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Thermally-broadened Lya absorbers (BLAs) offer an alternate method to usinghighly-ionized metal absorbers (OVI, OVII, etc.) to probe the warm-hotintergalactic medium (WHIM, T=10^5-10^7 K). Until now, WHIM surveys via BLAshave been no less ambiguous than those via far-UV and X-ray metal-ion probes.Detecting these weak, broad features requires background sources with awell-characterized far-UV continuum and data of very high quality. However, arecent HST/COS observation of the z=0.03 blazar Mrk421 allows us to perform ametal-independent search for WHIM gas with unprecedented precision. The datahave high signal-to-noise (S/N~50 per ~20 km/s resolution element) and thesmooth, power-law blazar spectrum allows a fully-parametric continuum model. Weanalyze the Mrk421 sight line for BLA absorbers, particularly for counterpartsto the proposed OVII WHIM systems reported by Nicastro et al. (2005a,b) basedon Chandra/LETG observations. We derive the Lya profiles predicted by the X-rayobservations. The signal-to-noise ratio of the COS data is high (S/N~25 perpixel), but much higher S/N can be obtained by binning the data to widthscharacteristic of the expected BLA profiles. With this technique, we aresensitive to WHIM gas over a large (N_H, T) parameter range in the Mrk421 sightline. We rule out the claimed Nicastro et al. OVII detections at their nominaltemperatures (T~1-2x10^6 K) and metallicities (Z=0.1 Z_sun) at >2 sigma level.However, WHIM gas at higher temperatures and/or higher metallicities isconsistent with our COS non-detections.

An HST/COS Search for Warm-Hot Baryons in the Mrk421 Sightline