Identification of X-ray lines in the spectrum of the arcsec-scale precessing jets of SS 433

The extended X-ray emission observed at arcsec scales along the propagation trajectory of the precessing relativistic jets of the Galactic microquasar SS 433 features a broad emission line, with the position of the centroid being significantly different for the approaching and receding jets (≈7.3 and ≈6.4 keV, respectively). These observed line positions are at odds with the predictions of the kinematic model for any of the plausible bright spectral lines in this band, raising the question of their identification. Here we address this issue by taking into account time delays of the emission coming from the receding regions of the jets relative to that from the approaching ones, which cause a substantial phase shift and distortion of the predicted line positions for the extended (~1017 cm) emission compared to the X-ray and optical lines observed from the central source (emitted at distances ~1011 and ~1015 cm, respectively). We demonstrate that the observed line positions are fully consistent with the Fe XXVI Lyα (E0 = 6.96 keV) line emerging from a region of size ~6 × 1016 cm along the jet. This supports the idea that intensive reheating of the jets up to temperatures >10 keV takes place at these distances, probably as a result of partial deceleration of the jets due to interaction with the surrounding medium, which might cause collisions between discrete dense blobs inside the jets.