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

We report OSSE observations of the bursting X-ray pulsar GRO J1744-28 made in 1995 December and 1996 January at hard X-ray energies >35 keV. The pulse profile of the persistent (i.e., nonbursting) pulsar emission is fitted with a sinusoid in the energy range 35-90 keV. Residuals reveal a second harmonic amplitude of 3.0% ± 0.5% of the fundamental. The distribution of time intervals between bursts measured in January is characterized by a broad flat-topped function with width 35 minutes and mean 33 minutes. The burst profile averaged over an ensemble of 104 bursts in the 35-60 keV energy range has FWHM width of 3.6 ± 0.3 s and displays a factor of 2 faster rise time than decay and a pronounced dip in persistent emission after the burst. The phase of the sinusoidal pulse profile during bursts lags the phase prior to bursts by 90 ms (1.2 radians), and a 29 ± 6 ms (0.39 ± 0.08 radians) lag persists following the burst. There are no statistically significant spectral differences between the hard X-ray spectra of the bursting and persistent emission in the OSSE energy range, nor is there any evidence of annihilation or neutron capture line emission or cyclotron absorption.