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 similar or equal to 35 minutes and mean similar or equal to 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 similar or equal to 90 ms (similar or equal to 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.