Star cluster buzzing with pulsars
A dense globular star cluster near the centre of our Milky Way galaxy holds a buzzing beehive of rapidly spinning pulsars, according to a team of astronomers from McGill and other research institutions.
The team reported these new findings today at the American Astronomical Society's meeting in San Diego, and in the online journal Science Express, with publication upcoming in the parent journal, Science.
"We hit the jackpot when we looked at this cluster," said lead researcher Scott Ransom, who was conducting postdoctoral research at McGill when many of the pulsars were discovered.
Using the National Science Foundation's Green Bank Telescope in West Virginia, Ransom and his colleagues discovered 21 new pulsars in the cluster. The cluster, called Terzan 5, now holds the record for pulsars, with 24.
"Not only does this cluster have a lot of pulsars but the pulsars in it are very interesting. They include at least 13 in binary systems and the four fastest-rotating pulsars known in any globular cluster, with the fastest two rotating nearly 600 times per second, roughly as fast as a household blender," added Ransom.
"Discovering these amazing pulsars was like finding lots of needles in a giant haystack, where the haystack is an enormous amount (terabytes!) of telescope data," said team member Victoria Kaspi, a professor of astrophysics at McGill University. "We couldn't have done it without the superb computing resources at McGill," she added.
Globular clusters contain up to millions of stars, all of which formed at about the same time. A neutron star is what is left after a massive star explodes at the end of its life. Pulsars are spinning, superdense neutron stars that whirl "lighthouse beams" of radio waves or light around as they spin.
When a neutron star pairs up with a "normal" companion star, its strong gravitational pull can draw material off the companion onto the neutron star. This also transfers some of the companion's spin to the neutron star, thereby "recycling" the neutron star into a rapidly rotating millisecond pulsar. In Terzan 5, all the pulsars discovered are rotating rapidly as a result of this process.
"All these exotic pulsars will keep us busy for years to come," said Jason Hessels, a PhD student in McGill's physics department.
Astronomers had previously discovered three pulsars in Terzan 5, some 28,000 light-years distant in the constellation Sagittarius, but suspected there were more. On July 17, 2004, Ransom and his colleagues found 14 new pulsars in six hours, the most ever found in a single observation. Eight more observations between July and November of 2004 discovered seven more pulsars. In addition, the astronomers' data show evidence for several more pulsars that still need to be confirmed.
Future studies of the pulsars in Terzan 5 will help scientists understand the nature of the cluster and the complex interactions of the stars at its dense core. Also, several of the pulsars offer a rich yield of new scientific information. The material in a neutron star is as dense as that in an atomic nucleus, which has implications for nuclear physics as well as astrophysics.
"Finding all these pulsars has been extremely exciting, but the excitement really has just begun," Ransom said. "Now we can start to use them as a rich and valuable cosmic laboratory," he added.
In addition to Ransom, now a scientist at the National Radio Astronomy Observatory in Charlottesville, Virginia, Kaspi and Hessels, the research team included Ingrid Stairs of the University of British Columbia, Paulo Freire of Arecibo Observatory in Puerto Rico, Fernando Camilo of Columbia University and David Kaplan of the Massachusetts Institute of Technology.
The pulsar research was supported by the Canada Foundation for Innovation, Science and Engineering Research Canada, the Quebec Foundation for Research on Nature and Technology, the Canadian Institute for Advanced Research, the Canada Research Chairs program, and the National Science Foundation.
