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We report the results of 1D particle-in-cell simulations of ultrarelativistic shock waves in proton-electron-positron plasmas. We consider magnetized shock waves, in which the upstream medium carries a large scale magnetic field, directed transverse to the flow. Relativistic cyclotron instability of each species as the incoming particles encounter the increasing magnetic field within the shock front provides the basic plasma heating mechanism. The most significant new results come from simulations with mass ratio $m_p/m_\pm = 100$. We show that if the protons provide a sufficiently large fraction of the upstream flow energy density (including particle kinetic energy and Poynting flux), a substantial fraction of the shock heating goes into the formation of suprathermal power-law spectra of pairs. Cyclotron absorption by the pairs of the high harmonic ion cyclotron waves, emitted by the protons, provides the non-thermal acceleration mechanism. As the proton fraction increases, the non-thermal efficiency increases and the pairs' power-law spectra harden. We suggest that the varying power law spectra observed in synchrotron sources powered by magnetized winds and jets might reflect the correlation of the proton to pair content enforced by the underlying electrodynamics of these sources' outflows, and that the observed correlation between the X-ray spectra of rotation powered pulsars with the X-ray spectra of their nebulae might reflect the same correlation., 32 pages, 13 figures, accepted for publication in ApJ

We report here the first polarimetric measurements of the pulsars in the J0737-3039 binary neutron star system using the Green Bank Telescope. We conclude both that the primary star (A) has a wide hollow cone of emission, which is an expected characteristic of the relatively open magnetosphere given its short spin period, and that A has a small angle between its spin and magnetic dipole axes, $4\pm 3$ degrees. This near alignment of axes suggests that A's wind pressure on B's magnetosphere will depend on orbital phase. This variable pressure is one mechanism for the variation of flux and profile shape of B with respect to the orbital phase that has been reported. The response of B to the A wind pressure will also depend on the particular side of its magnetosphere facing the wind at the spin phase when B is visible. This is a second possible mechanism for variability. We suggest that B may have its spin axis aligned with the orbital angular momentum owing to A's wind torque that contributes to its spindown. Monitoring the pulsars while geodetic precession changes spin orientations will provide essential evidence to test detailed theoretical models. We determine the Rotation Measures of the two stars to be $-112.3\pm 1.5$ and $-118\pm 12$ rad m$^{-2}$., Comment: Submitted to ApJ Letters

We present a model for magnetic energy dissipation in a pulsar wind nebula. Better understanding of this process is required to assess the likelihood that certain astrophysical transients may be powered by the spin-down of a "millisecond magnetar." Examples include superluminous supernovae, gamma-ray bursts, and anticipated electromagnetic counterparts to gravitational wave detections of binary neutron star coalescence. Our model leverages recent progress in the theory of turbulent magnetic relaxation to specify a dissipative closure of the stationary magnetohydrodynamic (MHD) wind equations, yielding predictions of the magnetic energy dissipation rate throughout the nebula. Synchrotron losses are treated self-consistently. To demonstrate the model's efficacy, we show that it can reproduce many features of the Crab Nebula, including its expansion speed, radiative efficiency, peak photon energy, and mean magnetic field strength. Unlike ideal MHD models of the Crab (which lead to the so-called sigma problem) our model accounts for the transition from ultra to weakly magnetized plasma flow, and for the associated heating of relativistic electrons. We discuss how the predicted heating rates may be utilized to improve upon models of particle transport and acceleration in pulsar wind nebulae. We also discuss implications for the Crab Nebula's gamma-ray flares, and point out potential modifications to models of astrophysical transients invoking the spin-down of a millisecond magnetar., Submitted to ApJ

We report the discovery of kilohertz fluctuations, including quasiperiodic oscillations (QPO) at {approx}330 Hz (260-407 Hz) and {approx}760 Hz (671-849 Hz) and a broadband kilohertz continuum in the power-density spectrum of the high-mass X-ray binary pulsar Centaurus X-3. These observations of Cen X-3 were carried out with the Rossi X-Ray Timing Explorer (RXTE). The fluctuation spectrum is flat from millihertz to a few hertz, then steepens to f{sup -2} behavior between a few hertz and {approx}100 Hz. Above 100 Hz, the spectrum shows the QPO features, plus a flat continuum extending to {approx}1200 Hz and then falling out to {approx}1800 Hz. These results, which required the co-adding three days of observations of Cen X-3, are at least as fast as the fastest known variations in X-ray emission from an accreting compact object (kilohertz QPO in LMXB sources) and probably faster since extension to {approx}1800 Hz is indicated by the most likely parameterization of the data. Multidimensional radiation hydrodynamics simulations of optically thick plasma flow onto the magnetic poles of an accreting neutron star show that the fluctuations at frequencies above 100 Hz are consistent with photon bubble turbulence and oscillations (PBO) previously predicted to be observable in this source. Wemore » show that previous observations of Cen X-3 constrain the models to depend on only one parameter, the size of the polar cap. For a polar cap opening angle of 0.25 radians (polar cap radius {approx}2.5 km and area {approx}20 km2, for a neutron star radius of 10 km), we show that the spectral form above 100 Hz is reproduced by the simulations, including the frequencies of the QPO and the relative power in the QPO and the kilohertz continuum. This has resulted in the first model-dependent measurement of the polar cap size of an X-ray pulsar. The simulations underpredict the overall amplitude of the observed spectrum, which we suggest is the consequence of a two-dimensional axisymmetric simulation of an intrinsically three-dimensional phenomenon. The power-density spectrum of Cen X-3 shows a dramatic decrease above {approx}1000 Hz, which suggests an optical depth {approx}30 across the accretion mound consistent with effects of radiative diffusion in the simulations. We identify this decline at high frequency as the first direct evidence of radiative diffusion near the surface of a neutron star (NS). We suggest the fluctuations observed at frequencies below 100 Hz, whose spectrum has a different form from that of the kilohertz phenomena, reflect intermittency in the mass transfer mechanism that carries plasma from the accretion disk to field aligned flow onto the neutron star's polar caps. Using simple estimates based on Rayleigh-Taylor instabilities, possibly modulated by intrinsic disk turbulence, we show that mass transfer in ''blobs'' forming through Rayleigh-Taylor disruption of the disk's inner edge can explain the large amplitude fluctuations required by the spectrum at frequencies f{approx}1 Hz, but only if magnetic pressure in the disk's innermost regions inflates the disk until its scale height is comparable to the magnetosphere's size {approx}4300 km. The observational results required the development of a procedure for the careful determination of the dead-time effects of the PCA. This procedure is described in Appendix A. As a consequence of the use of observations of Cyg X-1 for the estimation of dead-time corrections for the observations of Cen X-3, we have also demonstrated that the black hole Cyg X-1 shows clear evidence of variability up to a frequency of {approx}280 Hz. Also observations of GX 17+2 were used to validate the procedure for dead-time corrections. This analysis of GX 17+2 clearly indicates the presence of a kilohertz QPO and the absence of any significant simultaneous kilohertz continuum. (c) 2000 The American Astronomical Society.« less

An initially static, plane-parallel, radiation pressure supported exponential atmosphere in a strong magnetic field is found to be unstable to the growth of buoyant low-density regions or "photon bubbles" in linear stability analysis. Here we present a series of numerical studies of the photon bubbles in such an atmosphere carried out using a quasi-two-dimensional radiation hydrodynamical code. When a single-mode perturbation is applied to the atmosphere, we find that the growth of these bubbles is in good agreement with the linear theory. When the evolution becomes nonlinear, the growth of the photon bubbles is found to be an efficient mechanism of energy transport. The presence of the low-density regions helps to increase the photon diffusion speed by a factor of several, while the buoyancy of the bubbles serves to transport energy via advection. Multimode studies, consisting of a perturbation with the linear combination of two single modes and a random perturbation, suggest that there is a tendency toward merger of the photon bubbles in their transport properties. The small wavenumber modes eventually dominate in radiation pressure, while the large wavenumber modes, although having higher growth rates, also saturate more quickly, and their contribution to the energy transport can be truncated. A grid study and the multimode calculations indicate that the energy transport through the atmosphere is well represented as long as the photon bubble mode with optical depth of ~10 over a wavelength is well resolved. Possible applications of the photon bubble instability includes the settling layer in the accretion column of a neutron star undergoing super-Eddington accretion. The enhanced energy transport may manifest itself in the emergent spectrum from the accreting pulsars and in the short-time variability in the light curves.

We study the physical processes of the PSR B1259-63 system containing a 47 ms pulsar orbiting around a Be star in a highly eccentric orbit. Motivated by the results of a multiwavelength campaign during the January 1994 periastron passage of PSR B1259-63, we discuss several issues regarding the mechanism of high-energy emission. Unpulsed power law emission from the this system was detected near periastron in the energy range 1-200 keV. We find that the observed high energy emission from the PSR B1259-63 system is not compatible with accretion or propeller-powered emission. Shock-powered high-energy emission produced by the pulsar/outflow interaction is consistent with all high energy observations. By studying the evolution of the pulsar cavity we constrain the magnitude and geometry of the mass outflow outflow of the Be star. The pulsar/outflow interaction is most likely mediated by a collisionless shock at the internal boundary of the pulsar cavity. The system shows all the characteristics of a {\it binary plerion} being {\it diffuse} and {\it compact} near apastron and periastron, respectively. The PSR B1259-63 cavity is subject to different radiative regimes depending on whether synchrotron or inverse Compton (IC) cooling dominates the radiation of electron/positron pairs advected away from the inner boundary of the pulsar cavity. The highly non-thermal nature of the observed X-ray/gamma-ray emission near periastron establishes the existence of an efficient particle acceleration mechanism within a timescale shown to be less than $\sim 10^2-10^3$ s. A synchrotron/IC model of emission of e\pm-pairs accelerated at the inner shock front of the pulsar cavity and adiabatically expanding in the MHD flow provides an excellent explanation of the observed time variableX-ray flux and spectrum from the PSR, 68 pages, accepted for publication in the Astrophys. J. on Aug. 26, 1996

We discuss our recent Rossi X-Ray Timing Explorer (RXTE) observations of GRO J1744-28, which discovered quasi-periodic oscillations (QPOs) of the intensities in the energy band 3-12 keV during observations starting on 1996 January 18.77 UT. We have found that the power spectrum in the frequency band 5-1000 Hz consists of two red-noise components that can be characterized by two power laws, each with an index of -5/3 and a QPO peak centered at 40 Hz. We suggest that the power peak is due to a newly discovered form of turbulence in the accretion column of super-Eddington accretion-powered pulsars driven by photon bubble instabilities. These instabilities give rise to strong power peaks at frequencies characteristic of photon diffusion and bubble coalescence in the highly nonlaminar accretion column resulting in photon bubble oscillations (PBOs). The relationship between the rms amplitude of the PBOs and the intensity is in qualitative agreement with observations. Our calculations also suggest that the observed high-frequency red-noise component with a -5/3 power-law index from 40 to 600 Hz is the first evidence of photon bubble turbulence in the accretion column of an X-ray pulsar. Recent RXTE observations of Sco X-1 have found high-frequency QPOs at 1100 and 830 Hz. We show that PBOs at these frequencies are a natural consequence of photon bubble instabilities. We also show that the rms amplitudes of the calculated PBOs at these frequencies are consistent with the observations. We predict that further RXTE observations of Sco X-1 should reveal additional PBOs at 2000 and 2600 Hz as well as a broadband continuum spectrum with a -5/3 power law, extending from 3000 to several times 104 Hz.

One of the most intriguing results from the gamma-ray instruments in orbit has been the detection of powerful flares from the Crab Nebula. These flares challenge our understanding of pulsar wind nebulae and models for particle acceleration. We report on the portion of a multiwavelength campaign using Keck, HST, and Chandra concentrating on a small emitting region, the Crab's inner knot, located a fraction of an arcsecond from the pulsar. We find that the knot's radial size, tangential size, peak flux, and the ratio of the flux to that of the pulsar are correlated with the projected distance of the knot from the pulsar. A new approach, using singular value decomposition for analyzing time series of images, was introduced yielding results consistent with the more traditional methods while some uncertainties were substantially reduced. We exploit the characterization of the knot to discuss constraints on standard shock-model parameters that may be inferred from our observations assuming the inner knot lies near to the shocked surface. These include inferences as to wind magnetization, shock shape parameters such as incident angle and poloidal radius of curvature, as well as the IR/optical emitting particle enthalpy fraction. We find that while the standard shock model gives good agreement with observation in many respects, there remain two puzzles: (a) The observed angular size of the knot relative to the pulsar--knot separation is much smaller than expected; (b) The variable, yet high degree of polarization reported is difficult to reconcile with a highly relativistic downstream flow., Comment: 46 pages, 14 figures, submitted to the Astrophysical Journal

The afterglow emission from gamma-ray bursts (GRBs) is usually interpreted as synchrotron radiation from electrons accelerated at the GRB external shock, that propagates with relativistic velocities into the magnetized interstellar medium. By means of multi-dimensional particle-in-cell simulations, we investigate the acceleration performance of weakly magnetized relativistic shocks, in the magnetization range 0, Comment: 23 pages, 15 figures, submitted to ApJ

We describe time-dependent, two-dimensional axisymmetric radiation hydrodynamic calculations of locally super-Eddington accretion onto highly magnetized neutron stars appropriate to the flow onto the polar caps of high-luminosity X-ray pulsars. Our calculations show the development and nonlinear evolution of photon bubble instabilities in the settling plasma below the radiation-dominated shock that terminates free fall along the magnetic field. The photon bubbles develop as elongated, very low density, optically thin regions of local outflow forming typically a few kilometers above the neutron star's surface with outflow velocities of order 0.1c. These "holes" in the plasma reduce the total opacity of the accretion mound and through photon advection significantly enhance the efficiency of radiation transport. The number of bubbles varies from a few to 20 and results in significant fluctuations (4%-10%) in the emitted luminosity. These fluctuations appear as reasonably high quality (Q) "photon bubble oscillations" (PBOs) in the power spectrum of the emergent luminosity time series on timescales between 0.1 and 1 ms, a phenomenon not previously observed. The discovery of PBOs would provide a powerful probe into the physical characteristics of super-Eddington flows. We use our results to exhibit power spectra of the predicted luminosity oscillations and discuss the feasibility of their detection by the X-Ray Timing Explorer (XTE). We also use our results to calculate the emergent spectrum and show that it is a highly depleted modified blackbody distribution roughly consistent with known rotation phase-averaged observations in the 10-100 keV range. Thus we have shown that the spectra of X-ray pulsars are a natural consequence of the dynamics of polar cap accretion.

The PSR B1259-63 system (Johnston et al. 1992, 1994) was observed near periastron by the Compton Gamma-Ray Observatory in 1994 January. This system contains a rapidly rotating pulsar and a Be star in a highly eccentric binary orbit. We report the discovery by the OSSE instrument of unpulsed emission with a hard power-law spectrum between 50 and 200 keV from the direction of this system. Neither diffuse Galactic background emission nor nearby X-ray binaries contribute significantly to the detected flux. Our results are particularly important for the theory of interaction of pulsars with gaseous environments. We interpret the hard X-ray emission as synchrotron radiation from relativistic particles of the PSR B1259-63 wind being shocked and accelerated within the binary. Our results indicate, for the first time in a binary pulsar, that shock acceleration can increase the original energy of pulsar wind particles by a factor 10, despite the high synchrotron and inverse Compton cooling rates near periastron. The derived shock properties (efficiency, radiation spectrum, timescale) are relevant for a broad class of high-energy astrophysical sources characterized by shocked relativistic plasmas subject to strong radiative cooling.

We propose a model of a optical 'wisps' of the Crab Nebula, features observed in the nebular synchrotron surface brightness near the central pulsar, as manifestations of the internal structure of the shock terminating the pulsar wind. We assume that this wind is composed of ions and a much denser plasma of electrons and positrons, frozen together to a toroidal magnetic field and flowing relativistically. We construct a form of solitary wave model of the shock structure in which we self-consistently solve for the ion orbits and the dynamics of the relativistically hot, magnetized e(+/-) background flow. We ignore dispersion in the ion energies, and we treat the pairs as an adiabatic fluid. The synchrotron emission enhancements, observed as the wisps, are then explained as the regions where reflection of the ions in the self-consistent magnetic field causes compressions of the e(+/-).

PSR B1259-63 is a 47 ms radio pulsar in a wide, eccentric orbit with a Be star. We study the shock interaction between the pulsar and the companion's mass outflow and investigate the time evolution of radiative shock regimes. We find that for small values of the Be star's mass-loss rate, inverse-Compton scattering is likely to dominate the shock emission. Alternately, for a large mass-loss rate, synchrotron emission will dominate. Multifrequency X-ray and gamma-ray observations near periastron can distinguish between these cases and yield unique constraints on the pulsar and Be star winds. The PSR B1259-63 system provides a unique laboratory to study the time-dependent interaction of a pulsar wind with the circumbinary material from its companion star.

We show that the recently discovered optical and ultraviolet light from the X-ray burst source 4U 1820-30 in the globular cluster NGC 6624 is due to reprocessing of the X-rays in the outer regions of an optically thick, geometrically thin accretion disk. We suggest that observation of orbital modulation of the reprocessed light, due to the variable contribution made as the heated face of the companion turns toward and away from the observer, would provide constraints on the inclination of the binary orbit, and we suggest that detection in the reprocessed flux of the 'red noise' already observed in the X-rays would provide useful constraints on the geometry and physics of the accretion disk.

It is shown that the location of the spin-up line in the P-P˙ diagram is sensitive to the presence of large-scale magnetic anomalies (quadrupole,...) and that one can usefully constrain the surface strength of such fields to be not more than ∼40% of the surface strength of the dipole in spun-up pulsars. This constraint shows that if gamma-ray bursters are related to old Galactic neutron stars, those with strong cyclotron lines cannot have had the same magnetic history as spun-up millisecond pulsars. The spin-up line also constrains models in which the magnetic field is a single intense surface pole, although the present data set has too few very short period pulsars with measured P˙ to provide useful information

The properties of the high-energy emission expected from the eclipsing millisecond pulsar system PSR 1957+20 are investigated. Emission is considered by both the relativistic shock produced by the pulsar wind in the nebula surrounding the binary and by the shock constraining the mass outflow from the companion star of PSR 1957+20. On the basis of the results of microscopic plasma physical models of relativistic shocks it is suggested that the high-energy radiation is produced in the range from X-rays to MeV gamma rays in the binary and in the range from 0.01 eV to about 40 keV in the nebula. Doppler boost of the emission in the radiating wind suggests the flux should vary on the orbital time scale, with the largest flux observed roughly coincident with the pulsar's radio eclipse.

The formation of 'photon bubbles' in a convectively stable scattering atmosphere supported against gravity entirely by radiation pressure is studied by means of linear stability theory. A simple model is developed for the 2D structure of a plasma mound formed by laminar accretion onto the magnetic poles of a neutron star, in which upward photon diffusion balances downward photon advection with the plasma. It is shown that the vertical pressure and density structure is the same as in an isothermal atmosphere. Application of the stability theory to this model suggests photon bubbles would form in a polar accretion mound under the conditions expected in accretion-powered pulsars within a few tenths of a millisecond. Because long-wavelength modes have the largest rise speeds, eventual dominance by a few large bubbles is suggested, and possible connections between bubble formation and short-time variability in accretion-powered pulsars is discussed, as well as a possible connection of the photon bubble phenomenon to the rapid time variability observed in the Rapid Burster and in quasi-period oscillator sources.