Joseph K. Daugherty

Electromagnetic cascades in pulsars

The development of pair photon cascades initiated by high energy electrons above a pulsar polar cap is simulated numerically. The calculation uses the energy of the primary electron, the magnetic field strength, and the period of rotation as parameters and follows the curvature radiation emitted by the primary, the conversion of this radiation e(+) - e(-) pairs in the intense fields, and the quantized synchrotron radiation by the secondary pairs. A recursive technique allows the tracing of an indefinite number of generations using a Monte Carlo method. Gamma ray and pair spectra are calculated for cascades in different parts of the polar cap and with different acceleration models. It is found that synchrotron radiation from secondary pairs makes an important contribution to the gamma ray spectrum above 25 MeV, and that the final gamma ray and pair spectra are insensitive to the height of the accelerating region, as long as the acceleration of the primary electrons is not limited by radiation reaction.

Gamma-ray pulsars: emission from extended polar cap cascades

We have used a Monte Carlo simulation of a Polar Cap (PC) model of gamma-ray pulsars to estimate light curves and phase-resolved spectra for sources whose rotational and magnetic axes are oriented so that only one of the magnetic poles produces emission directed at the Earth. In this Single Polar Cap (SPC) scenario, even sources whose light curves have two distinct peaks (Crab, Vela, Geminga, PSR B1951+32) are due to emission concentrated near the rim of a single PC. If the inclination alpha is comparable to the half-width of the PC gamma-beam, alpha ~ theta_{b}, the peak-to-peak phase separation can have the large values (0.4 - 0.5) observed from these sources. In the model presented here we attribute the observed interpeak emission to pair cascades above the PC interior. Our simulation assumes the physics of conventional PC models, in which the gamma rays are due to photon-pair cascades initiated by curvature radiation from the acceleration of electrons above the PCs. In this work we assume that the acceleration occurs over a finite region which may extend up to several radii above the neutron star surface. We find that the combined effects of moderately enlarged PC dimensions and extended acceleration zones resolve a major difficulty with earlier PC models, namely their small beam widths (and hence small detection probabilities). Our best fits to the observed light curves are obtained from models in which the accelerated electrons have a uniform surface density over the PC interior and a sharp density increase of 3 - 5 near the rim.

Compton scattering in strong magnetic fields

The relativistic cross section for Compton scattering by electrons in strong magnetic fields is derived. The results confirm and extend earlier work which has treated only transitions to the lowest or first excited Landau levels. For the teragauss field strengths expected in neutron star magnetospheres, the relative rates for excited state transitions are found to be significant, especially for incident photon energies several times the cyclotron frequency. Since these transitions must result in the rapid emission of one or more cyclotron photons as well as the Compton-scattered photon, the scattering process actually becomes a photon splitting mechanism which acts to soften hard photon spectra, and also provides a specific mechanism for populating higher Landau levels in the electron distribution function. The results should be significant for models of gamma-ray bursters and pulsating X-ray sources. 15 references.

Polar cap models of gamma-ray pulsars: Emision from single poles of nearly aligned rotators

We compare a new Monte Carlo simulation of polar cap models for gamma-ray pulsars with observations of sources detected above 10 MeV by the Compton Observatory (CGRO). We find that for models in which the inclination of the magnetic axis is comparable to the angular radius of the polar cap, the radiation from a single cap may exhibit a pusle with either a single broad peak as in PSR 1706-44 and PSR 1055-52, or a doubly peaked profile comparable to those observed from the Crab, Vela and Geminga pulsars. In general, double pulses are seen by observers whose line of sight penetrates into the cap interior and are due to enhanced emission near the rim. For cascades induced by culvature radiation, increased rim emission occurs even when electrons are accelerated over the entire cap, since electrons from the interior escape along magnetic field lines with less curvature and hence emit less radiation. However, we obtain better fits to the duty cycles of observed profiles if we make the empirical assumption that acceleration occurs only near the rim. In either case, the model energy spectra are consistent with most of the observed sources. The beaming factors expected from nearly aligned rotators, based on standard estimates for the cap radius, imply that their luminosities need not be as large as in the case of orthogonal rotators. However, small beam angles are also a difficutly with this model because they imply low detection probablities. In either case the polar cap radius is a critical factor, and in this context we point out that plasma loading of the field lines should make the caps larger than the usual estimates based on pure dipole fields.

Cyclotron resonant scattering and absorption. [in gamma ray bursts

The relativistic cross-sections for first-order absorption and second-order scattering are compared to determine the conditions under which the absorption cross-section is a good approximation to the much more complex scattering cross-section for purposes of modeling cyclotron lines in gamma-ray bursts. Differences in both the cross-sections and the line profiles are presented for a range of field strengths, angles, and electron temperatures. The relative difference of the cross-sections at one line width from resonance was found to increase with field strength and harmonic number. The difference is also strongly dependent on the photon angle to the magnetic field. For the field strength, 1.7 {times} 10 to the 12th G, and the angle inferred from the Ginga burst features, absorption is an excellent approximation for the profiles at the first and second harmonics. 21 refs.

A measurement of cosmic-ray positron and negatron spectra between 50 and 800 MV

A balloon-borne magnetic spectrometer was used to measure the spectra of cosmic ray positrons and negatrons at energies between 50 and 800 MeV. Comparisons of the separate positron and negatron spectra observed near the earth with their expected intensities in interstellar space can be used to investigate the complex (and variable) interaction of galactic cosmic rays with the expanding solar wind. The present measurements, which have established finite values or upper limits for the positron and negatron spectral between 50 and 800 MeV, have confirmed earlier evidence for the existence of a dominant component of negatrons from primary sources in the galaxy. The present results are shown to be consistent with the hypothesis that the positron component is in fact mainly attributable to collisions between cosmic ray nuclei and the interstellar gas. The estimate of the absolute intensities confirm the indications from neutron monitors that in 1972 the interplanetary cosmic ray intensities were already recovering toward their high levels observed in 1965.

Comptonization of thermal photons by relativistic electron beams

This paper presents a numerical calculation of gamma-ray emission produced by Compton scattering of relativistic electron beams on background thermal radiation, which includes spatial dependence of electron energy losses and cyclotron resonance scattering in a strong magnetic field. In the first version, the scattering is described by the fully relativistic Klein-Nishina cross section, but the magnetic field is neglected. In the second version, the scattering is described by the magnetic resonant cross section in the Thomson limit. It is found that when the magnetic field is not included, electron energy losses are important only at higher neutron star surface temperatures (T about 3,000,000 K). In the presence of a strong magnetic field, (10 to the 12th G), resonant scattering greatly increases electron energy losses, making scattering very efficient even at lower surface temperatures. Resulting photon and electron spectra for both cases ae discussed in relation to models for pulsar X-ray and gamma-ray emission. 26 references.

Pair annihilation in superstrong magnetic fields

In the presence of superstrong external magnetic fields, electron-positron pairs may annihilate into single photons with energy E/sub ..gamma../>2mc/sup 2/, as well as the two or more photons normally allowed in free space. Even the normal annihilation modes are significantly affected by the ability of the rigid field lines to absorb the transverse momentum in particle interactions. In particular, for 2..gamma.. annihilation a rest, the familiar 511 keV line spectrum is broadened to an extent which is quite sensitive to the field strength and angle of emission. As B is increased beyond 10/sup 13/ gauss, the 2..gamma.. spectrum is eventually distorted and shifted into the range mc/sup 2/

On pair production in intense electromagnetic fields occurring in astrophysical situations

We show that the pair production rate in a strong magnetic field is substantially altered when an electric field is also included. We illustrate and emphasize this significant alteration by considering a few special cases. In the vicinity of the polar cap of a rotating magnetized neutron star it is currently though thatboth steady electric and magnetic fields must be present. The results presented here then indicate that some considerable degree of caution must be exercised in applying pair production rates calculated under the assumption of zero electric field to the problems of pulsar emission and the generation and maintence of pulsar magnetospheres. In general such rates are very different from the rate computed allowing for the existence of an electric field.

Pulse profiles and spectra of gamma ray pulsars in the polar cap model

Abstract We investigate predictions for pulse profiles and spectra of gamma-ray pulsars in the polar cap curvature radiation-initiated cascade model. In this model, the gamma-ray beam is a hollow cone centered on the magnetic pole, producing either double-peaked or single-peaked pulse profiles depending on observer orientation. We have computed simulated distributions of pulse peak phase separation seen by observers at random orientation as a function of gamma-ray beam width and obliquity distribution. The observed distribution of pulse peak phase separation, which favors double-peaked pulses with phase separation near 0.4, can be matched assuming that most gamma-ray pulsars have obliquity