AN X-RAY OUTBURST FROM THE RADIO EMITTING X-RAY BINARY LSI-DEGREES-303(61)

Coordinated X-ray and two-frequency radio observations were carried ou t over one cycle of the periodic, radio-emitting binary system LSI+61 degrees 303, using the ROSAT and VLA observatories. An X-ray outburst, with a duration of about 10 days, was detected. Both the radio and X- ray flux densities varied by a factor of 10, with the X-ray peak occur ring at phase 0.5 and the radio peak at phase 0.95. The range of phase of high X-ray emission matches the phase interval during which the so urce is, on average, active at radio. Timing analysis of the X-ray dat a failed to detect pulsed emission in the period range of 1.5 millisec onds to 3000 seconds. We discuss a model in which the X-ray emission i s produced in hot circumstellar gas shocked by the expanding plasmon o f relativistic electrons that is responsible for the radio outburst. W ith the known parameters of the circumstellar gas, the binary system a nd the radio plasmon, such a model is able to reproduce both the lumin osity and spectral properties of the keV X-rays, and the timing of the X-ray emission relative to the radio outburst. Inverse-Compton scatte ring of stellar photons off relativistic particles in the expanding pl asmon can account for the gamma-ray luminosity of LSI+61 degrees 303 a nd the low ratio of X-ray to gamma-ray emission.