Simultaneous X‐Ray and Radio Monitoring of the Unusual Binary LS I +61o303: Measurements of the Light Curve and High‐Energy Spectrum

The binary system, LSI+61 303, is unusual both because of the dramatic,periodic, radio outbursts, and because of its possible association with the 100MeV gamma-ray source, 2CG135+01. We have performed simultaneous radio and RossiX-ray Timing Explorer X-ray observations at eleven intervals over the 26.5 dayorbit, and in addition searched for variability on timescales ranging frommilliseconds to hours. We confirm the modulation of the X-ray emission onorbital timescales originally reported by Taylor et al. (1996), and in additionwe find a significant offset between the peak of the X-ray and radio flux. Weargue that based on these results, the most likely X-ray emission mechanism isinverse Compton scattering of stellar photons off of electrons accelerated atthe shock boundary between the relativistic wind of a young pulsar and the Bestar wind. In these observations we also detected 2 -- 150 keV flux from thenearby low-redshift quasar QSO~0241+622. Comparing these measurements toprevious hard X-ray and gamma-ray observations of the region containing bothLSI+61 303 and QSO~0241+622, it is clear that emission from the QSO dominates.Comment: 23 pages, 6 figures, Accepted for publication in the Astrophysical Journa