VSOP and stellar sources - the case of LS I +61 303

Abstract

Space-VLBI observations of stellar sources represent a challenge since there are few sources with sufficiently high brightness temperature for detection on space-ground baselines. X-ray binaries (XRB) are among the few types of stellar radio sources that can be detected. Observations of the unusual X-ray and gamma-ray binary system LS I 61 303 obtained with the HALCA satellite and a 20-element ground array are described. The data in this 48-hour experiment represent some of the best quality VLBI observations of LS I 61 303. No fringes were detected on HALCA baselines. 10-minute snapshot images were produced from the global ground array data and reveal an expansion velocity of 800 km/s. Some of these image data reveal hints of more extended emission but high-SNR closure phase data do not support relativistic outflow in the plane-of-the-sky in LSI 61 303. The largest closure phase rates are consistent with an outflow of ~1000 km/s as deduced from the image data. The closure phases also show no evidence of structure variation on size scales greater than ~10 mas. A number of issues related to VSOP2 observations of stellar radio sources are raised.