Astronomers Discover Fastest-Spinning Pulsar (2025)

Embargoed for Release: 2:00 p.m., EST, Thursday, January 12, 2006

Graphics: How Are Millisecond Pulsars Formed?

Astronomers using the National Science Foundation's Robert C. ByrdGreen Bank Telescope have discovered the fastest-spinning neutron starever found, a 20-mile-diameter superdense pulsar whirling faster thanthe blades of a kitchen blender. Their work yields important newinformation about the nature of one of the most exotic forms of matterknown in the Universe.

"We believe that the matter in neutron stars is denser than an atomicnucleus, but it is unclear by how much. Our observations of such arapidly rotating star set a hard upper limit on its size, and hence onhow dense the star can be.," said Jason Hessels, a graduate student atMcGill University in Montreal. Hessels and his colleagues presentedtheir findings to the American Astronomical Society's meeting inWashington, DC.

Pulsars are spinning neutron stars that sling "lighthouse beams" ofradio waves or light around as they spin. A neutron star is what isleft after a massive star explodes at the end of its "normal"life. With no nuclear fuel left to produce energy to offset thestellar remnant's weight, its material is compressed to extremedensities. The pressure squeezes together most of its protons andelectrons to form neutrons; hence, the name "neutron star."

"Neutron stars are incredible laboratories for learning about thephysics of the fundamental particles of nature, and this pulsar hasgiven us an important new limit," explained Scott Ransom, anastronomer at the National Radio Astronomy Observatory and one ofHessels' collaborators on this work.

The scientists discovered the pulsar, named PSR J1748-2446ad, in aglobular cluster of stars called Terzan 5, located some 28,000light-years from Earth in the constellation Sagittarius. Thenewly-discovered pulsar is spinning 716 times per second, or at 716Hertz (Hz), readily beating the previous record of 642 Hz from apulsar discovered in 1982. For reference, the fastest speeds of commonkitchen blenders are 250-500 Hz.

The scientists say the object's fast rotation speed means that itcannot be any larger than about 20 miles across. According toHessels, "If it were any larger, material from the surface would beflung into orbit around the star." The scientists' calculation assumedthat the neutron star contains less than two times the mass of theSun, an assumption that is consistent with the masses of all knownneutron stars.

The spinning pulsar has a companion star that orbits it once every 26hours. The companion passes in front of the pulsar, eclipsing thepulsar about 40 percent of the time. The long eclipse period, probablydue to bloating of the companion, makes it difficult for theastronomers to learn details of the orbital configuration that wouldallow them to precisely measure the masses of the pulsar and itscompanion.

"If we could pin down these masses more precisely, we could then get abetter limit on the size of the pulsar. That, in turn, would then giveus a better figure for the true density inside the neutron star,"explained Ingrid Stairs, an assistant professor at the University ofBritish Columbia and another collaborator on the work.

Competing theoretical models for the types and distributions ofelementary particles inside neutron stars make widely differentpredictions about the pressure and density of such an object.

"We want observational data that shows which models fit the reality ofnature," Hessels said.

If the scientists can't use PSR J1748-2446ad to do that, they arehopeful some of its near neighbors will yield the data theyseek. Using the GBT, the astronomers so far have found 30 new fast"millisecond pulsars" in the cluster Terzan 5, making 33 pulsars knownin the cluster in total. This is the largest number of such pulsarsever found in a single globular cluster.

Dense globular clusters of stars are excellent places to findfast-rotating millisecond pulsars. Giant stars explode as supernovaeand leave rotating pulsars which gradually slow down. However, if apulsar has a companion star from which it can draw material, thatincoming material imparts its spin, or angular momentum, to thepulsar. As a result, the pulsar spins faster. "In a dense cluster,interactions between the stars will create more binary pairs that canyield more fast-rotating pulsars," Ransom said.

The great sensitivity of the giant, 100-meter diameter GBT, along witha special signal processor, called the Pulsar Spigot, made possiblethe discovery of so many millisecond pulsars in Terzan 5. "We thinkthere are many more pulsars to be found in Terzan 5 and otherclusters, and given that the fast ones are often hidden by eclipses,some of them may be spinning even faster than this new one," Ransomsaid.

"We're excited about using this outstanding new telescope to answersome important questions about fundamental physics," he said.

In addition to Hessels, Ransom and Stairs, the research teamincludes Paulo Freire of Arecibo Observatory in Puerto Rico,Victoria Kaspi, of McGill University, and Fernando Camilo, ofColumbia University. Their report is being published in Science Express, the online version of the journal Science.

The National Radio Astronomy Observatory is a facility of theNational Science Foundation, operated under cooperative agreement by Associated Universities, Inc. The pulsar research also was supported by theCanada Foundation for Innovation, the Natural Sciences and EngineeringResearch Council of Canada, the Quebec Foundation for Research onNature and Technology, the Canadian Institute for Advanced Research,the Canada Research Chairs Program, and the National ScienceFoundation..

Graphics: How Are Millisecond Pulsars Formed?