The Transient 42 Second X-Ray Pulsar EXO 2030+375. II. The Luminosity Dependence of the Pulse Profile

Abstract

During an outburst from the transient, 42 s, X-ray pulsar EXO 2030+375, marked luminosity-dependent changes in pulsar profile were observed. At high luminosities the pulse profile consisted of a smooth asymmetric main pulse separated by about 180 deg in phase from a small interpulse. As the luminosity decreased by a factor of about 100, the relative strength of the two pulses reversed. During a second outburst, the phase-averaged X-ray light curve was punctuated by a series of flares that repeated about every four hours and involved a factor of six increase in intensity. The observed pulse profile is modeled using a simple geometric model where axially symmetric fan and pencil-beams of emission are emitted from two magnetic poles on a rotating neutron star. The profile shows a marked asymmetry which is modeled by offsetting the magnetic dipole axis from the rotation axis of the neutron star. The modeling indicates that, as the luminosity decreased, the dominant beam changed from a fan beam to a pencil-beam configuration.