Radio Imaging of SS433

web page by Michael Rupen

Last update: 12 January 2004

A quick introduction to SS433

SS433 is a binary star system in the constellation Aquila, consisting of an early type star and a compact object (neutron star or black hole). Material from the normal star is falling toward the compact object, either as the result of a strong stellar wind, or through Roche lobe overflow. The angular momentum of the material prevents it from falling directly onto the compact object; instead it forms an accretion disk which radiates at wavelengths ranging from the optical to the X-ray. This accretion is frequently (always?) associated with a collimated jet outflow moving at a substantial fraction of the speed of light. This basic picture is illustrated in the cartoon to the right, generated by Rob Hynes with his wonderful binary visualization software.
SS433 is unique in showing extremely large Doppler shifts in both optical and X-ray spectral lines. These lines originate in material in the jets moving at velocities of about 26% of the speed of light. The misalignment of the planes of the binary orbit and the accretion disk causes the latter to precess over a 162.5-day period; the jets are tied to the accretion disk, so they precess as well. The resulting wobble leads to changes in the Doppler shifts of the jet material, causing the spectroscopic lines to move up and down the spectrum by enormous amounts. Some of the original spectra are shown on Bruce Margon's Web site.
The relativistic jets are also seen directly in radio observations, which detect synchrotron emission from relativistic particles spiralling around strong magnetic fields. The first detailed observations were made with the Very Large Array (VLA) around the time of its formal dedication in 1980; some of these initial images, taken from Hjellming & Johnston (1981), are shown here to the left. The same precessing jet model which fits the optical line shifts gives the model superposed on these contour plots.

The importance of SS433

The fundamental process in astronomy is accretion: gravity collects matter to make stars, galaxies, and galaxy clusters, and the resulting energy powers the radiation we see. One of the fundamental results of the past few decades is that this accretion seems always to be accompanied by outflow, in objects ranging from proto-stellar disks to the supermassive black holes which define the centers of galaxies. The most spectacular of these outflows are the relativistic jets seen in active galactic nuclei (AGN), and more locally in microquasars and pulsars. SS433 is by far the best local laboratory for studying these phenomena.

SS433 is the only relativistic jet known which produces spectral lines. This has several major implications:

From the radio standpoint, the jets in SS433 also have two other important, unique characteristics: they evolve rapidly, and they precess over a large angle. The rapid evolution makes them easy to study on human timescales -- weeks to months, rather than the decades required to see interesting changes in extragalactic sources (active galactic nuclei). The large precession angle means that we can follow the evolution of the ejecta unambiguously over a substantial period, since previous and following ejections occur along different angles.

The VLBA Movie (Mioduszewski, Rupen, Walker, & Taylor 2004)

What we did

We obtained 39 two-hour observations of SS433 spread over 42 days (June 26 through August 6, 2003), covering roughly 1/4 of the precession period. These VLBA data were taken at 1.5 GHz, giving a spatial resolution along the jet of ~7 milliarcseconds (7 times better than the Hubble Space Telescope), corresponding to ~35 au at a distance of 5 kpc (i.e., roughly the size of the solar system).

The movie

Preliminary result #1: the brightening regions

There are several locations where individual ejecta brighten as they move out from the core. This brightening is substantial (factor 2-30), and occurs at symmetric distances/times about the core, after allowing for relativistic effects. However, even nearby ejecta do not necessarily brighten as they move through these same radii, suggesting that whatever causes the brightening is very restricted in azimuth. This is all in sharp contrast to Vermeulen et al.'s 1993 result that all ejecta brighten while moving out to ~50 milliarcseconds from the core.

Preliminary result #2: the anomalous emission

We also see substantial emission not associated with the jets, but centered on the binary system. This has been seen before (e.g., Paragi et al. 1999; Blundell et al. 2000) but this is the first time this "anomalous emission" has been followed in detail over many days. The anomalous emission moves out and brightens as the jet precesses away, with an average speed of ~10,000 km/s. The typical size of each of the emitting regions is ~200 au.

Other SS433 Links


Please send any questions, comments, or suggestions to Michael Rupen at the e-mail address given below.

Last modified 12 January 2004