The luminous early-type binary star LSI+61 is a variable radio source (Gregory et al. 1979) characterized by periodic, non-thermal radio outbursts with a period of 26.496 days (Taylor & Gregory 1984). In addition, the radio outburst peak exhibits a long-term modulation on a time scale of approximately 4 years, ranging from 60 mJy to over 300 mJy (Gregory et al 1989; Paredes et al 1990). The 26.5 day period was confirmed in the radial velocity of the stellar spectrum (Hutchings & Crampton 1981) and has been attributed to the eccentric orbital motion of a compact object, probably a neutron star, around a rapidly rotating B0 Ve star. Photometric observations at optical (Mendelson & Mazeh 1994) and infrared wavelengths (Paredes et al. 1994) show a clear 26.5 day modulation, with the degree of modulation increasing with longer wavelength. The system was detected as an x-ray source by Bignami et al. (1981) and has been proposed as the radio counterpart of the COS B (> 100 MeV) -ray source, CG135+01 (Gregory & Taylor 1978; Perotti et al. 1980). Detailed analysis of the COS B data by Pollock et al. (1981) and recent EGRET observations (Fichtel et al. 1994) support the association of the -ray source with LSI+61 .
Several models (Maraschi & Treves 1981; Taylor & Gregory 1982; Lipunov & Nazin 1994) have been proposed to account for the radio, optical and x-ray behaviour, but, to date, the observational constraints are insufficient to distinguish between the models. In particular, knowledge of the time dependence of the x-ray emission is very sparse. The earlier x-ray observations of Bignami et al. (1981) indicated an x-ray flux that was constant to within 30% of 2x10 erg cm s. However, that conclusion was based on only two observations, at radio phase 0.4 and 0.6, taken several cycles apart. Very recently, Goldoni & Mereghetti (1995) have reported the detection of x-ray variation by a factor of 3 over a period of a few days, with an average x-ray flux of 4x10 erg cm s. To more thoroughly investigate the x-ray properties of LSI+61 and the relationship of the x-ray emission to the periodic radio outbursts, we carried out coordinated ROSAT x-ray and VLA radio observations during one 26 day cycle in August-September 1992.