Using the High Resolution X-ray Spectrum of PSR B0656+14 to Constrain the Chemical Composition of the Neutron Star Atmosphere

Observations of PSR B0656+14 using the Chandra Low Energy Transmission Grating Spectrometer are presented. The zeroth order events are pulsed at an amplitude of 10 ± 2% and the image may be slightly extended. The extended emission is modelled as a Gaussian with a FWHM of about 0.75, for a linear size (at a distance of 760 pc) of 8.5 ×10 cm. In the absence of systematic errors in the detector point spread function, the extended emission comprises . 50% of the observed flux in the 0.2-2.0 keV band, for a luminosity of . 3× 10 erg s. The spectrum is well modelled by a dominant blackbody with T = 8.0±0.3×10 K and a size of 22.5 ± 2.1 km in addition to a harder component that is modelled as a hotter and much smaller blackbody. No significant absorption features are found in the spectrum that might be expected from ionization edges of H or He or bound-bound transitions of Fe in magnetized atmospheres. Such features are expected to be deep but could vary in position or strength with rotation phase. There are no strong absorption features in the pulse-phased spectra, however, so we conclude that the atmosphere is not dominated by Fe or other heavy elements that would be partially ionized at a temperature of 10K. Subject headings: stars:neutron — pulsars: individual (PSR B0656+14)