Astronomers have a new way to pin down the size and nature of stellar corpses called neutron stars, which are so dense they may boast exotic forms of matter seen nowhere else in the universe. The method relies on observations of effects predicted by Einstein鈥檚 theories of special and general relativity.
Neutron stars are the collapsed cores of massive stars that exploded as supernovae. They contain about as much matter as the Sun but are only about the size of a city, making them unique laboratories for studying how matter behaves under extreme densities and pressures.
鈥淭here could be exotic kinds of particles or states of matter, such as quark matter, in the centres of neutron stars, but it鈥檚 impossible to create them in the lab. The only way to find out is to understand neutron stars,鈥� says Sudip Bhattacharyya of NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland, US.
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Now, he and colleagues have found a new way of measuring the stars鈥� diameters 鈥� which, along with mass, is the key to understanding how dense 鈥� and exotic 鈥� the objects really are.
They used the European Space Agency鈥檚 XMM-Newton satellite to study a binary star system called Serpens X-1, which lies about 26,000 light years away and contains a neutron star that is sucking material from a stellar companion. The material forms a disc of hot gas around the neutron star, and the researchers made the best-ever spectral measurements of iron atoms in the disc.
The atoms are moving at about 40% the speed of light, fast enough to make an effect called beaming detectable by XMM. Predicted by special relativity, beaming boosts the intensity of light emitted from material that is moving at a high relative speed towards an observer. 鈥淭he portion of the disc that is approaching an observer emits a larger amount of radiation than the part moving away,鈥� team member Tod Strohmayer of Goddard told New 杏吧原创.
Warping space-time
But light from the disc also loses energy 鈥� and is shifted to longer, redder wavelengths 鈥� because it has to work hard to escape the gravitational pull of the dense neutron star. This effect, called gravitational redshift, is predicted by Einstein鈥檚 theory of general relativity, which posits that gravity bends the fabric of space-time.
By clocking the motion of iron atoms in the disc, researchers estimate the width of the disc 鈥� and thus the maximum diameter of the central neutron star 鈥� is about 30 km.
That is in line with previous estimates of the size of neutron stars, which were made by measuring the spectra of isolated neutron stars. Those measurements relied on estimates of the stars鈥� distances and models of how intrinsically bright their atmospheres are 鈥� quantities that are not known with high precision.
The new method thus adds to the arsenal of techniques needed to determine the exact nature of matter inside neutron stars 鈥� a goal that has remained elusive because it relies on accurate measurements of both the radius and the mass of the stars.
鈥淔or the neutron stars where we have good mass measurements, we don鈥檛 have any radius constraints,鈥� says Strohmayer. 鈥淔or objects like this one, where we now have some information on how big the radius can be, we don鈥檛 have an accurate mass measurement, so it鈥檚 kind of frustrating.鈥�
Understanding neutron stars is therefore 鈥渟till a mystery to solve鈥�, he says. 鈥淏ut we鈥檙e working on it.鈥�
