X-ray spectra of an isolated neutron star
Credit: V. Burwitz, F. Haberl, R. Neuhäuser, P. Predehl, J. Trümper, & V. E. Zavlin, Astronomy & Astrophysics, 2003, vol. 399, pg. 1109

How Compact are Compact Objects?

Neutron stars, those compressed remnants left over after a star explodes, must be extremely small objects. Astronomers realized this decades ago, after the discovery of rapidly spinning pulsars. But how small are they? Astronomers can use the radiation an object emits to determine how large it must be using simple physical laws, if the temperature of the object is known. Young neutron stars are so hot they emit most of their radiation as X-rays, so astronomers need to measure the X-ray spectrum (the brightness of the X-ray emission compared to the energy of the X-rays) to determine the temperature of the neutron star. Unfortunately these objects are so faint that only the largest X-ray observatories can measure their X-ray spectrum, but fortunately astronomers now have at their disposal 2 large X-ray observatories, the Chandra and the XMM-Newton X-ray Observatories. The image above shows X-ray spectra measured by Chandra and by XMM-Newton of an isolated, nearby neutron star called RX J1856.5-3754. The analysis of these spectra suggest that the emission from the star is not entirely uniform, but that there may be a "hot spot" on the neutron star. The analysis suggests that the neutron star is only about 12 kilometers in radius. An earlier analysis of another neutron star found a radius of about 8 to 14 kilometers for that object.

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Page Author: Dr. Michael F. Corcoran
Last modified April 21, 2003