Density and Velocity Structure of the Be Star Equatorial Disk in the Binary LS I +61o303, a Probable Microquasar

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Abstract
We explore a new method for investigating the density and velocity structure of the Be star equatorial disk in LS I +61°303, based on its variable radio emission. At a particular radius the method yields a circularly shaped solution locus in the radial (vr) and circular (vc) components of the disk gas velocity, which allows us to set limits on vr and vc at that radius. For a given model of the circular velocity, vc(r), we can derive vr(r) and the gas density, ρ(r), within a constant. While ρ(r) depends on the assumed model for vc(r), we find that the fractional change in gas density at any radius does not. This allows us to study the evolution in ρ(r) with time. The analysis indicates that the previously demonstrated ~4.6 yr modulation in radio properties results from an outward-moving density enhancement or shell in the equatorial disk with a velocity of ~1.0 km s-1. We propose that each new shell ejection may be triggered by the interaction of a short-lived relativistic wind (ejector phase) from the neutron star with the rapidly rotating Be star. Our best estimates of the mass accretion rate of the neutron star are in the range ~0.001 to ~0.01 of the Eddington accretion limit. This translates to an expected luminosity range of ~1035 to ~1036 ergs s-1, which is comparable to estimates of the total X-ray and γ-ray luminosity for LS I +61°303.