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

TL;DR

This study shows that the HeII emission line in SS433 does not trace the orbital motion of the compact object, challenging previous mass estimates based on this line.

Contribution

It demonstrates that the HeII emission region is larger and not eclipsed, making the HeII radial velocity curve unreliable for mass determination in SS433.

Findings

HeII emission region is larger than the donor star and accretion disk

HeII radial velocity curve is independent of precessional phase

HeII line does not reflect the orbital motion of the compact object

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.