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 theVela pulsar with the Chandra X-ray observatory. The spectrum of the pulsar doesnot show statistically significant spectral lines in the observed 0.25-8.0 keVband. Similar to middle-aged pulsars with detected thermal emission, thespectrum consists of two distinct components. The softer component can bemodeled as a magnetic hydrogen atmosphere spectrum - for the pulsar magneticfield $B=3\times 10^{12}$ G and neutron star mass $M=1.4 M_\odot$ and radius$R^\infty =13$ km, we obtain $\tef^\infty =0.68\pm 0.03$ MK, $L_{\rmbol}^\infty = (2.6\pm 0.2)\times 10^{32}$ erg s$^{-1}$, $d=210\pm 20$ pc (theeffective temperature, bolometric luminosity, and radius are as measured by adistant observer). The effective temperature is lower than that predicted bystandard neutron star cooling models. A standard blackbody fit gives $T^\infty=1.49\pm 0.04$ MK, $L_{\rm bol}^\infty=(1.5\pm 0.4)\times 10^{32} d_{250}^2$erg s$^{-1}$ ($d_{250}$ is the distance in units of 250 pc); the blackbodytemperature corresponds to a radius, $R^\infty =(2.1\pm 0.2) d_{250}$ km, muchsmaller than realistic neutron star radii. The harder component can be modeledas a power-law spectrum, with parameters depending on the model adopted for thesoft component - $\gamma=1.5\pm 0.3$, $L_x=(1.5\pm 0.4)\times 10^{31}d_{250}^2$ erg s$^{-1}$ and $\gamma=2.7\pm 0.4$, $L_x=(4.2\pm 0.6)\times10^{31} d_{250}^2$ erg s$^{-1}$ for the hydrogen atmosphere and blackbody softcomponent, respectively ($\gamma$ is the photon index, $L_x$ is the luminosityin the 0.2--8 keV band). The extrapolation of the power-law component of theformer fit towards lower energies matches the optical flux at $\gamma\simeq1.35$--1.45.