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/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 σ level, unresolved (FWHM ≤ 800 km s -1 at a 2 σ confidence level) O VII Kα and Ne IX Kα resonant absorption lines at 21.603 and 13.448 Å (i.e., cz = 14 km s -1 in the rest frame, from the O VII Kα line). O VIII Kα and O VII Kβ from the same system are also detected at a lower significance level (i.e., ~3 σ), while upper limits are set on O VIII Kβ, Ne X Kα, and Ne IX Kβ. The Far Ultraviolet Spectroscopic Explorer spectrum of this source shows complex O VI 2 s →2 p 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 ~2, with respect to the solar value. We propose several scenarios to account for this observation.