The X-ray spectrum of the vela pulsar resolved with the Chandra X-ray Observatory

We report the results of the spectral analysis of two observations of the Vela pulsar with the Chandra X-Ray Observatory. The spectrum of the pulsar does not show statistically significant spectral lines in the observed 0.25-8.0 keV band. Similar to middle-aged pulsars with detected thermal emission, the spectrum consists of two distinct components. The softer component can be modeled as a magnetic hydrogen atmosphere spectrum-for the pulsar magnetic field B = 3 x 10(12) G and neutron star mass M = 1.4 M. and radius R-infinity = 13 km, we obtain T-eff(infinity) = 0.68 +/- 0.03 MK, L-bol(infinity) = (2.6 +/- 0.2) x 10(32) ergs s(-1), and d = 210 +/- 20 pc (the effective temperature, bolometric luminosity, and radius are as measured by a distant observer). The effective temperature is lower than that predicted by standard neutron star cooling models. A standard blackbody tit gives T-infinity = 1.49 +/- 0.04 MK, L-bol(infinity) = (1.5 +/- 0.4) x 10(32)d(250)(2) ergs s(-1) (d(250) is the distance in units of 250 pc); the blackbody temperature corresponds to a radius R-infinity = (2.1 +/- 0.2)d(250) km, much smaller than realistic neutron star radii. The harder component can be modeled as a power-law spectrum, with parameters depending on the model adopted for the soft component: gamma = 1.5 +/- 0.3, L-X = (1.5 +/- 0.4) x 10(31)d(250)(2) ergs s(-1) and gamma = 2.7 +/- 0.4, L-X = (4.2 +/- 0.6) x 10(31)d(250)(2) ergs s(-1) for the hydrogen atmosphere and blackbody soft component, respectively (gamma is the photon index; L-X is the luminosity in the 0.2-8 keV band). The extrapolation of the power-law component of the former fit toward lower energies matches the optical flux at gamma = 1.35-1.45.