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Detection of Ne VIII in an Intervening Multi-Phase Absorption System Towards 3C 263

We report on the detection of Ne VIII in an intervening multiphase absorption line system at z=0.32566 in the FUSE spectrum of the quasar 3C 263. The Ne VIII 770 A line detection has a 3.9 sigma significance. At the same velocity we also find absorption lines from C IV, O III, O IV and N IV. The line parameter measurements yield log [N(Ne VIII), cm^-2] =13.98 (+0.10,-0.13) and b = 49.8 +/- 5.5 km/s. We find that the ionization mechanism in the gas phase giving rise to the Ne VIII absorption is inconsistent with photoionization. The absorber has a multi-phase structure, with the intermediate ions produced in cool photoionized gas and the Ne VIII most likely in a warm collisionally ionized medium in the temperature range (0.5 - 1.0) x 10^6 K. This is the second ever detection of an intervening Ne VIII absorption system. Its properties resemble the previous Ne VIII absorber reported by Savage et al. (2005). Direct observations of H I and O VI are needed to better constrain the physical conditions in the collisionally ionized gas phase of this absorber.