The Collimated Radiation in SS 433. Constraints from Spatially Resolved Optical Jets and $\texttt{Cloudy}$ Modeling of the Optical Bullets

Abstract

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.