Astronomers have found two new X-ray pulsars spinning in the Small Magellanic Cloud, the Milky Way’s neighbor. This brings the total number of pulsars there to 25 and drives home the fact that our neighboring galaxy has a much higher concentration of pulsars than we do, perhaps created during a burst of star formation a few million years ago when the two galaxies were at their closest.

Silas Laycock a graduate student at the University of Southampton, England, together with a team at NASA’s Goddard Space Flight Center, Greenbelt, Md., led by Dr. Robin Corbet, uncovered the pulsars with the Rossi X-ray Timing Explorer satellite. Laycock presents the team’s findings today at the Gamma Ray 2001 meeting in Baltimore.

“The discovery of so many X-ray pulsars has been a surprise to most astronomers,” said Laycock, who began his analysis of these pulsars last autumn while visiting Goddard. “A few years ago we only knew of one pulsar in the Small Magellanic Cloud. Now we are at 25 and still climbing. There seems to be 10-times the concentration of X-ray pulsars there compared to the Milky Way.”

An X-ray pulsar is a type of spinning neutron star, 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 ember, which still packs more than a sun’s worth of mass into a sphere about 10 miles wide, becomes the neutron star.

X-ray pulsars reside 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 pulsar’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% 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 the characters involved in this high-energy play — binary stars, exploding stars, eccentric orbits that bring companions within kissing distance — must be quite common in the Small Magellanic Cloud, or SMC.

The petite, irregularly shaped SMC is about 200,000 light years away and is the second closest galaxy to the Milky Way galaxy, visible to the naked eye from the Southern Hemi-sphere. The SMC’s total mass is only 1/50th that of the Milky Way galaxy, and yet there seems to be at least 10 times more X-ray pulsar systems than that ratio would suggest.

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, who is a scientist with the Universities Space Research Association. “That we are discovering so many may mean that they all formed at about the same time.”

Such a large star birth rate may well be related to the fierce gravitational interactions that have taken place between the SMC and the Milky Way galaxy. The SMC slowly orbits around the Milky Way in an elliptical path and, a few million years ago, the SMC and the Milky Way 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 test-bed for investigating evolutionary theories. Trying to observe such systems in the Milky Way galaxy is fraught with difficulties arising from the extremely variable interstellar absorption effects in different directions.

Identification of the massive young binary companions was obtained through optical ground-based imaging carried out at the 1.0-meter telescope of the South African Astronomical Observatory by Dr. Malcolm Coe, Laycock’s thesis advisor.

Corbet and his team have approved monitoring observations on RXTE every two weeks till February 2002. Further ground-based observations are planned from South Africa later in the year to study in detail the precise spectral type of the young stars and the extent to which the massive stellar disk varies. Observations by upcoming high-energy observatories such as INTEGRAL will permit direct measurements of the magnetic fields around the neutron stars from their higher energy coverage.

Other X-ray pulsars in the SMC have been detected by the Rossi Explorer, the Italian-Dutch BeppoSAX observatory, and the Japan-U.S. ASCA observatory.

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