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."
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