The X-Ray Spectrum of the Vela Pulsar Resolved with the [ITAL]Chandra X-Ray Observatory[/ITAL]
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Abstract
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 $B=3\\times 10^{12}$ 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, $L_{\\rm bol}^\\infty = (2.6\\pm 0.2)\\times 10^{32}$ erg s$^{-1}$, $d=210\\pm 20$ 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 $T^\\infty =1.49\\pm 0.04$ 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 blackbody temperature corresponds to a radius, $R^\\infty =(2.1\\pm 0.2) d_{250}$ 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 - $\\gamma=1.5\\pm 0.3$, $L_x=(1.5\\pm 0.4)\\times 10^{31} d_{250}^2$ erg s$^{-1}$ and $\\gamma=2.7\\pm 0.4$, $L_x=(4.2\\pm 0.6)\\times 10^{31} d_{250}^2$ erg 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 towards lower energies matches the optical flux at $\\gamma\\simeq 1.35$--1.45.