Long-Term Flux Monitoring of LSI +61 303 at 2.25 and 8.3 GHz

Abstract

LSI +61 303 is an exotic binary system consisting of a ~10 Msun B star and a compact object which is probably a neutron star. The system is associated with the interesting radio source GT0236+610 that exhibits bright radio outbursts with a period of 26.5 days. We report the results of continuous daily radio interferometric observations of GT0236+610 at 2.25 and 8.3 GHz from 1994 January to 1996 February. The observations cover 25 complete (and 3 partial) cycles with multiple observations each day. We detect substantial cycle-to-cycle variability of the radio emission characterized by a rapid onset of the radio flares followed by a more gradual decrease of the emission. We detect a systematic change of the radio spectral index alpha which typically becomes larger than zero at the onset of the radio outbursts. This behavior is suggestive of expansion of material initially optically thick to radio frequencies, indicating either that synchrotron or inverse Compton cooling are important or that the free-free optical depth to the source is rapidly changing. After two years of observations, we see only weak evidence for the proposed 4-year periodic modulation in the peak flux of the outbursts. We observe a secular trend in the outburst phases according the the best published ephemeris. This trend indicates either orbital period evolution, or a drift in outburst orbital phase in response to some other change in the system.Comment: 23 pages, LaTex, 7 figures, to appear in ApJ, v491, Dec 10th issue, for associated info and preprints see http://www.srl.caltech.edu/personnel/paulr/lsi.htm