More on the circumbinary disk of SS 433

TL;DR

This paper presents a simple yet effective model of a circumbinary disk around SS 433, strongly supporting its existence and implying a massive black hole in the system based on spectral analysis over multiple orbital periods.

Contribution

The authors develop a new excitation model for the circumbinary disk that matches observations and reinforces the case for a high-mass binary system with a massive black hole.

Findings

Model accurately describes spectral variations with orbital phase.

Supports the existence of a circumbinary disk with >200 km/s orbital speed.

Suggests the binary system's mass exceeds 40 solar masses.

Abstract

Certain lines in spectra of the Galactic microquasar SS 433, in particular the brilliant H alpha line, have been interpreted as emission from a circumbinary disk. In this interpretation the orbital speed of the glowing material is in excess of 200 km/s and the mass of the binary system in excess of 40 solar masses. A very simple model of excitation of disk material is in remarkable agreement with the observations, yet it seems that the very existence of a circumbinary disk is regarded as controversial. Published spectra, taken almost nightly over two orbital periods of the binary system, show H alpha and He I lines; these were analysed as superpositions of Gaussian components. A model in which the excitation of any given patch of putative circumbinary material is proportional to the inverse square of its instantaneous distance from the compact object was constructed and compared with observations. The new model provides an excellent description of the observations. The variation of the H alpha and He I spectra with orbital phase are described quantitatively provided the radius of the emitting ring is not much greater than the radius of the closest stable circumbinary orbit. The new analysis has greatly strengthened the case for a circumbinary disk orbiting the SS 433 system with a speed of over 200 km/s and presents supposed alternative explanations with major difficulties. If the circumbinary disk scenario is essentially correct, the mass of the binary system must exceed 40 solar masses and the compact object must be a rather massive black hole. The case is so strong that this possibility should be taken seriously.