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 Timing 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 confirm the modulation of the X-ray emission on orbital timescales originally reported by Taylor et al., and in addition we find a significant offset between the peak of the X-ray and radio flux. We argue that based on these results, the most likely X-ray emission mechanism is inverse Compton scattering of stellar photons off 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 2-150 keV flux 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.