Optical Interferometry of a Microquasar: Resolving Super-Eddington Outflows in SS 433

SS 433 rose to astrophysical stardom in the late 1970s with the discovery of its moving optical emission lines of hydrogen and helium which were shown to originate in a mildly relativistic (0.26c), precessing baryonic jet and which established it as the first microquasar. It remains the only known super-Eddington accretor in the Galaxy, possibly an edge-on Galactic ULX. The connection between SS 433 and ULXs, the latter discovered through X-ray observations since the 1980s, has been strengthened by the recent discovery of massive outflows in ULXs akin to those seen in SS 433 and even a second system containing a baryonic relativistic jet. The VLTI instrument GRAVITY achieves sub-milliarcsecond spatial resolution in the near-infrared, sufficient to spatially resolve both SS 433's optical massive winds and baryonic jets for the first time. Over multiple epochs, we find evidence for both a circumbinary disk and polar outflow. The jet structures are emitted closer to the compact object than predicted from optical spectroscopy, but with a consistent exponential emission profile. Using spectro-interferometry, we can further isolate discrete jet ejections and resolve the activity of the central jet in both space and time. We will also present the first XSHOOTER observations of SS 433, which for the first time provide up to 20 pairs of jet lines simultaneously and allow to constrain the conditions and heating mechanism of the optical emitting jet gas as it travels through a dense surrounding medium. We will discuss the implications of these measurements for the accretion-ejection structure of SS 433.