Einstein's general theory of relativity explained for us that the universe is elastic and gravity distorts space-time like we distort a couch when we sit on it. John Wheeler explained this perfectly when he wrote, "Matter tells space how to curve, and curved space tells matter how to move."

Now 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 cram more than an entire Sun’s worth of material into a sphere the size of a city. A cup of neutron-star stuff would outweigh Mount Everest. Astronomers use these collapsed stars as natural laboratories to study how tightly matter can be compacted under the most extreme pressure that nature can offer.

"This is fundamental physics," says Sudip Bhattacharyya at NASA’s Goddard Space Flight Center, USA. "There could be exotic kinds of particles or states of matter, such as quark matter, in the centres 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 do that, scientists must accurately and precisely measure the diameters and masses of neutron stars. In two concurrent studies, astronomers have taken a big step forward.

Using XMM-Newton, Bhattacharyya and his 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 disc, just beyond the neutron star’s surface, at 40% the speed of light.

Artist's concept of a rare explosion on a neutron star, which is the dead core of a massive star. The neutron star's strong gravity pulls gas from a nearby companion star. The gas forms a disk as it flows on to the neutron star, like water pouring down a drain (red area). Since a neutron star has about the mass of the Sun compressed into a sphere only about 16 km across, it is incredibly dense. This gives the star tremendous gravity, about 300 000 times greater than Earth's surface gravity, which compresses the gas as it builds up on the surface of the neutron star. Eventually, pressure and heat in the gas on the surface becomes so high that the gas detonates in a tremendous nuclear explosion. The explosion distorts and illuminates the gas disk. Credits: NASA/ Dana Berry

Previous X-ray observatories detected iron lines around neutron stars, but they lacked the sensitivity to measure the shapes of the lines in detail.

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 have 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 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.

Artist's concept of a thermonuclear burst consuming an entire neutron star. The neutron star (blue sphere) is part of a binary star system, and its neighbouring star (yellow-red sphere) supplies the fuel for the thermonuclear bursts. During solar outbursts or when the orbit brings the stars closer together, gas from the companion star flows toward the neutron star, attracted by its strong gravity. The flow of gas forms a swirling disk around the neutron star, called an accretion disk (multi-coloured swirl around the blue sphere). Thermonuclear bursts arise as gas moving at close to the speed of light crashes onto the neutron star surface. The gas, pinned to the neutron star by gravity, spreads across the surface. As more and more gas rains down, pressure builds and temperature climbs until there is enough energy for nuclear fusion. This ignites a chain reaction that engulfs the entire neutron star within a second. Bursts last for one to two minutes and can occur several times per hour. Credits: NASA

"We’re seeing the gas whipping around just outside the neutron star’s surface," says Cackett. "And since the inner part of the disc obviously cannot 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 29 to 33 km across, results that agree with other types of measurements."

"Now that we have 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 use them to probe conditions in the inner part of a neutron star’s accretion disc.

"Evidence for a Broad Relativistic Iron Line from the Neutron Star Low Mass X-ray binary Serpens X-1", by Bhattacharyya and Strohmayer,Astrophysical Journal Letters, 1 August 2007.

"Relativistic Iron emission lines in neutron star low-mass X-ray binaries as probes of neutron star radii" by E. Cackett, J. Milleri, S. Bhattacharya, J. Grindlay, J. Homan, M. van der Klis, T. Strohmayer and R. Wijnands has been submitted for publication in the Astrophysical Journal Letters.