Super-Keplerian Equatorial Outflows in SS 433. Centrifugal Ejection of
  the Circumbinary Disk
  The microquasar SS 433 is the only known steady supercritical accretor in the
Galaxy. It is well-known for its relativistic baryonic jets, but the system
also drives equatorial outflows. These have been routinely detected in radio
images, and components associated with a circumbinary disk have also been
suggested in optical emission lines. We wish to spatially resolve the regions
producing the stationary emission lines of SS 433 to shed light on its
circumbinary structure and outflows. With an estimated binary orbit size
$\lesssim 0.1 \text{ mas}$, this requires optical interferometry. We use the
optical interferometer VLTI+GRAVITY to spatially resolve SS 433 in the
near-infrared K band at high spectral resolution ($R\approx 4000$) on three
nights in July 2017. The stationary Br$\gamma$ line is clearly dominated by an
extended $\sim 1 \text{ mas} \sim 5 \text{ AU}$ circumbinary structure
perpendicular to the jets and with a strong rotation component. The rotation
direction is retrograde relative to the jet precession, in accordance with the
slaved disk precession model. The structure has a very high specific angular
momentum and is too extended to be a stable circumbinary disk in Keplerian
rotation; interpreting it as such leads to a very high enclosed mass $M \gtrsim
400 M_{\odot}$. We instead interpret it as the centrifugal ejection of the
circumbinary disk, with the implication that there must be an efficient
transfer of specific angular momentum from the binary to the disk. We suggest
that the equatorial outflows sometimes seen in radio images result from similar
episodes of circumbinary disk centrifugal ejection. In addition to the
equatorial structure, we find a very extended $\sim 6 \text{ mas} \sim 30
\text{ AU}$ spherical wind component to the Br$\gamma$ line: the entire binary
is engulfed in an optically thin spherical line emission envelope.
