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 December 1995 and January 1996 at hard X-ray energies ? 35 keV. The pulse profile of the persistent (i.e. non-bursting) pulsar emission is fit with a sinusoid in the energy range 35--90 keV. Residuals reveal a second harmonic amplitude of 3:0 \Sigma 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 min and mean ' 33 min. The burst profile averaged over an ensemble of 104 bursts in the 35--60 keV energy range has FWHM width of 3:6 \Sigma 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 \Sigma 6 ms (0:39 \Sigma 0:08 radians) lag persists following the burst. There are no statistically significant spectral dif..