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skyandtelescope.com • SEPTEMBER 2019 57
Microquasars
They may be challenging to observe, but the visible
companions in microquasars are exciting to pursue.
E
very generation of astronomers is
dealt a deep and profound mystery
to solve. In the late 1970s, the mystery
was the discovery of a “star” with very
unusual properties, leading astronomers
to suspect they were onto a new class of
phenomena. This object, SS 433, was the
433rd listed in a 1977 catalog of Milky
Way H-alpha emission stars compiled
by C. Bruce Stephenson and Nicholas
Sanduleak, astronomers at Case Western
Reserve University. Subsequent observa-
tions showed that its optical spectrum
exhibited peculiar emission lines; it also
turned out to be a variable source at radio
and X-ray wavelengths. But what clinched
the mystery was the discovery of a some-
what elongated feature in the radio data.
This combination of properties couldn’t
be reconciled to any single known model.
At the time, no one could imagine that a
miniature version of a remote quasar was
sitting in our backyard.Going Deep by Dave TostesonBy the end of the 1970s, a model
attributing bipolar jets to explain the
elongated radio emission was proposed.
However, it wasn’t until the early
1990s, when a double-sided radio jet
was imaged emanating from a source
near the galactic center, that the term
“microquasar” fi rmly took hold to
describe this particular class of object.
At the time of the discovery of
SS 433, astronomers contemplated the
following scenario to explain the com-
plexity of its emission: a binary stellar
pair in which a compact object, either
a black hole or a neutron star, pulls in
matter from its unevolved, or donor,
companion. This matter spirals into an
accretion disk, which heats up due to
friction, around the compact object. The
system’s intense gravitational energy
and (possibly) coiled magnetic fi eld
lines help launch a pair of bipolar jets.
The jets can be accelerated to a signifi -cant fraction of the speed of light, and,
when the geometry is just right, can
appear to be traveling faster than the
speed of light, a phenomenon described
as superluminal. Once a working model
was available, researchers set out to fi nd
as many examples of this type of object
as possible to test their theories.
Only the companion stars are acces-
sible to amateur astronomers (although
accretion discs around the compact
objects can contribute to the visible
light output). To my knowledge, nei-
ther jets nor other features associated
with microquasars can be seen in the
eyepiece, one exception being SS 433’s
supernova remnant, W50. Neverthe-
less, bearing in mind how remarkablepCYGNUS X-1 The microquasar with the
brightest companion in our sample, Cyg X-1
was the fi rst galactic system in which a black
hole was identifi ed. This Palomar Digital Sky
Survey image is 1° across.η CygCygnus X-1Sh 2-101