spectra from Gamma-Ray Burst GRB870303

We demonstrate that models of resonant cyclotron radiation transfer in a strong field (i.e. cyclotron scattering) can account for spectral lines seen at two epochs, denoted S1 and S2, in the Ginga data for GRB870303. Using a generalized version of the Monte Carlo code of Wang et al. (1988,1989b), we model line formation by injecting continuum photons into a static plane-parallel slab of electrons threaded by a strong neutron star magnetic field (~ 10^12 G) which may be oriented at an arbitrary angle relative to the slab normal. We examine two source geometries, which we denote "1-0" and "1-1," with the numbers representing the relative electron column densities above and below the continuum photon source plane. We compare azimuthally symmetric models, i.e. models in which the magnetic field is parallel to the slab normal, with models having more general magnetic field orientations. If the bursting source has a simple dipole field, these two model classes represent line formation at the magnetic pole, or elsewhere on the stellar surface. We find that the data of S1 and S2, considered individually, are consistent with both geometries, and with all magnetic field orientations, with the exception that the S1 data clearly favor line formation away from a polar cap in the 1-1 geometry, with the best-fit model placing the line-forming region at the magnetic equator. Within both geometries, fits to the combined (S1+S2) data marginally favor models which feature equatorial line formation, and in which the observer's orientation with respect to the slab changes between the two epochs. We interpret this change as being due to neutron star rotation, and we place limits on the rotation period.Comment: LaTeX2e (aastex.cls included); 45 pages text, 17 figures (on 21 pages); accepted by ApJ (to be published 1 Nov 1999, v. 525

On the Possibility of the Detection of Extinct Radio Pulsars

We explore the possibilities for detecting pulsars that have ceased to radiate in the radio band. We consider two models: the model with hindered particle escape from the pulsar surface (first suggested by Ruderman and Sutherland 1975) and the model with free particle escape (Arons 1981; Mestel 1999). In the model with hindered particle escape, the number of particles that leave the pulsar magnetosphere is small and their radiation cannot be detected with currently available instruments. At the same time, for the free particle escape model, both the number of particles and the radiation intensity are high enough for such pulsars to be detectable with the presently available receivers such as GLAST and AGILE spacecrafts. It is also possible that extinct radio pulsars can be among the unidentified EGRET sources.Comment: 5 pages, 1 figure corrected version of the paper that was published in Astronomy Letter

A comparison of weak-turbulence and PIC simulations of weak electron-beam plasma interaction

Quasilinear theory has long been used to treat the problem of a weak electron beam interacting with plasma and generating Langmuir waves. Its extension to weak-turbulence theory treats resonant interactions of these Langmuir waves with other plasma wave modes, in particular ion-sound waves. These are strongly damped in plasma of equal ion and electron temperatures, as sometimes seen in, for example, the solar corona and wind. Weak turbulence theory is derived in the weak damping limit, with a term describing ion-sound wave damping then added. In this paper we use the EPOCH particle-in-cell code to numerically test weak turbulence theory for a range of electron-ion temperature ratios. We find that in the cold ion limit the results agree well, but increasing ion temperature the three-wave resonance becomes broadened in proportion to the ion-sound wave damping rate. This may be important in, for example, the theory of solar radio bursts, where the spectrum of Langmuir waves is critical. Additionally we establish lower limits on the number of simulation particles needed to accurately reproduce the electron and wave distributions in their saturated states, and to reproduce their intermediate states and time evolution.Comment: Accepted by PO

A Chandra Search for Coronal X Rays from the Cool White Dwarf GD 356

We report observations with the Chandra X-ray Observatory of the single, cool, magnetic white dwarf GD 356. For consistent comparison with other X-ray observations of single white dwarfs, we also re-analyzed archival ROSAT data for GD 356 (GJ 1205), G 99-47 (GR 290 = V1201 Ori), GD 90, G 195-19 (EG250 = GJ 339.1), and WD 2316+123 and archival Chandra data for LHS 1038 (GJ 1004) and GD 358 (V777 Her). Our Chandra observation detected no X rays from GD 356, setting the most restrictive upper limit to the X-ray luminosity from any cool white dwarf -- L_{X} < 6.0 x 10^{25} ergs/s, at 99.7% confidence, for a 1-keV thermal-bremsstrahlung spectrum. The corresponding limit to the electron density is n_{0} < 4.4 x 10^{11} cm^{-3}. Our re-analysis of the archival data confirmed the non-detections reported by the original investigators. We discuss the implications of our and prior observations on models for coronal emission from white dwarfs. For magnetic white dwarfs, we emphasize the more stringent constraints imposed by cyclotron radiation. In addition, we describe (in an appendix) a statistical methodology for detecting a source and for constraining the strength of a source, which applies even when the number of source or background events is small.Comment: 27 pages, 4 figures, submitted to the Astrophysical Journa

Enhancing Acceleration Radiation from Ground-State Atoms via Cavity Quantum Electrodynamics

When ground state atoms are accelerated through a high Q microwave cavity, radiation is produced with an intensity which can exceed the intensity of Unruh acceleration radiation in free space by many orders of magnitude. The cavity field at steady state is described by a thermal density matrix under most conditions. However, under some conditions gain is possible, and when the atoms are injected in a regular fashion, the radiation can be produced in a squeezed state