Popping with pulsars

Radio astronomers find 21 new, fast-spinning pulsars in a single globular cluster.
By | Published: January 14, 2005
The majority of a globular cluster’s stars are old and cool, but several hot blue stars show up in this Hubble Space Telescope image of the center of NGC 6397. Many of these stars are believed to be “blue stragglers” or cataclysmic variables, which are the products of stellar mergers or close encounters, respectively. NGC 6397 lies 8,200 light-years from Earth in the constellation Ara. This image is roughly 2 arcminutes (3.8 light-years) wide.
NASA / Hubble Heritage Team (STScI/AURA)
January 14, 2005
Terzan 5, a massive, densely populated globular cluster in the constellation Sagittarius contains a record number of quickly rotating “millisecond” pulsars, say astronomers who discovered 21 new ones there. With three already known, Terzan 5 has a new total of 24, beating the former record-holder, 47 Tucanae, which has 22.

A pulsar is a spinning neutron star, the dense remains of a massive star that exploded as a supernova. Pulsars contain powerful magnetic fields, up to trillions of times stronger than Earth’s field. A pulsar’s field, tilted to the spin axis, may beam jets of particles like a lighthouse toward Earth. While an ordinary pulsar rotates in the once-per-second range, millisecond pulsars spin a hundred times faster or more. These are old, ordinary pulsars that have been spun up by accreting material from binary companion stars.

“We hit the jackpot when we looked at this cluster,” says Scott Ransom of the National Radio Astronomy Observatory. He led the team that used the 100-meter Green Bank Telescope in West Virginia to make the discovery. The 24 pulsars include at least 13 binary systems (two of them eclipsing) and the four fastest-rotating pulsars in any globular cluster. The swiftest of these spins nearly 600 times a second. “That’s faster than a kitchen blender,” says Ransom.

“Everyone had been wondering where Terzan 5’s pulsars were,” says Ransom, noting that theorists predicted some 60 to 200 there. With about a million stars, Terzan 5 lies 28,000 light-years from Earth and appears about 1′ across. The Green Bank Telescope’s beam, however, is 6′ across. In a single 6-hour observation of the whole cluster at once, Ransom’s team discovered 14 pulsars. Terzan 5 is not visible to the eye, however, because it lies in the galactic plane, hidden by dust.

The dense stellar environment in Terzan 5 helped create the high number of stars, says Ransom. Crowded conditions — with 100 to 10,000 stars per cubic light-year — mean neutron stars interact gravitationally with evolving ordinary stars, thereby gaining and losing partners and generally settling toward the cluster’s center. In a typical situation, gas from an evolved red giant star drops onto the neutron star, triggering X-ray flares and speeding up the pulsar’s spin. The more gas, the faster the spin climbs — up to a point.

“General relativity imposes a limit to how fast spins can get,” Ransom notes. When a pulsar is turning once every half a millisecond, its surface will be traveling at the speed of light.

Ransom says the team is now going to study each of the pulsars in detail, probing their extreme conditions and properties. The analysis requires a lot of supercomputer power. The team also plans, Ransom says, to look for more pulsars in Terzan 5, while checking out other massive globular clusters.