The radial velocity curve for HeII emission cannot be used for component mass determination in SS433

TL;DR

This study tests whether the He II 4686 Å emission in SS433 traces the orbital motion of the compact object. Using a six-year, homogeneous spectroscopic dataset from the Transient Double-beam Spectrograph on the 2.5 m telescope, the authors demonstrate that the He II line forms in an extended circumbinary environment and its flux varies sinusoidally with the orbital period, while its radial-velocity curve is not tied to the orbital motion and is largely independent of precessional phase. They show that the He II RV curve cannot reliably reflect the compact object's motion, calling into question prior mass-function estimates based on this line. These findings imply that circumbinary gas dominates He II emission in SS433 and highlight the need for alternative methods to determine component masses in this system.

Abstract

More than 150 measurements of the HeII 4686A emission line in spectra of SS433 were obtained during 388 nights in 2020-2025 with the Transient Double-beam Spectrograph on the 2.5 m telescope of Caucasian Mountain Observatory of Sternberg Astronomical Institute. We found that the HeII emission line formation region is not eclipsed and is significantly larger than both the donor star and the photosphere of the supercritical accretion disk. The HeII radial velocity curve was found to be independent of the precessional phase and inconsistent with the photometric curve. These findings suggest that the HeII line does not reflect the orbital motion of the compact object. Therefore, spectroscopic estimates of the masses of the components in SS433 based on the HeII emission line can be unrealistic.

Figures (5)

• Figure 1: Examples of TDS spectra of SS433 at dates with double-peaked He2 4686 Å line.
• Figure 2: The appearance of double-peaked profiles as a function of the precessional phase. Red and blue dots mark the velocities of each peak. Gray lines mark the position and FWHM of all single-peaked profiles used for constructing the radial velocity curve shown in Figure \ref{['fig:orbcurve']}. Crossover phases (disk edge-on) are marked with "c" labels at the top.
• Figure 3: Examples of TDS spectra of SS433 used for measurements of radial velocities of He2.
• Figure 4: Orbital phase curves of He2 4686 Å line variability. (a) The radial velocity curve: black circles show our TDS observations at phases $-0.2<\varphi_{\rm prec}<0.2;$ gray triangles show our TDS observations at phases $0.2<\varphi_{\rm prec}<0.8;$ crosses "$\times$" are our measurements from the X-shooter spectra, plus signs "+" are measurements from 2010ApJ...709.1374K, the blue line is a formal fit to the radial velocities ($e=0.30,$$K_{\rm X}=144$ km s$^{-1}$, $\omega=142^\circ,$$\gamma=71$ km s$^{-1}$); the red dashed curve corresponds to the radial velocities for the orbital parameters from Cher21 ($e=0.05,$$\omega=40^\circ$) scaled and shifted with $K_{\rm X}=140$ km s$^{-1}$ and $\gamma=40$ km s$^{-1}$; (b) The photometric B light curve at phases $-0.2<\varphi_{\rm prec}<0.2$, the red curve is the Gaussian fits of the primary and secondary minima. Blue vertical lines indicate the would-be positions of the primary and secondary minima for the formal fit to the He2 radial velocity curve shown in blue in panel (a). (c) The He2 4686 Å line height relative to the continuum expressed in stellar magnitudes for ease of comparison with the light curve. The red solid curve is the "upside-down" red light curve shown in panel (b), scaled to best match with the observed change in the line height, which corresponds to the 6% eclipsed fraction of the He2 line region. The upper dashed curve shows the expected change in the line height in the absence of an eclipse. The lower dashed curve corresponds to the 30% eclipsed fraction of the He2 line flux. (d) The same as in panel (c) for the line equivalent width; the solid curve corresponds to no line eclipse, but takes into account a sinelike flux variability shown in panel (e). (e) Orbital variability of the He2 line flux with a sinelike fit (red curve).
• Figure 5: Deviations of the radial velocity of He2 line from the formal fit in Figure \ref{['fig:orbcurve']}a as a function of the precessional phase. No regular phase dependence is seen. Crossover phases are marked with "c" labels at the top.