Discovery of Absorption Features in the X-Ray Spectrum of an Isolated Neutron Star

Abstract

We observed 1E 1207.4-5209, a neutronstar in the center of the supernova remnant PKS 1209-51/52, with theACIS detector aboard the Chandra X-Ray Observatory and detected twoabsorption features in the source spectrum. The features are centerednear 0.7 and 1.4 keV; their equivalent widths are about 0.1 keV. Wediscuss various possible interpretations of the absorption features andexclude some of them. A likely interpretation is that the features areassociated with atomic transitions of once-ionized helium in theneutron star atmosphere with a strong magnetic field. The first cleardetection of absorption features in the spectrum of an isolated neutronstar provides an opportunity to measure the mass-to-radius ratioand to constrain the equation of state of the superdense matter.