A HUBBLE SPACE TELESCOPE/COSMIC ORIGINS SPECTROGRAPH SEARCH FOR WARM-HOT BARYONS IN THE Mrk 421 SIGHT LINE

Thermally broadened Ly alpha absorbers (BLAs) offer an alternate method to using highly ionized metal absorbers (O VI, OVII, etc.) to probe the warm-hot intergalactic medium (WHIM, T = 10(5)-10(7) K). Until now, WHIM surveys via BLAs have been no less ambiguous than those via far-UV and X-ray metal-ion probes. Detecting these weak, broad features requires background sources with a well-characterized far-UV continuum and data of very high quality. However, a recent Hubble Space Telescope/Cosmic Origins Spectrograph (COS) observation of the z = 0.03 blazar Mrk 421 allows us to perform a metal-independent search for WHIM gas with unprecedented precision. The data have high signal-to-noise ratio (S/N approximate to 50 per similar to 20 km s(-1) resolution element) and the smooth, power-law blazar spectrum allows a fully parametric continuum model. We analyze the Mrk 421 sight line for BLA absorbers, particularly for counterparts to the proposed OVII WHIM systems reported by Nicastro et al. based on Chandra/Low Energy Transmission Grating observations. We derive the Ly alpha profiles predicted by the X-ray observations. The S/N of the COS data is high (S/N approximate to 25 pixel(-1)), but much higher S/N can be obtained by binning the data to widths characteristic of the expected BLA profiles. With this technique, we are sensitive to WHIM gas over a large (N(H), T) parameter range in the Mrk 421 sight line. We rule out the claimed Nicastro et al. OVII detections at their nominal temperatures (T similar to 1-2 x 10(6) K) and metallicities (Z = 0.1 Z(circle dot)) at greater than or similar to 2 sigma level. However, WHIM gas at higher temperatures and/or higher metallicities is consistent with our COS non-detections.