SS 433: The accretion disk revealed in Hα

Abstract

Context. The Galactic microquasar SS 433 is very luminous and ejects opposite jets at approximately one quarter the speed of light. It is regarded as a super-Eddington accretor but until recently there were no observations of accretion. Aims. We present an analysis of spectroscopic optical data obtained before and during a major flare, which yield in Hα unambiguous evidence for the accretion disk. Methods. Already published high resolution spectra, taken with a 3.6-m telescope almost nightly over 0.4 of a precession cycle, are analysed. Results. The spectra, taken almost nightly in August and September 2004, revealed a period of quiescence followed by activity which culminated in the accretion disk of SS 433 becoming visible. The visible material in the accretion disk orbited the compact object at greater than 500 km s-1, implying that the mass of the compact object is less than 37 ⊙. Evidence that an accretion stream joins the disk at over 700 km s-1 suggests that the mass is considerably below this upper limit. The accretion disk clearly orbits the centre of mass of the binary system with the compact object, sharing its speed of approximately 175 km s-1. The mass of the companion lies between 20 and 30 ⊙ and it probably does not fill its Roche lobe. © 2010 ESO.