Simultaneous X-ray and Radio Monitoring of the Unusual Binary Ls I ]61¡303 : Measurements of the Light Curve and High-energy Spectrum

The binary system, LS I ]61¡303, is unusual both because of the dramatic, periodic, radio outbursts, and because of its possible association with the 100 MeV gamma-ray source, 2CG 135]01. We have performed simultaneous radio and Rossi X-Ray T iming Explorer X-ray observations at 11 intervals over the 26.5 day orbit, and in addition searched for variability on timescales ranging from milliseconds to hours. We conÐrm the modulation of the X-ray emission on orbital timescales originally reported by Taylor et al., and in addition we Ðnd a signiÐcant o†set between the peak of the X-ray and radio Ñux. We argue that based on these results, the most likely X-ray emission mechanism is inverse Compton scattering of stellar photons o† of electrons accelerated at the shock boundary between the relativistic wind of a young pulsar and the Be star wind. In these observations we also detected 2È150 keV Ñux from the nearby low-redshift quasar QSO 0241]622. Comparing these measurements to previous hard X-ray and gamma-ray observations of the region containing both LS I ]61¡303 and QSO 0241]622, it is clear that emission from the QSO dominates. Subject headings : binaries : close È gamma rays : observations È radio continuum: stars È stars : individual (LS I ]61¡303)