  The microquasar SS 433 is well-known for its precessing, relativistic
baryonic jets. Depending on their heating mechanism, the optical jet bullets
may serve as a probe of the collimated radiation coming from the inner region
close to the compact object. The optical interferometer VLTI/GRAVITY has
allowed to spatially resolved the optical jets in SS 433 for the first time. We
present here the second such observation taken over three nights in July 2017.
In addition, we use the multi-wavelength XSHOOTER spectrograph at VLT to study
the optical bullets in SS 433 in detail. GRAVITY reveals elongated
exponential-like spatial profiles for the optical jets, suggestive of a heating
mechanism acting throughout a long portion of the jet and naturally explained
by photoionization by the collimated radiation. We also spatially resolve the
movement of the optical bullets for the first time, detecting extended jet
components corresponding to previous ejections. \texttt{Cloudy} photoionization
models explain both the spatial intensity profiles measured with GRAVITY and
the line ratios from XSHOOTER, and constrain the properties of the optical
bullets and the ionizing radiation. We find that the latter must peak in the UV
with an isotropic luminosity (as inferred by a face on observer) $\approx
10^{41}$ erg/s. Provided that the X-ray SED is sufficiently hard, the
collimated X-ray luminosity could still be high enough so that the face on
observer would see SS 433 as an ULX ($L_X \lesssim 10^{40}$ erg/s) and it would
still be compatible with the H/He/He+ ionization balance of the optical
bullets. The kinetic power in the optical jets is constrained to $3-20 \times
10^{38}$ erg/s, and the extinction in the optical jets to $A_V = 6.7 \pm 0.1$.
We suggest there may be substantial $A_V \gtrsim 1$ and structured
circumstellar extinction in SS 433, likely arising from dust formed in
equatorial outflows.
