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Tentative detection of warm intervening gas towards PKS 0548-322 with XMM–Newton
Authors:X. Barcons  F. B. S. Paerels  F. J. Carrera  M. T. Ceballos   M. Sako
Affiliation:Instituto Física de Cantabria (CSIC-UC), 39005 Santander, Spain;Columbia Astrophysics Laboratory, Columbia University, 538W, 120th Street, New York NY 10027, USA;Stanford Linear Accelerator Centre, 2575 Sand Hill Road M/S 29, Menlo Park, CA 94025, USA
Abstract:We present the results of a long (∼93 ks) XMM–Newton observation of the bright BL-Lac object  PKS 0548-322 ( z = 0.069)  . Our Reflection Grating Spectrometer (RGS) spectrum shows a single absorption feature at an observed wavelength  λ= 23.33 ± 0.01 Å  , which we interpret as O  vi Kα absorption at   z = 0.058  , i.e. ∼3000 km s−1 from the background object. The observed equivalent width of the absorption line, ∼30 mÅ, coupled with the lack of the corresponding absorption edge in the EPIC pn data, implies a column density of   N O VI∼ 2 × 1016 cm−2  and turbulence with a Doppler velocity parameter   b > 100 km s−1  . Within the limitations of our RGS spectrum, no O  vii or O  v Kα absorption are detected. Under the assumption of ionization equilibrium by both collisions and the extragalactic background, this is only marginally consistent if the gas temperature is  ∼2.5 × 105 K  , with significantly lower or higher values being excluded by our limits on O  v or O  vii . If confirmed, this would be the first X-ray detection of a large amount of intervening warm absorbing gas through O  vi absorption. The existence of such a high column density absorber, much stronger than any previously detected one in O  vi , would place stringent constraints on the large-scale distribution of baryonic gas in the Universe.
Keywords:techniques: spectroscopic    galaxies: active    X-rays: galaxies
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