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

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Abstract

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 2155304. The Chandra HRCS/LETG spectrum of this z 0:116 source clearly shows, at 5 level, unre-solved (FWHM 800 km s1 at a 2 confidence level) O viiK and Ne ix K resonant absorption lines at 21:603þ0:0140:024 and 13:448 þ0:022 0:024 A ̊ (i.e., cz 14þ190330 km s1 in the rest frame, from the O vii K line). O viii K and O viiK from the same system are also detected at a lower significance level (i.e.,3 ), while upper lim-its are set on O viiiK, Ne xK, and Ne ixK. 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 s1) and slightly redshifted (cz 36 6 km s1), and one broader (FWHM 158 26 km s1) and blueshifted (cz 135 14 km s1). 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 2, with respect to the solar value. We propose several scenarios to account for this observation. Subject headings:Galaxy: halo — intergalactic medium — quasars: absorption lines — X-rays: ISM 1.

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