Hard X-Ray Spectroscopy and Pulsar Phase Analysis of the Bursting X-Ray Pulsar GRO J1744−28 with OSSE

Abstract

We report OSSE observations ofthe bursting X-ray pulsar GRO J1744--28 made in December 1995 andJanuary 1996 at hard X-ray energies > 35 keV. The pulse profile of thepersistent (i.e. non-bursting) pulsar emission is fit with a sinusoidin the energy range 35--90 keV. Residuals reveal a second harmonicamplitude of 3.0+/-0.5% of the fundamental. The distribution of timeintervals between bursts measured in January is characterized by abroad flat-topped function with width =~ 35 min and mean =~ 33 min.The burst profile averaged over an ensemble of 104 bursts in the35--60 keV energy range has FWHM width of 3.6+/- 0.3 s and displaysa factor of 2 faster rise time than decay and a pronounced dip inpersistent emission after the burst. The phase of the sinusoidalpulse profile during bursts lags the phase prior to bursts by =~ 90ms ( =~ 1.2 radians), and a 29+/- 6 ms (0.39+/-0.08 radians) lagpersists following the burst. There are no statistically significantspectral differences between the hard X-ray spectra of the bursting andpersistent emission in the OSSE energy range, nor is there any evidence ofannihilation or neutron capture line emission or cyclotron absorption.