Rotation as a source of asymmetry in the double-peak lightcurves of the bright EGRET pulsars

We investigate the role of rotational effects in inducing asymmetry present above ~5 GeV in the double-peak lightcurves of the bright EGRET pulsars: Vela, Crab, and Geminga. According to Thompson 2001, the trailing peak dominates over the leading peak above ~5 GeV consistently for all three pulsars, even though this is not the case over the entire energy range of EGRET, i.e. above ~100 MeV. We present the results of Monte Carlo simulations of electromagnetic cascades in a pulsar magnetosphere within a single-polar-cap scenario with rotationally-induced propagation effects of the order of v/c (where v is the local corotation velocity). We find that even in the case of nearly aligned rotators with spin periods of P ~ 0.1 s rotation may lead to asymmetric (with respect to the magnetic axis) magnetic photon absorption which in turn leads to asymmetric gamma-ray pulse profiles. The resulting features - softer spectrum of the leading peak and the dominance of the trailing peak above ~5 GeV - agree qualitatively with the EGRET data of the bright gamma-ray pulsars.Comment: 12 pages, 11 figs, latex2e, accepted for publication by A&

Model of peak separation in the gamma lightcurve of the Vela pulsar

The separation \del between two peaks in the gamma-ray pulse profile is calculated as a function of energy for several polar cap models with curvature-radiation-induced cascades. The Monte Carlo results are interpreted with the help of analytical approximations and discussed in view of the recent data analysis for the Vela pulsar (Kanbach 1999). We find that the behaviour of \del as a function of photon energy $\epsilon$ depends primarily on local values of the magnetic field, $B_{\rm local}$, in the region where electromagnetic cascades develop. For low values of $B_{\rm local}$ ($< 10^{12}$ G), \del(\epsilon) is kept constant. However, for stronger magnetic fields (\ga 10^{12} G) in the hollow-column model \del decreases with increasing photon energy at a rate dependent on maximum energy of beam particles as well as on viewing geometry. There exists a critical photon energy \et above which the relation \del(\epsilon) changes drastically: for \epsilon > \et, in hollow-column models the separation \del increases (whereas in filled-column model it decreases) rapidly with increasing $\epsilon$, at a rate of $\sim 0.28$ of the total phase per decade of photon energy. The existence of critical energy \et is a direct consequence of one-photon magnetic absorption effects. In general, \et is located close to the high-energy cutoff of the spectrum, thus photon statistics at \et should be very low. That will make difficult to verify the existence of \et in real gamma-ray pulsars. Spectral properties of the Vela pulsar would favour those models which use low values of magnetic field in the emission region (B_{\rm local} \simless 10^{11} G) which in turn implies a constant value of the predicted \del within EGRET range.Comment: 7 pages, 3 figures, 1 table, latex, mn.sty, epsf.sty, MNRAS, in pres