Rotationally-induced asymmetry in the double-peak lightcurves of the bright EGRET pulsars?

Abstract

Pulsed emission from the bright EGRET pulsars - Vela, Crab, and Geminga - extends up to 10 GeV. The generic gamma lightcurve features two peaks separated by 0.4 to 0.5 in phase. According to Thompson (2001) the lightcurve becomes asymmetrical above 5 GeV in such a way that the trailing peak dominates over the leading peak. We attempt to interpret this asymmetry within a single-polar-cap scenario. We investigate the role of rotational effects on the magnetic one-photon absorption rate in inducing such asymmetry. Our Monte Carlo simulations of pulsar gamma-ray beams reveal that in the case of oblique rotators with rotation periods of a few millisecond the rotational effects lead to the asymmetry of the requested magnitude. However, the rotators relevant for the bright EGRET pulsars must not have their inclination angles too large in order to keep the two peaks at a separation of 0.4 in phase. With such a condition imposed on the model rotators the resulting effects are rather minute and can hardly be reconciled with the magnitude of the observed asymmetry.