Chandra Phase-resolved X-Ray Spectroscopy of the Crab Pulsar

We present the first phase-resolved study of the X-ray spectral properties of the Crab pulsar that covers all pulse phases. The superb angular resolution of the Chandra X-Ray Observatory enables distinguishing the pulsar from the surrounding nebulosity, even at pulse minimum. Analysis of the pulse-averaged spectrum measures interstellar X-ray extinction due primarily to photoelectric absorption and secondarily to scattering by dust grains in the direction of the Crab Nebula. We confirm previous findings that the line of sight to the Crab is underabundant in oxygen, although more so than recently measured. Using recent abundances and cross sections from Wilms, Allen, and McCray, we find [O/H] = (3.33 ± 0.25) × 10^-4. Analysis of the spectrum as a function of pulse phase measures the low-energy X-ray spectral index even at pulse minimum—albeit with large statistical uncertainty—and we find marginal evidence for variations of the spectral index. The data are also used to set a new (3 σ) upper limit to the temperature of the neutron star of log T_∞ < 6.30.