Probing the WHIM through OVII K-alpha Fluctuations
Fabrizio Nicastro (SAO/INAF-OAR) , M. Elvis (SAO), J. Drake (SAO), Y. Krongold (UNAM), S. Mathur (OSU), F. Fiore (INAF-OAR), R. Williams (University of Leiden)
According to hydrodynamical simulations the local Universe is permeated of tenuous warm-hot baryonic matter, distributed in filaments throughout the intergalactic space. These filaments imprint extremely weak high-ionization metal absorption in the X-ray spectra of background sources. The strongest of these transitions is the OVII Kalpha resonant, at lambda=21.6 A. Due to the steepness of the number density distribution of OVII WHIM absorbers with an equivalent width larger than a given threshold, however, less than one OVII Kalpha absorber with individually detectable (with current spectrometers) equivalent width (EW > 10 mA) is expected to be found along a random line of sight up to z=1. Many more numerous of these absorbers, instead, are expected to produce non-resolvable absorption lines, which still, when integrated, give rise to broad and shallow residual opacity. This signature can be searched for in the binned stacked spectra of the brightest blazars in the Chandra archive, up to the minimum common redshift (the redshift of Mkn 421, z=0.03). We have performed this search, and found a 3 ignal which we attribute to unresolved WHIM absorption (but note that Chandra-LETG calibrations are also known to a similar accuracy). This signal is in excellent agreement with theoretical expectations: 45 random lines of sight up to z=0.03 extracted from the latest Cen & Ostriker (2006) simulations, show average WHIM opacity of the order of -1.5