27 August 2007

This image series is an 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

Astronomers using XMM-Newton and Suzaku have seen Einstein’s predicted distortion of space-time and pioneered a ground-breaking technique for determining the properties of neutron stars.

ESA’s XMM-Newton and the JAXA/NASA Suzaku X-ray observatories have been used to see the distortion of space-time around three neutron stars. These objects contain the densest observable matter in the Universe.

Neutron stars cram more than a Sun’s worth of material into a city-sized sphere. This means that 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 address this mystery, scientists must accurately and precisely measure the diameters and masses of neutron stars. In two concurrent studies, one with XMM-Newton and the other with Suzaku, 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.

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

This image is an 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

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 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.

An assembly of 51 mirrors, carefully sized, formed and nested one inside another, makes XMM-Newton the most sensitive X-ray telescope ever built. ESA's XMM-Newton derives its name from its X-ray multi-mirror design and honours Sir Isaac Newton. This unique X-ray observatory was launched by Ariane 5 from the European spaceport in French Guiana on 10 December 1999.

Credits: ESA

"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.


Notes:

The paper that deals with results from XMM-Newton titled ‘Evidence for a Broad Relativistic Iron Line from the Neutron Star Low Mass X-ray binary Serpens X-1’, by Bhattacharyya and Strohmayer, appeared in the Astrophysical Journal Letters on 1 August 2007.

The Suzaku paper ‘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 same journal (Astrophysical Journal).

XMM-Newton, ESA’s space-borne X-ray observatory is the biggest scientific satellite ever built in Europe. Its telescope mirrors are the most sensitive ever developed in the world, and with its sensitive detectors, it sees much more than any previous X-ray satellite.

XMM-Newton was designed and built to return data for at least a decade. It has detected more X-ray sources than any previous satellite and is helping solve many cosmic mysteries of the violent Universe, from what happens in and around black holes to the formation of galaxies in the early Universe.

The satellite uses over 170 wafer-thin cylindrical mirrors spread over three telescopes. Its orbit takes it almost a third of the way to the Moon, so that astronomers can enjoy long, uninterrupted views of celestial objects.

Suzaku is the fifth Japanese X-ray astronomy satellite. It was developed at the Institute of Space and Astronautical Science of Japan Aerospace Exploration Agency (ISAS/JAXA) in collaboration with the USA (NASA/Goddard Space Flight Center, MIT Massachusetts Institute of Technology) and Japanese institutions and launched on 10 July 2005, from JAXA Uchinoura Space Centre.


For more information:

Tod Strohmayer, NASA Goddard Space Flight Center, USA
Email: Stroh @ milkyway.gsfc.nasa.gov

Sudip Bhattacharyya, University of Maryland, USA
Email: Sudip @ milkyway.gsfc.nasa.gov

Norbert Schartel, ESA XMM-Newton Project Scientist
Email: Norbert.Schartel @ sciops.esa.int

Source: ESA - News

The University of Michigan press release is reproduced below:

Aug. 27, 2007

ANN ARBOR, Mich.—Einstein's predicted distortion of space-time occurs around neutron stars, University of Michigan astronomers and others have observed. Using European and Japanese/NASA X-ray observatory satellites, teams of researchers 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 nature can offer.

Researchers who study neutron stars are seeking answers to fundamental physics questions. Their centers could hold exotic particles or states of matter that are impossible to create in a lab.

The first step in addressing these mysteries is to accurately and precisely measure the diameters and masses of neutron stars. A U-M study is one of two that have recently done just that.

Like neutron stars themselves, the region around these stars is also extreme. The motions of gas in this environment are described by Einstein's general theory of relativity. Scientists are now exploiting general relativity to study neutron stars.

U-M research fellow Edward Cackett and assistant professor Jon Miller are lead authors of a paper on the research that has been submitted to Astrophysical Journal Letters. Independent work reported by Sudip Bhattacharyya and Tod Strohmayer of NASA's Goddard Space Flight Center bolsters the results reported by Cackett and Miller, and together the results signal that an accessible new method for probing neutron stars has been found.

NASA describes the findings as "a big step forward."

Cackett and Miller used the Japanese/NASA Suzaku X-ray observatory satellite to survey three neutron-star binaries: Serpens X-1, GX 349+2, and 4U 1820-30. The team studied the spectral lines from hot iron atoms that are whirling around in a disk just beyond the neutron stars' surface at 40 percent light speed.

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

Cackett and Miller, along with the Goddard astronomers, were able to determine that the iron line is broadened asymmetrically by the gas's extreme velocity. The line is smeared and distorted 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.

The iron line Cackett and Miller observed in Serpens X-1 was nearly identical to the one Bhattacharyya and Strohmayer observed with a different satellite: the European Space Agency's XMM-Newton. In the other star systems, Cackett and Miller observed similarly-skewed iron lines.

"We're seeing the gas whipping around just outside the neutron star's surface," Cackett said. "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."

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.

"Now that we've seen this relativistic iron line around three neutron stars, we have established a new technique," Miller said. "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."

The paper authored by Cackett and Miller is titled, "Relativistic iron emission lines in neutron star low-mass x-ray binaries as probes of neutron star radii."

Source: U-M press release