High-Altitude Particle Acceleration and Radiation in Pulsar Slot Gaps

We explore the pulsar slot gap electrodynamics up to very high altitudes, where for most relatively rapidly rotating pulsars both the standard small-angle approximation and the assumption that the magnetic field lines are ideal stream lines break down. We address the importance of the electrodynamic conditions at the slot gap boundaries and the occurrence of a steady-state drift of charged particles across the slot gap field lines at very high altitudes. These boundary conditions and the deviation of particle trajectories from stream lines determine the asymptotic behavior of the scalar potential at all radii from the polar cap to near the light cylinder. As a result, we demonstrate that the steady-state accelerating electric field must approach a small and constant value at high altitude above the polar cap. This parallel electric field is capable of maintaining electrons moving with high Lorentz factors (a few times 10^7) and emitting curvature gamma-ray photons up to nearly the light cylinder. By numerical simulations, we show that primary electrons accelerating from the polar cap surface to high altitude in the slot gap along the outer edge of the open field region will form caustic emission patterns on the trailing dipole field lines. Acceleration and emission in such an extended slot gap may form the physical basis of a model that can successfully reproduce some pulsar high-energy light curves.Comment: 26 pages, 2 figures, to appear in the Astrophysical Journal, May 10, 200

A Photon Splitting Cascade Model of Soft Gamma-Ray Repeaters

The spectra of soft gamma-ray repeaters (SGRs), with the exception of the March 5, 1979 main burst, are characterized by high-energy cutoffs around 30 keV and low-energy turnovers that are much steeper than a Wien spectrum. Baring (1995) found that the spectra of cascades due to photon splitting in a very strong, homogeneous magnetic field can soften spectra and produce good fits to the soft spectra of SGRs. Magnetic field strengths somewhat above the QED critical field strength $B_{\rm cr}$, where $B_{\rm cr} = 4.413 \times 10^{13}$ G, is required to produce cutoffs at 30-40 keV. We have improved upon this model by computing Monte Carlo photon splitting cascade spectra in a neutron star dipole magnetic field, including effects of curved space-time in a Schwarzschild metric. We investigate spectra produced by photons emitted at different locations and observer angles. We find that the general results of Baring hold for surface emission throughout most of the magnetosphere, but that emission in equatorial regions can best reproduce the constancy of SGR spectra observed from different bursts.Comment: 5 pages in LATEX using REVTEX aipbook.sty + 4 figures (uuencoded, compressed postscript), to appear in the proceedings of the Third Huntsville Workshop on Gamma-Ray Bursts, eds. C. Kouveliotou, M. S. Briggs and G. J. Fishman (New York, AIP

Photon Splitting in Magnetar Models of Soft Gamma Repeaters

The recent association of soft gamma repeaters (SGRs) with counterparts in other wavebands has sparked much interest in these sources. One of the recent models for these objects is that they originate in the environs of neutron stars with fields much stronger than the quantum critical field \teq{B_{cr}=4.413\times 10^{13}} Gauss. Near such neutron stars, dubbed magnetars, the exotic quantum process of magnetic photon splitting becomes prolific. Its principal effect is to degrade photon energies and thereby soften gamma-ray spectra from neutron stars; it has recently been suggested that splitting may be responsible for limiting the hardness of emission in SGRs, if these sources originate in neutron stars with supercritical surface fields. Seed photons in supercritical fields efficiently generate soft gamma-ray spectra, typical of repeaters. In this paper, the influence of the curved dipole field geometry of a neutron star magnetosphere on the photon splitting rate is investigated. The dependence of the attenuation length on the location and angular direction of the seed photons is explored.Comment: 5 pages including 3 encapsulated figures, as a compressed, uuencoded, Postscript file. To appear in Proc. of the 1995 La Jolla workshop ``High Velocity Neutron Stars and Gamma-Ray Bursts'' eds. Rothschild, R. et al., AIP, New Yor

A New Class of Radio Quiet Pulsars

The complete absence of radio pulsars with periods exceeding a few seconds has lead to the popular notion of the existence of a high $P$ death line. In the standard picture, beyond this boundary, pulsars with low spin rates cannot accelerate particles above the stellar surface to high enough energies to initiated pair cascades through curvature radiation, and the pair creation needed for radio emission is strongly suppressed. In this paper we postulate the existence of another pulsar ``death line,'' corresponding to high magnetic fields $B$ in the upper portion of the $\dot{P}$--$P$ diagram, a domain where few radio pulsars are observed. The origin of this high $B$ boundary, which occurs when $B$ becomes comparable to or exceeds $10^{13}$ Gauss, is again due to the suppression of magnetic pair creation $\gamma\to e^+e^-$, but in this instance, primarily because of ineffective competition with the exotic QED process of magnetic photon splitting. This paper describes the origin, shape and position of the new ``death line,'' above which pulsars are expected to be radio quiet, but perhaps still X-ray and $\gamma$-ray bright.Comment: 5 pages, including 1 eps figure, to appear in Proc. 4th Compton Symposium, (1997) ed. Dermer, C. D. & Kurfess, J. D. (AIP, New York

Two-photon annihilation of thermal pairs in strong magnetic fields

The annihilation spectrum of pairs with 1-D thermal distributions in the presence of a strong magnetic field is calculated. Numerical analysis of the spectrum are performed for mildly relativistic temperatures and for different angles of emission with respect to field lines. Teragauss magnetic fields are assumed so that conditions are typical of gamma ray burst and pulsar environments. The spectra at each viewing angle reveal asymmetric line profiles that are signatures of the magnetic broadening and red shifting of the line: these asymmetries are more prominent for small viewing angles. Thermal Doppler broadening tends to dominate in the right wing of the line and obscures the magnetic broadening more at high temperatures and smaller viewing angles. This angular dependence of the line asymmetry may prove a valuable diagnostic tool. For low temperatures and magnetic field strengths, useful analytic expressions are presented for the line width, and also for the annihilation spectrum at zero viewing angle. The results presented find application in gamma ray burst and pulsar models, and may prove very helpful in deducing field strengths and temperatures of the emission regions of these objects from line observations made by Compton GRO and future missions

Evaluation of moire techniques for wind tunnel metrology

The development of a moire technique suitable for the analysis of object deflections in a cryogenically cooled, transonic wind tunnel is described. The operating environment for the wind tunnel has a temperature range of 77 to 3390 k, pressure to 91390 Kgs/sq m, and noise to 150 dB SPL. Efforts were made to accomplish the following: to demonstrate projection moire as it would be used to study structural deflections; to use optical processing to multiply the sensitivity of the moire; and to investigate a system design based on the requirements of the wind tunnel geometry