Nearly-Aligned Rotator Model of Gamma Ray Pulsars
Session 21 -- Pulsars
Oral presentation, Monday, 30, 1994, 2:00-3:30

## [21.06] Nearly-Aligned Rotator Model of Gamma Ray Pulsars

J.K. Daugherty (UNCA), A.K. Harding (NASA/GSFC)

We have derived new pulse profiles and energy spectra for gamma ray pulsars, using a polar cap (PC) cascade model in which the inclination $\alpha$ of the magnetic axis is comparable to the cap half-angle $\theta_{pc}$. The model assumes acceleration of electrons above the pulsar surface to energies $\sim10$ TeV. The electrons emit curvature radiation (CR) which is converted by the pulsar B-field to photon-pair cascades in the deep magnetosphere. Since CR-induced PC cascades produce hollow-cone emission regardless of the primary electron beam geometry, observers whose viewing angles $\zeta$ intersect the rim of a single PC see double pulses, which for $\alpha \sim \theta_{pc}$ may vary from 0 to $\sim 180$ degrees. We have examined the $(\alpha,\zeta,\theta_{pc})$ parameter space which can produce doubly peaked model profiles consistent with those observed from the Crab, Vela, and Geminga. We have also considered the effect of the primary beam geometry on peak duty cycles. Our best fits are obtained from models in which acceleration occurs only around the PC rim. The model profiles also show emission between the peaks, corresponding to the phase interval for which the observer viewpoint is outside the PC rim, which may account for the interpulse emission in Vela and Geminga. The same model parameters have produced phase-resolved spectra which are generally consistent with observations, especially for Vela. Our model also allows the possibility of two closely separated peaks, as well as a single broad peak (seen by observers whose viewpoints do not penetrate the PC rim). Since the model beaming factors and detection probabilities are sensitive to PC size, we have considered the dependence of cascade properties on $\theta_{pc}$. One major effect is that the B-field curvature increases with $\theta_{pc}$, leading to enhanced CR and hence more extensive cascades. The high-energy spectral cutoffs are also lowered by more efficient pair conversion. For these enlarged-PC models, our best spectral fits require either a reduced B-field or an extended acceleration zone above the NS surface.