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Taking the Heavens' Pulse

20 October 2000 7:00 pm
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A pulsar's home. M62 (NGC 6266), one of the globular clusters in which new pulsars have been found.

Radio astronomers have discovered pulsars in four more globular clusters--the giant, spherical collections of stars surrounding the center of the Milky Way galaxy--bringing the total of pulsar-bearing clusters to 16. The discoveries may help solve the puzzle of why some globular clusters contain lots of pulsars, whereas others lack them altogether. This, in turn, will shed light on the evolution of the clusters, which are the oldest objects in the Milky Way. Surprisingly, one of the new pulsars also seems to have a big planet orbiting it.

Since 1967, astronomers have found more than 1000 pulsars--rapidly spinning, compact neutron stars that spew out two beams of radio waves--in the flattened disk of the Milky Way galaxy. But pulsars weren't discovered in globular clusters until the early 1990s, and finding new ones is difficult because of the unknown "smearing" of pulses by the interstellar medium. Moreover, most globular cluster pulsars are part of a binary system, so the pulse frequency constantly changes due to the Doppler effect.

Now, a team led by Nichi D'Amico of Bologna Observatory, Italy, has used supercomputers to perform a wholesale search for new pulsars in globular clusters. The researchers found a total of four new binary millisecond pulsars, each in a different globular cluster, they report in a paper accepted by Astrophysical Journal Letters; recently, they have discovered two more in one of the clusters, says team member Andrew Lyne of the University of Manchester, United Kingdom.

"The discovery of these new systems represents a new stage in pulsar searches," says Ben Stappers of the University of Amsterdam. Thanks to the new search technique, many more globular cluster pulsars are expected to be found in the near future, says Stappers. Not only do the recent pulsar observations make it possible for researchers to do better statistics on pulsars, but they also reveal details about the three-dimensional distribution of interstellar gas in the galaxy.

Another find worth delving into, Stappers says, is the new pulsar in NGC 6544, a globular cluster in the constellation Sagittarius the Archer, whose companion has a very low mass: It weighs in at just 10 times the mass of Jupiter. This mysterious companion doesn't have enough mass to be a star, Stappers says, so it may be a giant planet. "But it's not clear how a planetary system could survive in the dense environment of a globular cluster," he adds.

Related sites

Paper by D'Amico et al.

Pulsar tutorial

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