Making an extra effort to image a faint, gigantic corkscrew
traced by fast protons and electrons shot out from a
mysterious microquasar paid off for a pair of
astrophysicists who gained new insights into the beast’s
inner workings and also resolved a longstanding dispute over
the object’s distance.

The astrophysicists used the National Science Foundation’s
Very Large Array (VLA) radio telescope to capture the
faintest details yet seen in the plasma jets emerging from
the microquasar SS 433, an object once dubbed the “enigma of
the century.” As a result, they have changed scientists’
understanding of the jets and settled the controversy over
its distance “beyond all reasonable doubt,” they said.

SS 433 is a neutron star or black hole orbited by a “normal”
companion star. The powerful gravity of the neutron star or
black hole draws material from the stellar wind of its
companion into an accretion disk of material tightly
circling the dense central object prior to being pulled onto
it. This disk propels jets of fast protons and electrons
outward from its poles at about a quarter of the speed of
light. The disk in SS 433 wobbles like a child’s top,
causing its jets to trace a corkscrew in the sky every 162
days.

The new VLA study indicates that the speed of the ejected
particles varies over time, contrary to the traditional
model for SS 433.

“We found that the actual speed varies between 24 percent to
28 percent of light speed, as opposed to staying constant,”
said Katherine Blundell, of the University of Oxford in the
United Kingdom. “Amazingly, the jets going in both
directions change their speeds simultaneously, producing
identical speeds in both directions at any given time,”
Blundell added. Blundell worked with Michael Bowler, also
of Oxford. The scientists’ findings have been accepted
by the Astrophysical Journal Letters.

The new VLA image shows two full turns of the jets’ corkscrew
on both sides of the core. Analyzing the image
showed that if material came from the core at a constant
speed, the jet paths would not accurately match the details
of the image.

“By simulating ejections at varying speeds, we were able to
produce an exact match to the observed structure,” Blundell
explained. The scientists first did their match to one of
the jets. “We then were stunned to see that the varying
speeds that matched the structure of one jet also exactly
reproduced the other jet’s path,” Blundell said. Matching
the speeds in the two jets reproduced the observed structure
even allowing for the fact that, because one jet is moving
more nearly away from us than the other, it takes light
longer to reach us from it, she added.

The astrophysicists speculate that the changes in ejection
speed may be caused by changes in the rate at which material
is transferred from the companion star onto the accretion
disk.

The detailed new VLA image also allowed the astrophysicists
to determine that SS 433 is nearly 18,000 light-years
distant from Earth. Earlier estimates had the object, in
the constellation Aquila, as near as 10,000 light-years.
An accurate distance, the scientists said, now allows
them to better determine the age of the shell of debris
blown out by the supernova explosion that created the
dense, compact object in the microquasar. Knowing the
distance accurately also allows them to measure the
actual brightness of the microquasar’s components, and
this, they said, improves their understanding of the
physical processes at work in the system.

The breakthrough image was made using 10 hours of observing
time with the VLA in a configuration that maximizes the
VLA’s ability to see fine detail. It represents the longest
“time exposure” of SS 433 at radio wavelengths, and thus
shows the faintest details. It also represents the best such
image that can be done with current technology. Because the
jets in SS 433 are moving, their image would be “smeared” in
a longer observation. In order to see even fainter details
in the jets, the astrophysicists must await the greater
sensitivity of the Expanded VLA, set to become available in
a few years.

SS 433 was the first example of what now are termed microquasars,
binary systems with either a neutron star or black hole orbited
by another star, and emitting jets of material at high speeds.
The strange stellar system received a wealth of media coverage
in the late 1970s and early 1980s. A 1981 Sky & Telescope
article was entitled, “SS 433 — Enigma of the Century.”

Because microquasars in our own Milky Way Galaxy are thought to
produce their high-speed jets of material through processes similar
to those that produce jets from the cores of galaxies, the nearby
microquasars serve as a convenient “laboratory” for studying the
physics of jets. The microquasars are closer and show changes
more quickly than their larger cousins.

Katherine Blundell is a University Research Fellow funded by
the UK’s Royal Society.

The National Radio Astronomy Observatory is a facility of the
National Science Foundation, operated under cooperative agreement
by Associated Universities, Inc.

NOTE: This release, with graphics, is available on the NRAO Web
site, at: http://www.nrao.edu/pr/2004/ss433corkscrew/