Identification of the Mass Donor Star’s Spectrum in SS 433

Abstract

We present spectroscopy of the microquasar SS 433 obtained near primary eclipse and disk precessional phase Ψ = 0.0, when the accretion disk is expected to be most "face-on". The likelihood of observing the spectrum of the mass donor is maximized at this combination of orbital and precessional phases since the donor is in the foreground and above the extended disk believed to be present in the system. The spectra were obtained over four different runs centered on these special phases. The blue spectra show clear evidence of absorption features consistent with a classification of A3-7 I. The behavior of the observed lines indicates an origin in the mass donor. The observed radial velocity variations are in anti-phase to the disk, the absorption lines strengthen at mid-eclipse when the donor star is expected to contribute its maximum percentage of the total flux, and

SS 433: a phenomenon imitating a Wolf-Rayet star

“…In order to get a black body emission negligible compared to this optically thin emission, we had to set the radius of the A7 I star to R = 30 R or less. Then this constraint is compatible with the result of Hillwig et al (2004), who find the radius of the Roche lobe volume for the mass donor star to be R L = 28 ± 2 R . Note that this corresponds to a star with an intermediate size between type I (supergiant) and type II (bright giant) stars (Venn 1995).…”

“…Note that Hillwig et al (2004) find a binary separation of a = 0.26 ± 0.02 AU = 56 ± 4 R , which is of the order of the typical radius of the free-free emitting region at 5 µm R 5 µm = 50−67 R , which we calculated in Sect. 4.5.…”

“…However, the recent works of Gies et al (2002a) and Hillwig et al (2004) suggest that the donor star is an A3-7 I supergiant star with 10.9 ± 3.1 M . This result seems to be confirmed by the preliminary study of blue spectra of SS 433 by Charles et al (2004), who find absorption features typically seen in late B-A spectra.…”

Spectroscopic Observations of the Mass Donor Star in SS 433

“…These authors present spectra of SS 433 that show features found in an A-type supergiant but that do not follow a coherent orbital radial velocity curve. Their observations show a larger scatter, were performed over a greater variety of orbital and precessional phases, and have a much lower average velocity than those presented by Hillwig et al (2004). Barnes et al (2006) and Clark et al (2007) suggest that these A-type supergiant absorption lines arise in an accretion-driven outflow.…”

“…Finding these lines is difficult, not only due to the small relative flux contribution of the mass donor star but also to the active emission spectrum from the accretion disk, jets, and strong disk wind in the system. The most convincing observations to date of the mass donor spectrum were obtained by Hillwig et al (2004). Their spectra suggested that the donor star has a spectral classification of A3-7 I, and their radial velocity measurements led to component masses of M p O and (where the subscripts 10.9 ‫ע‬ 3.1 M M p 2.9 ‫ע‬ 0.7 M represent the optical companion star O and the X-ray-emitting compact object X).…”