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 × 10¹² G and neutron star mass M = 1.4 M_⊙ and radius R^∞ = 13 km, we obtain T^∞_eff = 0.68 ± 0.03 MK, L^∞_bol = (2.6 ± 0.2) × 10³² 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 fit gives T^∞ = 1.49 ± 0.04 MK, L^∞_bol = (1.5 ± 0.4) × 10³²d²₂₅₀ ergs s^-1 (d₂₅₀ is the distance in units of 250 pc); the blackbody temperature corresponds to a radius R^∞ = (2.1 ± 0.2)d₂₅₀ 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: γ = 1.5 ± 0.3, L_x = (1.5 ± 0.4) × 10³¹d²₂₅₀ ergs s^-1 and γ = 2.7 ± 0.4, L_x = (4.2 ± 0.6) × 10³¹d²₂₅₀ ergs s^-1 for the hydrogen atmosphere and blackbody soft component, respectively (γ 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 γ ≃ 1.35-1.45.