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 V ela pulsar with the Chandra X-Ray Observatory. The spectrum of the pulsar d oes 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 ca n be modeled as a magnetic hydrogen atmosphere spectrum-for the pulsar magn etic field B = 3 x 10(12) G and neutron star mass M = 1.4 M. and radius R-i nfinity = 13 km, we obtain T-eff(infinity) = 0.68 +/- 0.03 MK, L-bol(infini ty) = (2.6 +/- 0.2) x 10(32) ergs s(-1), and d = 210 +/- 20 pc (the effecti ve temperature, bolometric luminosity, and radius are as measured by a dist ant observer). The effective temperature is lower than that predicted by st andard neutron star cooling models. A standard blackbody tit gives T-infini ty = 1.49 +/- 0.04 MK, L-bol(infinity) = (1.5 +/- 0.4) x 10(32)d(250)(2) er gs s(-1) (d(250) is the distance in units of 250 pc); the blackbody tempera ture corresponds to a radius R-infinity = (2.1 +/- 0.2)d(250) km, much smal ler 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, re spectively (gamma is the photon index; L-X is the luminosity in the 0.2-8 k eV band). The extrapolation of the power-law component of the former fit to ward lower energies matches the optical flux at gamma = 1.35-1.45.