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 TransmissionGrating Spectrometer are presented. The zeroth order events are pulsed at anamplitude of 10 +/- 2% and the image may be slightly extended. The extendedemission is modelled as a Gaussian with a FWHM of about 0.75", for a linearsize (at a distance of 760 pc) of 8.5e15 cm. In the absence of systematicerrors 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 <~3e32 erg/s. The spectrum is well modelled by a dominant blackbody with T =8.0e5 +/- 3e4 K and a size of 22.5 +/- 2.1 km in addition to a harder componentthat is modelled as a hotter and much smaller blackbody. No significantabsorption features are found in the spectrum that might be expected fromionization edges of H or He or bound-bound transitions of Fe in magnetizedatmospheres. Such features are expected to be deep but could vary in positionor strength with rotation phase. There are no strong absorption features in thepulse-phased spectra, however, so we conclude that the atmosphere is notdominated by Fe or other heavy elements that would be partially ionized at atemperature of 1e6 K.Comment: 14 pages, 5 figures, accepted for publication in the Astrophysical Journa