The physical parameters of the microquasar S26 in the Sculptor Group galaxy NGC 7793

NGC 7793-S26 is an extended source (350pc × 185pc) previously studied in the radio, optical and X-ray domains. It has been identified as a microquasar which has inflated a superbubble. We used integral-field spectra from the Wide-Field Spectrograph on the ANU 2.3-m telescope to analyse spectra between 3600 and 7000Å. This allowed us to derive fluxes and line ratios for selected nebular lines. Applying radiative shock model diagnostics, we estimate shock velocities, densities, radiative ages and pressures across the object. We show that S26 is just entering its radiative phase, and that the northern and western regions are dominated by partially radiative shocks due to a lower density interstellar medium in these directions. We determine a velocity of expansion along the jet of 330kms^-1, and a velocity of expansion of the bubble in the minor axis direction of 132kms^-1. We determine the age of the structure to be 4.1 × 10⁵yr, and the jet energy flux to be (4-10) × 10⁴⁰ergs^-1. The jet appears to be collimated within ∼0.25°, and to undergo very little precession. If the relativistic β ∼ 1/3, then some 4M_⊙ of relativistic matter has already been processed through the jet. We conclude that the central object in S26 is probably a black hole with a mass typical of the ultraluminous X-ray source population which is currently consuming a fairly massive companion through Roche lobe accretion.