Chandra discovery 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/hot local gas either in our Galaxy, or in the intergalactic space surrounding our Galaxy, along the line of sight toward the blazar PKS 2155-304. The Chandra HRCS/LETG spectrum of this z = 0 : 116 source clearly shows, at 5 level, unresolved (FWHM less than or equal to 800 km s(-1) at a 2 sigma confidence level) O VII Kalpha and Ne IX Kalpha resonant absorption lines at 21.603(-0.024)(+0.014) and 13.448(-0.024)(+0.022) Angstrom (i.e., cz = 14(-330)(+190) km s(-1) in the rest frame, from the O VII Kalpha line). O VIII Kalpha and O VII K from the same system are also detected at a lower significance level (i.e., similar to3 sigma), while upper limits are set on O VIII Kbeta, Ne x Kalpha, and Ne IX Kbeta. The Far Ultraviolet Spectroscopic Explorer spectrum of this source shows complex O VI 2s-->2p absorption at the same redshift as the X-ray system, made by at least two components: one relatively narrow (FWHM = 106 +/- 9 km s(-1)) and slightly redshifted (cz = 36 +/- 6 km s(-1)), and one broader ( FWHM = 158 +/- 26 km s(-1)) and blueshifted (cz = 135 +/- 14 km s(-1)). We demonstrate that the physical states of the UV and X-ray absorbers are hard to reconcile with a single, purely collisionally ionized, equilibrium plasma. We propose instead that the X-ray and at least the broader and blueshifted UV absorber are produced in a low-density intergalactic plasma, collapsing toward our Galaxy, consistent with the predictions of a warm-hot intergalactic medium from numerical simulations. We find that any reasonable solution requires overabundance of Ne compared to O by a factor of similar to2, with respect to the solar value. We propose several scenarios to account for this observation.