• DOI:10.1086/340489
• Corpus ID: 17387111

Chandra Discovery of a Tree in the X-Ray Forest toward PKS 2155–304: The Local Filament?

@article{Nicastro2002ChandraDO,
  title={Chandra Discovery of a Tree in the X-Ray Forest toward PKS 2155–304: The Local Filament?},
  author={Fabrizio Nicastro and Andreas Zezas and Jeremy J. Drake and Martin Elvis and Fabrizio Fiore and Antonella Fruscione and Massimo Marengo and Smita Mathur and S. Bianchi Harvard-Smithsonian Center for Astrophysics and Cambridge and Usa and Osservatorio Astronomico di Roma and Monteporzio and Italy. and The Ohio State University and Columbus and Universita' di RomaTre and Roma},
  journal={The Astrophysical Journal},
  year={2002},
  volume={573},
  pages={157-167}
}

• F. Nicastro, A. Zezas, +15 authors Roma
• Published 4 January 2002
• Physics
• The Astrophysical Journal

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 A (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 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 ~2, with respect to the solar value. We propose several scenarios to account for this observation.