High-energy spectral and temporal characteristics of GRO J1008-57

A transient X-ray source, GRO J1008-57, was discovered by the Burst and Tra nsient Source Experiment (BATSE) on board the Compton Gamma Rag, Observator y (CGRO) in 1993 July. It reached a maximum intensity of about 1.4 times th at of the Crab, in the 20-60 keV energy band. Pulsations in the X-ray inten sity were detected at a period of 93.5 s. It has subsequently been determin ed 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 pointe d high-energy experiments: ROSAT, ASCA, and CGRO/OSSE. These nonsimultaneou s but contemporaneous observations took place near and shortly after the pe ak of the outburst light curve. We report for the first time on a combined analysis of the CGRO and ASCA data sets. We have attempted to model the bro adband high-energy continuum distribution and phase-resolved spectra. The b roadband, phase-averaged continuum is well approximated by a power law with an exponential cutoff. Evidence for 6.4 keV line emission due to Fe is pre sented based on our spectral analysis. The energy dependence of the pulse p rofiles is examined in order to determine the energy at which the low-energ y double-peaked profile detected by ASCA evolves into single-peaked pulse p rofile detected by BATSE. We discuss the implications of this pulse profile for the magnetic field and beam distribution for GRO J1008 - 57. Analysis of the BATSE and Rossi X-Ray Timing Explorer/ASM flux histories suggests th at P-orbital similar to 135 days. We further suggest that a transient disk is likely to form during episodes of outbursts.