NRI astronomer observes Einsteins time-space warp around neutron stars

An Indian origin astronomer has pioneered a groundbreaking technique for determining the properties of neutron stars.

Paris, Aug 28 : An Indian origin astronomer has pioneered a groundbreaking technique for determining the properties of neutron stars.

Using the ESA's XMM -Newton and the NASA/JAXA Suzaku X-ray observatories, Sudip Bhattacharyya and his team observed Einstein's predicted distortion of space time around three neutron stars.

Neutron stars contain the densest observable matter in the Universe. A cup of neutron star material 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. 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," said Sudip Bhattacharyya of NASA's Goddard Space Flight Center, US.

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 whirls around in a disc, 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.

With the 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," said Strohmayer.

Another team of scientists led by Edward Cackett and Jon Miller of the University of Michigan, which also includes Bhattacharyya and Strohmayer, used the NASA/JAXA Suzaku's superb spectral capabilities to survey three neutron-star binaries: Serpens X-1, GX 349+2, and 4U 1820-30.

They 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, said an ESA statement.

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

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

The XMM-Newton study, 'Evidence for a Broad Relativistic Iron Line from the Neutron Star Low Mass X-ray binary Serpens X-1', appears in the August 1 issue of the Astrophysical Journal Letters.

The Suzaku paper 'Relativistic Iron emission lines in neutron star low-mass X-ray binaries as probes of neutron star radii' has been submitted for publication in the same journal.

ANI