-- the core remains of a star once several times more massive than our Sun. Such a massive star, upon depleting its nuclear fuel, violently expels much of its mass into space in an event called a supernova explosion. The remaining ember, which still packs more than a Sun's worth of mass into a sphere about 10 miles (16 kilometers) wide, becomes the neutron star.X-ray pulsars occur in binary star systems. The compact pulsar orbits around a young and unstable hydrogen-burning star, which is surrounded by a large disk of hydrogen that provides a reservoir of fuel to power the X-ray pulsar. The tiny pulsar becomes visible and "pulses" in X-ray radiation when the orbit brings the two stars close to each other.
Gas from the "normal" star's equatorial disk spills over onto the pulsar, attracted by the latter's strong gravity. The pulsar's enormous magnetic field then channels the gas toward the magnetic poles of the pulsar where the gas attains speeds up to 20 percent that of light and heats to temperatures far hotter than when it was part of the healthy star. This gas now glows predominantly in the X-ray band.
Laycock said that binary stars and supernovas with eccentric orbits must be quite common in the Small Magellanic Cloud.
An unusual galaxy
The SMC is about 200,000 light-years away and is the second-closest galaxy to the Milky Way, visible to the naked eye in the Southern Hemisphere. Its total mass is only one-fiftieth that of our galaxy's, and yet it seems to contain at least 10 times more X-ray pulsar systems than the Milky Way.
In the SMC astronomers have detected a very high proportion of hot young stars, along with the still-glowing remains of many recent supernova explosions. These are just two of the clues that provide a "smoking gun" in the hunt for what produces the SMC's pulsars, suggesting a burst of new-star formation as recent as about 5 million years ago.
"X-ray pulsars of the kind we are finding in the SMC have a very limited lifetime," said Corbet, of the Universities Space Research Association. "That we are discovering so many may mean that they all formed at about the same time."
The role of tidal forces
Such a large star birth rate may well be related to the fierce gravitational interactions that have taken place between the SMC and Milky Way Galaxy. The SMC slowly orbits the Milky Way in an elliptical path and, a few million years ago, the two galaxies were at their closest point.
Corbet suspects that the Milky Way caused large tidal forces to occur in the SMC, which resulted in the birth of numerous bright, massive stars. These stars then later exploded to form the X-ray pulsars we see today.
The SMC represents an exceptional target for astronomers to study X-ray pulsar systems. All the objects are at essentially the same distance and are situated far above the gas- and dust-strewn plane of the Milky Way, hence providing an excellent testbed for investigating evolutionary theories.
Trying to observe such systems in the Milky Way is fraught with difficulties arising from the extremely variable interstellar absorption effects in different directions.
Identification of the massive young binary companions was obtained by using the 39-inch (1-meter) telescope at the South African Astronomical Observatory.
Past X-ray pulsars in the SMC were detected by the Rossi Explorer, the Italian-Dutch BeppoSAX Observatory and the Japan-U.S. ASCA Observatory.
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