Cosmic Origins Spectrograph Observations of Warm Intervening Gas Towards 3C263

We present HST/COS high S/N observations of the z = 0.32566 multi-phase absorber towards 3C263. The COS data shows absorption from H I, O VI, C III, N III, Si III and C II. The Ne VIII in this absorber is detected in the FUSE spectrum. The low and intermediate ions are kinematically aligned with each other and H I and display narrow line widths of 6 km/s. The O VI lines are kinematically offset by 12 km/s from the low ions and are a factor of four broader. All metal ions except O VI and Ne VIII are consistent with an origin in gas photoionized by the extragalactic background radiation. The bulk of the observed H I is also traced by this photoionized medium. The carbon abundance in this gas phase is near-solar. The O VI and Ne VIII favor an origin in collisionally ionized gas at T = 5.2 x 10^5 K. The H I absorption associated with this warm absorber is a BLA marginally detected in the COS spectrum. This warm gas phase has total hydrogen column density of N(H) ~ 3 x 10^19 which is 2 dex higher than what is traced by the photoionized gas. Simultaneous detection of O VI, Ne VIII and BLAs in an absorber can be a strong diagnostic of gas with temperature in the range of 10^5 - 10^6 K corresponding to the warm phase of the WHIM or shock-heated gas in the extended halos of galaxies.