DETECTION OF Ne VIII IN AN INTERVENING MULTIPHASE ABSORPTION SYSTEM TOWARD 3C 263

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We report the detection of Ne viii in an intervening multiphase absorption line system at z = 0.32566 in the Far Ultraviolet Spectroscopic Explorer spectrum of the quasar 3C 263 (zem = 0.646). The Ne viii λ770 Å detection has a 3.9σ 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−1. 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 multiphase 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) × 106 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 and colleagues. 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.