The Accretion Curtain and Pulse Phase Variations of the Bursting X-Ray Pulsar GRO J1744−28

Abstract

The X-ray pulsar GRO J1744-28 accretes so rapidly that thegas flow onto its magnetic poles should be decelerated by radiationforces rather than gas pressure gradients. The vertical scale of thedeceleration region, where the bulk of the flow's kinetic energyis transformed to radiation, is large compared to the size of theaccretion flow's footprint on the stellar surface, and radiation escapesin a fan beam through the sides of the decelerating gas column. Ifone assumes that the X-ray bursts observed from GRO J1744-28 aretriggered in the accretion disk, this picture of the accretion columngeometry and a consideration of the interaction between the disk andthe stellar magnetic field at the magnetospheric boundary lead to anatural explanation for the phase lags of the X-ray pulses observedduring bursts: The accretion column is not round but flattened, so itsfootprint forms an arc on the stellar surface close to the magneticpole. The broad side of the arc faces the magnetic pole. When theaccretion rate rises, the gas flow shifts to a different set of fieldlines, and the accretion footprint pivots through a moderate anglearound the magnetic pole. Although the footprint has remained close tothe magnetic pole, the direction it faces has changed. The time ofpulse maximum, when an observer sees the broad side of the accretioncolumn most face-on, changes accordingly, and a phase-lag results.