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

We observed 1E 1207.4-5209, a neutron star in the center of the supernova remnant PKS 1209-51/52, with the ACIS detector aboard the Chandra X-Ray Observatory and detected two absorption features in the source spectrum. The features are centered near 0.7 and 1.4 keV; their equivalent widths are about 0.1 keV. We discuss various possible interpretations of the absorption features and exclude some of them. A likely interpretation is that the features are associated with atomic transitions of once-ionized helium in the neutron star atmosphere with a strong magnetic field. The first clear detection of absorption features in the spectrum of an isolated neutron star provides an opportunity to measure the mass-to-radius ratio and to constrain the equation of state of the superdense matter.