Astronomy & Astrophysics

Issue		A&A Volume 446, Number 2, February I 2006
Page(s)		439 - 446
Section		Extragalactic astronomy
DOI		10.1051/0004-6361:20041963

A&A 446, 439-446 (2006)
DOI: 10.1051/0004-6361:20041963

Long observations of the BALQSO LBQS 2212-1759 with XMM-Newton

J. Clavel¹, N. Schartel² and L. Tomas²

¹ Research & Scientific Support Department, ESTEC, SCI-SA, Postbus 299 2200 AG - Noordwijk, The Netherlands
e-mail: Jean.Clavel@esa.int
² XMM-Newton Science Operations Centre, ESAC, Apartado 50727, 28080 Madrid, Spain
e-mail: [nscharte;ltomas]@xmm.vilspa.esa.es

(Received 6 September 2004 / Accepted 9 September 2005 )

Abstract
Very long (172 ks effective exposure time) observations of the BALQSO LBQS 2212-1759 with XMM-Newton yield a stringent upper-limit on its 0.2-10 keV (rest- frame 0.64-32.2 keV) flux, $F_ \leq 6\times10^~{\rm erg\,cm^\,s^}$ , while simultaneous UV and optical observations reveal a rather blue spectrum extending to 650 Åin the source rest frame. These results are used to set a tight upper-limit on its optical to X-ray spectral index ${\rm\alpha_{ox} \leq -2.56}$ . Given the HI-BAL nature of LBQS 212-1759 , its X-ray weakness is most likely due to intrinsic absorption. If this is the case, and assuming that the intrinsic ${\rm\alpha_{ox}}$ of LBQS 2212-1759 is -1.63 - a value appropriate for a radio-quiet quasar of this luminosity - one can set a lower limit on the X-ray absorbing column $N_{\rm H} \geq 3.4\times10^\; {\rm cm^}$ . Such a large column has a Thomson optical depth to electron scattering ${\rm\tau_ \geq 23}$ , sufficient to extinguish the optical and UV emission. The contradiction becomes even more acute if the gas is neutral since the opacity in the Lyman continuum becomes extremely large, ${\rm\tau_\,\geq\,2\times 10^$ , conflicting with the source detection below 912 Å. This apparent contradiction probably means that our lines-of-sight to the X-ray and to the UV emitting regions are different, such that the gas completely covers the compact X-ray source but only partially the more extended source of ultraviolet photons. An extended ( ${\simeq} 1\arcmin$ ) X-ray source is detected ${\sim} 2\arcmin$ to the south-east of the QSO. Given its thermal spectrum and temperature ( $1.5\,\leq\,T\,\leq\,3.0~ {\rm keV}$ ), it is probably a foreground ( $0.29 \leq z \leq 0.46$ ) cluster of galaxies.

Key words: quasars: absorption lines -- quasars: individual: LBQS 2212-1759 -- X-rays: individuals: LBQS 2212-1759 -- galaxies: active -- X-rays: galaxies -- ultraviolet: galaxies -- galaxies: active

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