SS 433: the wiggle of the wind

Context. The brilliant Balmer Hα line in the stationary emission spectrum of the Galactic microquasar SS 433 has a broad component (~1000 km s-1). This is formed in the wind blowing from the accretion disk of the compact object, which orbits the centre of mass of the binary at ~175 km s-1 with a 13-day period. The centroid of the Hα emission line formed in the wind has an imperfect memory of the motion of its source. Aims. The aim is to understand how this emission line is left with a blurred memory of the motion of the source of the wind. Methods. We analysed stationary Hα spectra, taken almost nightly over two orbital periods of the binary system. Results. The loss of memory by emission lines from the wind is easily understood quantitatively, if any parcel of wind that has been newly conditioned to radiate in Hα continues its emission for a period of days. If the decay is exponential, a mean lifetime of about two days is all that is required. This timescale is very close to what is required for the decay of Hα from the putative circumbinary disk to be responsible for the narrow components of the Hα emission line. It is also close to the timescales observed for the individual bolides that radiate Hα in the relativistic jets. Conclusions. The structure of the stationary Hα emission line is now well understood. The broad component is undoubtedly formed in the wind from the accretion disk and the two narrow components carry no such signature. Their properties are entirely consistent with an origin in a circumbinary disk. The characteristic lifetime of Hα emitting material is a few days, as observed for the relativistic bolides in the jets