FUSE and STIS Observations of the Warm-Hot Intergalactic Medium towards PG1259+593

We use FUSE and STIS spectra to study intergalactic absorption towards the quasar PG1259+593 (z=0.478). We identify 135 intergalactic absorption lines with equivalent widths >10mA, tracing 78 absorption components in 72 Ly alpha/beta absorption-line systems. We concentrate on the distribution and physical properties of the WHIM as sampled by OVI and intrinsically broad Ly alpha lines. The number of intervening OVI absorbers for equivalent widths W>24 mA is 3-6 over an unobscured redshift path of dz=0.368. This implies a number density of OVI systems, dN/dz, of ~8-16 along this sight line. This range is consistent with estimates from other sight lines, supporting the idea that intervening intergalactic OVI absorbers contain an substantial fraction of the baryonic mass in the low-redshift Universe. We identify a number of broad Ly alpha absorbers with large Doppler parameters (b~40-200 km/s) and low column densities (N(HI)<10^14 cm^-2). For pure thermal broadening, these widths correspond to temperatures of ~1x10^5 to 3x10^6 K. While these broad absorbers could be caused by blends of multiple, unresolved lines, continuum undulations, or by kinematic flows and Hubble broadening, we consider the possibility that some of these features are single-component, thermally broadened Ly alpha lines. These systems could represent WHIM absorbers that are too weak, too metal-poor, and/or too hot to be detected in OVI. If so, their widths and their frequency in the PG1259+593 spectrum imply that these absorbers trace an even larger fraction of the baryons in the low-redshift Universe than the OVI absorbing systems (abridged version).