2006A&A...446..439C

Very long (172ks effective exposure time) observations of the BALQSO LBQS 2212-1759 with XMM-Newton yield a stringent upper-limit on its 0.2-10keV (rest-frame 0.64-32.2keV) flux, F_0.64–32.2≤6x10^–17erg/cm²/s, while simultaneous UV and optical observations reveal a rather blue spectrum extending to 650 Å in the source rest frame. These results are used to set a tight upper-limit on its optical to X-ray spectral index α_ox≤-2.56. Given the HI-BAL nature of LBQS 2212-1759, its X-ray weakness is most likely due to intrinsic absorption. If this is the case, and assuming that the intrinsic α_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_H≥3.4x10²⁵cm^–2. Such a large column has a Thomson optical depth to electron scattering τ_Th≥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, τ_Ly≥2x10⁸, conflicting with the source detection below 912Å. 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 (≃1') X-ray source is detected ∼2' to the south-east of the QSO. Given its thermal spectrum and temperature (1.5≤T≤3.0keV), it is probably a foreground (0.29≤z≤0.46) cluster of galaxies.