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Session 73 - White Dwarfs, Neutron Stars and Pulsars.
Display session, Thursday, June 13
Great Hall,
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\pm0.5% of the fundamental. The distribution of time intervals between bursts measured in January is characterized by a broad flat-topped function with width \simeq 35 min and mean \simeq 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\pm 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 \simeq 90 ms (\simeq 1.2 radians), and a 29\pm 6 ms (0.39\pm0.08 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.
