The X-Ray Spectrum of the Vela Pulsar Resolved with the [ITAL]Chandra X-Ray Observatory[/ITAL]

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 \^\∞ =0.68\± 0.03 MK, L\ bol^\∞ = (2.6\± 0.2)\× 10³² erg s⁻¹, 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)\× 10³² d₂₅₀² erg 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, Lₓ=(1.5\± 0.4)\× 10³¹ d₂₅₀² erg s⁻¹ and \γ=2.7\± 0.4, Lₓ=(4.2\± 0.6)\× 10³¹ d₂₅₀² erg s⁻¹ for the hydrogen atmosphere and blackbody soft component, respectively (\γ is the photon index, Lₓ is the luminosity in the 0.2--8 keV band). The extrapolation of the power-law component of the former fit towards lower energies matches the optical flux at \γ\ 1.35--1.45.