Neutron stars warp space-time

Astronomers have pioneered a technique for determining the properties of ultradense objects.

Provided by the Goddard Space Flight Center

August 28, 2007

Using European and Japanese/NASA X-ray satellites, astronomers have seen Einstein's predicted distortion of space-time around three neutron stars, and in doing so they have pioneered a groundbreaking technique for determining the properties of these ultradense objects. Neutron stars contain the densest observable matter in the universe. They cram more than a sun's worth of material into a city-sized sphere, meaning a few cups of neutron-star stuff would outweigh Mount Everest. Astronomers use these collapsed stars as natural laboratories to study how tightly matter can be crammed under the most extreme pressures that nature can offer."This is fundamental physics," says Sudip Bhattacharyya of NASA's Goddard Space Flight Center in Greenbelt, Md. and the University of Maryland, College Park. "There could be exotic kinds of particles or states of matter, such as quark matter, in the centers of neutron stars, but it's impossible to create them in the lab. The only way to find out is to understand neutron stars. "To address this mystery, scientists must accurately and precisely measure the diameters and masses of neutron stars.

In two concurrent studies, one with the European Space Agency's XMM-Newton X-ray Observatory and the other with the Japanese/NASA Suzaku X-ray observatory, astronomers have taken a big step forward.Using XMM-Newton, Bhattacharyya and his NASA Goddard colleague Tod Strohmayer observed a binary system known as Serpens X-1, which contains a neutron star and a stellar companion. They studied a spectral line from hot iron atoms that are whirling around in a disk just beyond the neutron star's surface at 40 percent the speed of light.Previous X-ray observatories detected iron lines around neutron stars, but they lacked the sensitivity to measure the shapes of the lines in detail. Thanks to XMM-Newton's large mirrors, Bhattacharyya and Strohmayer found that the iron line is broadened asymmetrically by the gas's extreme velocity, which smears and distorts the line because of the Doppler Effect and beaming effects predicted by Einstein's special theory of relativity. The warping of space-time by the neutron star's powerful gravity, an effect of Einstein's general theory of relativity, shifts the neutron star's iron line to longer wavelengths. "We've seen these asymmetric lines from many black holes, but this is the first confirmation that neutron stars can produce them as well.

It shows that the way neutron stars accrete matter is not very different from that of black holes, and it gives us a new tool to probe Einstein's theory," says Strohmayer. A group led by Edward Cackett and Jon Miller of the University of Michigan, which includes Bhattacharyya and Strohmayer, used Suzaku's superb spectral capabilities to survey three neutron-star binaries:Serpens X-1, GX 349+2, and 4U 1820-30. This team observed a nearly identical iron line in Serpens X-1, confirming the XMM-Newton result. It detected similarly skewed iron lines in the other two systems as well."We're seeing the gas whipping around just outside the neutron star's surface," says Cackett. "And since the inner part of the disk obviously can't orbit any closer than the neutron star's surface, these measurements give us a maximum size of the neutron star's diameter. The neutron stars can be no larger than 18 to 20.5 miles across, results that agree with other types of measurements. Now that we've seen this relativistic iron line around three neutron stars, we have established a new technique", adds Miller. "It's very difficult to measure the mass and diameter of a neutron star, so we need several techniques to work together to achieve that goal. "Knowing a neutron star's size and mass allows physicists to describe the "stiffness," or "equation of state," of matter packed inside these incredibly dense objects. Besides using these iron lines to test Einstein's general theory of relativity, astronomers can probe conditions in the inner part of a neutron star's accretion disk.The XMM-Newton paper appeared in the August 1 Astrophysical Journal Letters. The Suzaku paper has been submitted for publication in the same journal.

Acknowledgements: Astronomy newsletter

srini

Neutron star - the Earth's closest neighbour

Calvera - the eighth neutron star discovery

Using NASA's Swift satellite, McGill University and Penn State University astronomers have identified an object that is possibly the closest neutron star to Earth.The object, located in the constellation Ursa Minor, is nicknamed 'Calvera,' after the villain in the movie "The Magnificent Seven." If confirmed, it would be only the eighth known isolated neutron star (a neutron star not associated with a supernova remnant, a binary companion, or radio pulsations). "The seven previously known isolated neutron stars are known collectively as 'The Magnificent Seven' within the community, and so the name Calvera is a bit of an inside joke on our part," says co-discoverer Derek Fox of Penn State.

Robert Rutledge of McGill University in Montreal, Quebec, originally called attention to the source. He compared a catalogue of 18,000 X-ray sources from the German-American ROSAT satellite, which operated from 1990 to 1999, with catalogues of objects that appear in visible light, infrared light, and radio waves. He realized that the ROSAT source 1RXS J141256.0+792204 did not appear to have a counterpart at any other wave length.

The group aimed Swift at the object in August 2006. Swift's X-ray Telescope showed that the source was still there, and emitting about the same amount of X-ray energy as it had during the ROSAT era. The Swift observations enabled the group to pinpoint the object's position more accurately, and showed that it was not associated with any known object."The Swift observation of this source is what got the show going," says Penn State undergraduate Andrew Shevchuk. "As soon as I saw the data, I knew Calvera was a great neutron-star candidate”.

The team next targeted Calvera with the 8.1-meter Gemini North Telescope in Hawaii. These observations, along with a short observation by NASA's Chandra X-ray Observatory, showed that the object is not associated with any optical counterpart down to a very faint magnitude. Chandra's sharper X-ray vision sees the object as point-like, consistent with the neutron-star interpretation.

According to Rutledge, there are no widely accepted alternate theories for objects bright in X-rays and faint in visible light, like Calvera. Exactly which type of neutron star it is, however, remains a mystery. As Rutledge says, "Either Calvera is an unusual example of a known type of neutron star, or it is some new type of neutron star, the first of its kind."Calvera's location, high above the plane of our Milky Way Galaxy, is part of its mystery. In all likelihood, the neutron star is the remnant of a star that lived in our galaxy's starry disk before exploding as a supernova. In order to reach its current position, it had to wander some distance out of the disk. But exactly how far? "The best guess is that it is still close to its birthplace, and therefore close to Earth," says Rutledge. If this interpretation is correct, the object is 250 to 1,000 light-years away. This would make Calvera one of the closest known neutron stars — possibly the closest. "Because it is so bright, and probably close to Earth, it is a promising target for many types of observations," says Fox. Indeed, to clear up the mysteries surrounding Calvera, the team will be taking a longer observation with Chandra to see if the source pulsates in X-rays, and to measure its spectrum. They also joined a group using a radio telescope to search for radio

pulsations, which were not seen.

Calvera could represent the tip of the iceberg for isolated neutron stars. "There could easily be dozens," says Fox. "The key point is that until our Swift survey, no one was able to refine the X-ray positions of large numbers of ROSAT sources to the point where it became clear which ROSAT sources were 'missing' their optical counterparts."

source: Astronomy newsletter to me.

srini

Getting denser!

For gold, r(critical) becomes 0.7*10**9km (a shining blackhole!); for platinum, it is 0.55*10**9km.
Now going a bit celestial, the density of a neutron star is known to be in the range of 0.5*10**15gm/cc as they are supposed to contain one solar mass per ball of 20km diameter!
Still one takes the core at which gravitation collapses to a steady state, it is about 10**12 kg/cc or 10**15 gm/cc, said to be close to the density of a typical nucleus! i.e double that of a neutron star. Compare it with that of the neutron I had estimated to be! 5*10**136 gm/cc (at least).

srini