* Astronomy

Credit: Chandra: NASA/CXC/Middlebury College/F.Winkler et al; ROSAT: NASA/GSFC/S.Snowden et al.; Optical: NOAO/AURA/NSF/Middlebury College/F.Winkler et al.
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Position(2000): RA 08h 23m 08.16s | Dec -42º 41' 41.40''

This graphic shows a wide-field view of the Puppis A supernova remnant along with a close-up image of the neutron star, known as RX J0822-4300, that is moving at a blistering pace. The larger field-of-view is a composite of X-ray data from the ROSAT satellite (pink) and optical data (purple), from the Cerro Tololo Inter-American Observatory 0.9-meter telescope, which highlights oxygen emission. Astronomers think Puppis A was created when a massive star ended its life in a supernova explosion about 3,700 years ago, forming an incredibly dense object called a neutron star and releasing debris into space.
The neutron star was ejected by the explosion. The inset box shows two observations of this neutron star obtained with the Chandra X-ray Observatory over the span of five years, between December 1999 and April 2005. By combining how far it has moved across the sky with its distance from Earth, astronomers determined the cosmic cannonball is moving at over 3 million miles per hour, one of the fastest moving stars ever observed. At this rate, RX J0822-4300 is destined to escape from the Milky Way after millions of years, even though it has only travelled about 20 light years so far.

Labelled Composite Image of RX J0822-4300 in Puppis A

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A neutron star has been clocked travelling at more than 1500 kilometres per second. It joins the ranks of other fast moving neutron stars, deepening the puzzle over how these dense stellar corpses are accelerated to such astonishing velocities.

Neutron stars are the city-sized spheres that remain after stars are destroyed in supernova explosions. They are incredibly dense – a teaspoonful of neutron star material would weigh a billion tonnes.
Many neutron stars are now known to travel at speeds of hundreds of kilometres per second, with one shown in 2005 to be moving at 1100 km/s. Some others have been estimated as travelling faster than 1500 km/s but with less certain measurements: their speeds were measured in an indirect way, based on observations of their effect on the gaseous medium that they move through.
Astronomers have had a hard time figuring out how neutron stars get accelerated to such blistering speeds. Their theoretical models can produce speeds of a few hundred kilometres per second, but these suggest that neutron stars should rarely, if ever, reach more than 1000 kilometres per second.
The neutron star now found to be zipping along at 1500 km/s is providing an even bigger challenge for the models. Its speed was measured by Frank Winkler of Middlebury College in Vermont and Robert Petre of NASA's Goddard Space Flight Centre in Greenbelt, Maryland, both in the US.
Dramatic recoil

Using NASA's orbiting Chandra X-ray Observatory, they took snapshots five years apart of the neutron star RX J0822-4300 in the Puppis A supernova remnant. They found it to be moving 44 millionths of a degree per year. Given the estimated distance of 6500 light years to the supernova remnant, this translates into about 1500 kilometres per second.
The neutron star and some nearby clumps of gas leftover from the explosion are moving in opposite directions. The motion of the gas was measured in earlier studies.

"You've got the outer layers of the star going in one direction and the neutron star going in the other direction and that's the first time anyone has seen this so dramatically" - Robert Petre.

Such opposite motions are predicted by one popular model of how neutron stars get kicked to high speed, which says that the gas in the explosion gets spewed out preferentially in one direction, causing the neutron star to recoil the opposite way.
Subject to error

"It’s a great measurement. It shows that high velocity neutron stars may be even more common than we think" - Shami Chatterjee of Cornell University in New York, US, who led the previous study of the 1100-km/s neutron star.

The uncertainty in the distance to Puppis A means the neutron star's velocity is subject to error.
The more such neutron stars are found, the harder it will be for current models to explain, says Chris Fryer of the Los Alamos National Laboratory in New Mexico, US. Such high speeds sometimes occur in the simulations, but only rarely.

"Every time you find a neutron star that’s really fast you're pushing all the models to the limits" - Chris Fryer.

Astronomers C Y Hui and Werner Becker of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, have made similar measurements of RX J0822-4300, but put its velocity at about 1100 km/s. Petre argues that his team's calculation of over 1500 km/s is more precise, however, because it is based on more measurements.

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Astronomers using the National Science Foundation's Robert C. Byrd Green Bank Telescope have discovered the fastest-spinning neutron star ever found, a 20-mile-diameter superdense pulsar whirling faster than the blades of a kitchen blender. Their work yields important new information about the nature of one of the most exotic forms of matter known in the Universe.

"We believe that the matter in neutron stars is denser than an atomic nucleus, but it is unclear by how much. Our observations of such a rapidly rotating star set a hard upper limit on its size, and hence on how dense the star can be" - Jason Hessels, graduate student at McGill University in Montreal. Hessels and his colleagues presented their findings to the American Astronomical Society's meeting in Washington, DC.

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

"Neutron stars are incredible laboratories for learning about the physics of the fundamental particles of nature, and this pulsar has given us an important new limit" - Scott Ransom, astronomer at the National Radio Astronomy Observatory and one of Hessels' collaborators on this work.

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

The scientists say the object's fast rotation speed means that it cannot be any larger than about 20 miles across.

"If it were any larger, material from the surface would be flung into orbit around the star" - Jason Hessels

The scientists' calculation assumed that the neutron star contains less than two times the mass of the Sun, an assumption that is consistent with the masses of all known neutron stars.
The spinning pulsar has a companion star that orbits it once every 26 hours. The companion passes in front of the pulsar, eclipsing the pulsar about 40 percent of the time. The long eclipse period, probably due to bloating of the companion, makes it difficult for the astronomers to learn details of the orbital configuration that would allow them to precisely measure the masses of the pulsar and its companion.

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

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

"We want observational data that shows which models fit the reality of nature" - Jason Hessels.

If the scientists can't use PSR J1748-2446ad to do that, they are hopeful some of its near neighbours will yield the data they seek. Using the GBT, the astronomers so far have found 30 new fast "millisecond pulsars" in the cluster Terzan 5, making 33 pulsars known in the cluster in total. This is the largest number of such pulsars ever found in a single globular cluster.
Dense globular clusters of stars are excellent places to find fast-rotating millisecond pulsars. Giant stars explode as supernovae and leave rotating pulsars which gradually slow down. However, if a pulsar has a companion star from which it can draw material, that incoming material imparts its spin, or angular momentum, to the pulsar. As a result, the pulsar spins faster.

"In a dense cluster, interactions between the stars will create more binary pairs that can yield more fast-rotating pulsars" - Scott Ransom.

The great sensitivity of the giant, 100-meter diameter GBT, along with a special signal processor, called the Pulsar Spigot, made possible the discovery of so many millisecond pulsars in Terzan 5.

"We think there are many more pulsars to be found in Terzan 5 and other clusters, and given that the fast ones are often hidden by eclipses, some of them may be spinning even faster than this new one. We're excited about using this outstanding new telescope to answer some important questions about fundamental physics" - Scott Ransom.

In addition to Hessels, Ransom and Stairs, the research team includes Paulo Freire of Arecibo Observatory in Puerto Rico, Victoria Kaspi, of McGill University, and Fernando Camilo, of Columbia 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 the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc. The pulsar research also was supported by the Canada Foundation for Innovation, the Natural Sciences and Engineering Research Council of 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..

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