The unusual double-star system Circinus X-1 is being investigated by Kinwah Wu and collaborators to verify the theories regarding its formation and evolution.
Multiple star systems, in which two or more stars orbit each other, are common. X-ray binaries form a class of double-star systems in which the primary star is a neutron star or black hole, while its companion is an ordinary star. The companion loses matter to the primary star, giving rise to X-ray emission. These systems can be divided further into high-mass and low-mass X-ray binaries, depending on the mass of the companion star. Systems in these subclasses share distinct characteristics, but Circinus X-1, having characteristics of both types, defies classification.
The picture that has formed is one of a neutron star orbiting a low-mass star every 17 days. The orbit of the neutron star is highly elliptical, the most extreme of all known X-ray binaries, whereas low-mass X-ray binaries usually have circular orbits. As the neutron star swings past its companion at periastron, tidal interactions induce rapid matter outflow from the companion and tear apart the accretion disc around the neutron star, resulting in large X-ray fluctuations. As the neutron star pulls away from its companion, matter transfer ceases, and X-ray emission becomes steady.
Circinus X-1 also has other unusual features: it is one of only a few known X-ray binaries with jets of high-speed material flowing from it, and it lies very close to a supernova remnant (SNR), an expanding bubble of material driven off as a massive star exploded. The origin of the jets of Circinus X-1 is unknown. Since a neutron star is the collapsed core of a star that goes supernova, the nearby SNR suggests that Circinus X-1 might be the remains of a runaway supernova which exploded asymmetrically, kicking the system out of the expanding bubble of material.
This idea is supported by recent observations by Helen
Johnston (AAO), Rob Fender (U. Amsterdam) and Kinwah Wu. Optical
spectra of Circinus X-1 show a very broad line which probably arises
from infalling matter near the neutron star, and some very narrow
lines, probably originating near the companion star. These lines are not at
the frequencies they would be if the emitting material were at
rest. This means that the emitting material is moving, the amount
by which the lines are shifted indicating its speed. Kinwah and his collaborators assume that the shift of the
center of the broad line is due to the motion of the neutron star in its orbit and that the shifts in the centers of the
narrow lines are due to the motion of the companion star. By comparing
the line shifts they have found that the system is traveling at an
unusually high speed through space - 375 
- as would
be expected for a runaway system.
Circinus X-1 is indeed a very rare object - a low-mass binary with jets, so young that its orbit has not yet had time to become circular. Kinwah and his collaborators will continue to study this system in order to obtain a consistent theory of its origin and evolution.
A cartoon of Circinus X-1 showing the neutron star and accretion disc (not to scale) together with the companion star. The colored regions around the elliptical orbit show where the indicated processes are expected to dominate.