ChandraDiscovery of a Tree in the X‐Ray Forest toward PKS 2155−304: The Local Filament?

We present the first X-ray detection of resonant absorption from warm/hotlocal gas either in our Galaxy, or in the Intergalactic space surrounding ourGalaxy, along the line of sight toward the blazar PKS 2155-304. The ChandraHRCS-LETG spectrum of this z=0.116 source clearly shows unresolved OVII(Ka) andNeIX(Ka) resonant absorption lines at 21.603 A and 13.448 A (i.e. cz =[-340--200] km/s in the rest frame, from the OVII line). OVIII(Ka) and OVII(Kb)from the same system are also detected at a lower significance level, whileupper limits are set on OVIII(Kb), NeX(Ka) and NeIX(Kb). The FUSE spectrum ofthis source shows complex OVI(2s-->2p) absorption at the same redshift as theX-ray system, made by at least two components: one relatively narrow andslightly redshifted, and one broader and blueshifted (cz = -135 km/s). Wedemonstrate that the physical states of the UV and X-ray absorbers are hard toreconcile with a single, purely collisionally ionized, equilibrium plasma. Wepropose, instead, that the X-ray and, at least the broader and blueshifted UVabsorber are produced in a low density intergalactic plasma (partlyphotoionized by the diffuse extragalactic X-ray background), collapsing towardsour Galaxy, consistent with the predictions of a Warm-Hot Intergalactic Medium(WHIM) from numerical simulations. We find that any reasonable solutionrequires overabundance of Ne compared to O by a factor of ~2, with respect tothe solar value and propose several scenarios to account for this observation.Comment: 25 pages, 6 figures, re-submited to ApJ after minor referee's comment