Einstein X-ray identification of the variable radio star LS I +61 303.

Two X-ray observations are presented for the star LSI plus 61 deg 303, located well inside the approximately 3 sq deg error box of the high-energy gamma-ray source 2CG 135 plus 01, using IPC and HRI instruments. The X-ray flux from the observed star in the IPC spectral interval of 0.2 to approximately 5 keV is approximately 2.1 x 10 to the -12th ergs/sq cm/s, assuming that 1 IPC count equals 2.2 x 10 to the 11th ergs. The HRI flux does not show any significant difference to within the accuracy with which absolute fluxes from the two instruments can be compared (approximately 30 percent). Fourier analysis of the data shows no evidence for periodic variation for periods ranging between 0.18 and 700 s. X-ray data do not support any evidence for temporal variation of LSI plus 61 deg 303 as an X-ray source. The possible connection between the star, now a recognized X-ray source, with the high-energy COS B gamma-ray source 2CG 135 plus 01 is discussed on the basis of the derived quantitative astrophysical parameters.