[Paper Review] Super-Keplerian Equatorial Outflows in SS 433. Centrifugal Ejection of the Circumbinary Disk

Idel Waisberg, Jason Dexter+3 more|arXiv (Cornell University)|Nov 30, 2018

Astrophysical Phenomena and Observations40 references4 citations

TL;DR

Using VLTI/GRAVITY interferometry, this study resolves the near-infrared Brγ line in SS 433, revealing a super-Keplerian equatorial outflow with extreme specific angular momentum, interpreted as centrifugal ejection of a circumbinary disk. The outflow is retrograde relative to jet precession, implying efficient angular momentum transfer from the binary, and explains both the extended line emission and radio-observed equatorial structures.

ABSTRACT

The microquasar SS 433 is the only known steady supercritical accretor in the Galaxy. It is well-known for its relativistic baryonic jets, but the system also drives equatorial outflows. These have been routinely detected in radio images, and components associated with a circumbinary disk have also been suggested in optical emission lines. We wish to spatially resolve the regions producing the stationary emission lines of SS 433 to shed light on its circumbinary structure and outflows. With an estimated binary orbit size $\lesssim 0.1 ext{ mas}$, this requires optical interferometry. We use the optical interferometer VLTI+GRAVITY to spatially resolve SS 433 in the near-infrared K band at high spectral resolution ($R\approx 4000$) on three nights in July 2017. The stationary Br$γ$ line is clearly dominated by an extended $\sim 1 ext{ mas} \sim 5 ext{ AU}$ circumbinary structure perpendicular to the jets and with a strong rotation component. The rotation direction is retrograde relative to the jet precession, in accordance with the slaved disk precession model. The structure has a very high specific angular momentum and is too extended to be a stable circumbinary disk in Keplerian rotation; interpreting it as such leads to a very high enclosed mass $M \gtrsim 400 M_{\odot}$. We instead interpret it as the centrifugal ejection of the circumbinary disk, with the implication that there must be an efficient transfer of specific angular momentum from the binary to the disk. We suggest that the equatorial outflows sometimes seen in radio images result from similar episodes of circumbinary disk centrifugal ejection. In addition to the equatorial structure, we find a very extended $\sim 6 ext{ mas} \sim 30 ext{ AU}$ spherical wind component to the Br$γ$ line: the entire binary is engulfed in an optically thin spherical line emission envelope.

Motivation & Objective

• To spatially resolve the origin of stationary emission lines in SS 433 to probe its circumbinary structure and outflows.
• To determine the kinematics and geometry of the Brγ line emission in the near-infrared using high-resolution interferometry.
• To test whether the extended emission arises from a Keplerian disk or a non-Keplerian, centrifugally ejected structure.
• To investigate the connection between radio-observed equatorial outflows and the near-infrared line-emitting structures.
• To constrain the enclosed mass and angular momentum of the circumbinary region, challenging standard Keplerian disk models.

Proposed method

• Conducted high-spectral-resolution (R ≈ 4000) optical interferometry with VLTI/GRAVITY in the K band during three nights in July 2017.
• Performed model-independent centroid analysis across the Brγ line to determine the spatial origin of emission components.
• Fitted the data with a combination of a point source (accretion disk + donor star), an equatorial disk/outflow, and a spherical wind component.
• Used kinematic modeling to derive rotation velocity, position angle, and velocity dispersion of the equatorial structure.
• Computed enclosed mass assuming Keplerian rotation to test consistency with observed kinematics.
• Compared results with the 2016 GRAVITY observation to assess variability in the outflow structure.

Experimental results

Research questions

• RQ1 What is the spatial origin and kinematic structure of the stationary Brγ emission in SS 433?
• RQ2 Is the extended equatorial emission consistent with a Keplerian circumbinary disk or a non-Keplerian outflow?
• RQ3 What is the specific angular momentum of the equatorial structure, and how does it compare to the binary system?
• RQ4 How does the geometry and kinematics of the outflow relate to the precessing relativistic jets?
• RQ5 What mechanism drives the ejection of material with super-Keplerian rotation, and how does it relate to radio-observed equatorial outflows?

Key findings

• The Brγ line emission is dominated by an extended equatorial structure of ~1 mas (~5 AU) perpendicular to the jets, with a retrograde rotation relative to jet precession.
• The equatorial structure exhibits a specific angular momentum ~10 times higher than that of the binary components, ruling out a stable Keplerian disk.
• Interpreting the structure as a Keplerian disk implies an implausibly high enclosed mass of ~400 M☉, indicating non-Keplerian dynamics.
• The data support a centrifugal ejection model for the circumbinary disk, with efficient angular momentum transfer from the binary to the disk.
• A separate, extended spherical wind component (~6 mas, ~30 AU) is detected, responsible for high-velocity line wings (>1000 km/s).
• The 2016 observation showed a jet-aligned bipolar outflow, while the 2017 data show a dominant equatorial structure, indicating episodic disk ejection.