High-energy Spectral and Temporal Characteristics of Gro J1008[57

A transient X-ray source, GRO J1008[57, was discovered by the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory (CGRO) in 1993 July. It reached a maximum intensity of about 1.4 times that of the Crab, in the 20È60 keV energy band. Pulsations in the X-ray intensity were detected at a period of 93.5 s. It has subsequently been determined to be a member of the Be star subclass of X-ray transients. In addition to BATSE, GRO J1008[57 was observed during its outburst by several pointed high-energy experiments : ROSAT , ASCA, and CGRO/OSSE. These nonsimultaneous but contemporaneous observations took place near and shortly after the peak of the outburst light curve. We report for the Ðrst time on a combined analysis of the CGRO and ASCA data sets. We have attempted to model the broadband high-energy continuum distribution and phase-resolved spectra. The broadband, phase-averaged continuum is well approximated by a power law with an exponential cuto†. Evidence for 6.4 keV line emission due to Fe is presented based on our spectral analysis. The energy dependence of the pulse proÐles is examined in order to determine the energy at which the low-energy double-peaked proÐle detected by ASCA evolves into single-peaked pulse proÐle detected by BATSE. We discuss the implications of this pulse proÐle for the magnetic Ðeld and beam distribution for GRO J1008[57. Analysis of the BATSE and Rossi X-Ray T iming Explorer/ASM Ñux histories suggests that days. We further suggest that a transient disk is likely to form during episodes of PorbitalD 135 outbursts. Subject headings : binaries : close È pulsars : individual (GRO J1008[57) È stars : neutron È X-rays : stars