  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 keV 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 constrain the equation of
state of the superdense matter.

Astrophysics