DETECTION OF Ne viii IN AN INTERVENING MULTI - PHASE ABSORPTION SYSTEM TOWARDS 3 C 263 1

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 (zem = 0.646). The Ne viii λ770 Å detection has a 3.9 σ 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] = 13.98 −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)×10 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 Ovi are needed to better constrain the physical conditions in the collisionally ionized gas phase of this absorber. Subject headings: cosmology: observations – intergalactic medium — quasars: absorption lines – ultraviolet: general – quasars: individual (3C263)