The Signature of a Windy Radio Supernova Progenitor in a Binary System

Type II supernova progenitors are expected to emit copious amounts of mass in a dense stellar wind prior to the explosion. When the progenitor is a member of a binary, the orbital motion modulates the density of this wind. When the progenitor explodes, the high-velocity ejecta collides with the modulated wind, which in turn produces a modulated radio signal. In this Letter we derive general analytic relations between the parameters of the radio signal modulations and binary parameter in the limit of large member-mass ratio. We use these relations to infer the semimajor axis of SN 1979c and a lower bound for the mass of the companion. We further constrain the analytic estimates by numerical simulations using the Astrophysical Multipurpose Software Environment framework. In these calculations we simulate the progenitor binary system including the wind and the gravitational effect of a companion star. The simulation output is compared to the observed radio signal in supernova SN 1979C. We find that it must have been a binary with an orbital period of about 2000 yr. If the exploding star evolved from a ∼18 M⊙ zero-age main sequence at solar metallicity, we derive a companion mass of 5–12 M⊙ in an orbit with an eccentricity lower than about 0.8.