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

¹ Copernicus Astronomical Center, Toruń, Poland
² Dept. of Astronomy and Astrophysics, Nicholas Copernicus University, Toruń, Poland

Corresponding author: J. Dyks, jinx@ncac.torun.pl

Received: 10 December 2001
Accepted: 4 July 2002

Abstract

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 ([CITE]), 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 β (where β is the dimensionless local corotation velocity). We find that even in the case of nearly aligned rotators with spin periods of  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.