Gamma-Rays From Nearly-Aligned Pulsars
Session 93 -- Pulsars
Oral presentation, Friday, January 14, 10:15-11:45, Crystal Forum Room (Crystal City Marriott)

## [93.05] Gamma-Rays From Nearly-Aligned Pulsars

Steven J. Sturner & Charles D. Dermer (Naval Research Laboratory, Washington, DC 20375)

We discuss a nearly-aligned pulsar model with polar cap acceleration in order to explain the energetics, number, spectra, and pulse profiles of the known gamma-ray pulsars. In this model, the nonthermal energy of electrons flowing outward along field lines connected to the light cylinder is converted to gamma-rays via magnetic Compton scattering of optical and soft X-ray photons. The scattered photons initiate a pair cascade in the pulsar magnetosphere through magnetic pair production followed by synchrotron emission.

We find that the efficiency for converting spin-down luminosity to outflowing particle luminosity (and thus gamma-ray luminosity) increases with decreasing spin-down luminosity, a trend recently emphasized by Ulmer. The predicted gamma-ray flux is $\propto \dot{P}^{3/4}/P^{5/4} d^{2}$, where P is the period, $\dot{P}$ is the period derivative, and d is the distance to the pulsar.

For initial spin periods between 10 and 30 ms and neutron star polar magnetic fields between 1 and 4 teragauss, this model accounts for the number and age distribution of the five pulsars which have been observed to emit >100 MeV gamma-rays. The calculated spectra are used to fit gamma-ray pulsar observations. This model produces a hollow cone of emission that can reproduce both the broad single-peaked and narrow double-peaked pulse profiles observed from gamma-ray pulsars. The resulting polarization angle swing of the radio emission is examined under the assumption that it is produced as the pulsar polar cap crosses the observers line of sight. We will also discuss the implications of this model for the observability of millisecond pulsars.