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F. Nicastro, M. Elvis (SAO), T. Fang (Carn. Mell. University), S. Mathur (OSU), A. Siemiginowska, A. Zezas (SAO)
In this contribution we present the brightest high resolution X-ray spectrum ever taken for an extragalactic source. Following our ToO request, Chandra observed the blazar Mkn 421 (z=0.03) during an exceptionally high-luminosity flare. The observation lasted about 100 ks, during which the source reached a flux level of > 0.1 Crab in the 0.5-2 keV band This allowed us to collect 4.2 million counts in the 1st-order ACIS-LETG spectrum of Mkn 421, and more than 3000 counts per resolution elements at the rest frame wavelength of the OVII K\alpha resonant transitions (21.6 Å). A forest of very weak (EW=3.1-10 mÅ) resonant absorption lines is detected from the rest frame wavelength of the OVII K\alpha all the way down to the position of the OVII K\alpha line at the source redshift. We identify these lines as due to: (a) Local Group Warm-Hot Intergalactic Medium (WHIM) absorption, (b) intervening WHIM absorption at redshifts z=0.01 (associated with faint H Ly\alpha absorption) and z=0.025, and (c) intrinsic source absorption. The strongest of these systems is associated with the local WHIM first discovered along the line of sight to PKS 2155-304 (Nicastro et al., 2002, 2003), and now observed (always with consistent gas properties) along all the lines of sight for which Chandra high resolution spectra with sufficient signal to noise ratio are available. The faintest systems (probing OVII column densities as low as 1015 cm-2) are those identified as due to the two intervening WHIM systems. If both these identification are correct this discovery implies a number of intervening OVII WHIM systems per unit redshift of dN/dz(NOVII>1015) = 75, about 3-4 times larger than the corresponding number estimated for OVI systems in the local Universe (down to OVI EW of 60 mÅ). The WHIM baryon fraction implied depends slightly on the ionization correction applied, and ranges between 40 % and 60 % of the total baryons at z<2, so confirming hydrodynamical simulation predictions and accounting for all of the missing baryons in the local Universe.
F.N. aknowledges support by Chandra grants GO2-3122A and GO2-3128X, and by the Chandra CXC contract NAS8-39073.
Bulletin of the American Astronomical Society,
© 2003. The American Astronomical Soceity.