A&A 409, 57-64 (2003)
DOI: 10.1051/0004-6361:20031018

The X-ray absorber of PKS 2126-158

F. Fiore¹, M. Elvis², R. Maiolino³, F. Nicastro², A. Siemiginowska², G. Stratta^{1, 4} and V. D'Elia<sup>1

1</sup>  INAF - Osservatorio Astronomico di Roma, via Frascati 33, Monteporzio-Catone (RM), 00040, Italy
    e-mail: stratta@mporzio.astro.it; delia@mporzio.astro.it
²  Harvard-Smithsonian Center for Astrophysics, Cambridge MA 02138, USA
    e-mail: elvis@head-cfa.harvard.edu; aneta@head-cfa.harvard.edu
³  INAF - Osservatorio Astrofisico di Arcetri, largo E. Fermi, 5. 50125, Firenze, Italy
    e-mail: maiolino@arcetri.astro.it
⁴  Universitá di Roma "La Sapienza", Piazzale Aldo Moro 19050 5, 00185 Roma, Italy

(Received 8 January 2003 / Accepted 16 June 2003 )

Abstract
BeppoSAX observed the z=3.27 quasar PKS 2126-158 on May 24-28 1999 when its 2-10 keV and 0.1-2.5 keV fluxes were and    respectively, a factor of 2 higher than in all previous ROSAT and ASCA observations and 40% higher than in two more recent Chandra and XMM-Newton observations. The shortest detected rest frame variability timescale is of a few months, comparable to the causal timescale associated with an emission region of ~ 10 Schwarzschild radii around a few black hole. The source is detected with a signal to noise ratio  up to ~50 keV, 215 keV rest frame. The BeppoSAX observations confirm the presence of low energy absorption along the line of sight, independent of the continuum model adopted, at a high confidence level. Despite the limited spectral resolution of the BeppoSAX LECS and MECS it is possible to put constraints on different absorption and continuum models, but not to unambiguously determine the redshift of the absorber. If the absorber is not significantly ionized the BeppoSAX data do prefer an absorber at  . Strong and complex metal line systems along the line of sight to PKS 2126-158 have been found at  z=0.6631 and at  2.64<z<2.82. They could well be associated with the X-ray absorption. Conversely, an ionized ("warm") absorber at the quasar redshift provides a good fit only if the iron abundance is smaller than ~ 0.3 solar, while that of the other elements is fixed to the solar value. Lower iron metallicity would imply a lower dust to gas ratio, since iron aggregates easily in dust. This can help in solving the apparent paradox of the lack of significant ultraviolet reddening in this source while strong absorption is detected in X-rays. Low iron abundance would be at odds with the supersolar abundances derived from the broad emission lines.

Key words: quasars: general -- X-rays: galaxies

Offprint request: F. Fiore, fiore@mporzio.astro.it

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