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

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_0.64-32.2 ≤ 6×10^-17 erg cm^{-2 s^{-1}}, 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 α_{ox ≤ -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 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 ≥ 3.4×10²⁵ cm^-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 ≥ 2× 10⁸}, 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 (≃ 1 arcmin) X-ray source is detected ∼ 2 arcmin to the south-east of the QSO. Given its thermal spectrum and temperature (1.5 ≤ T ≤ 3.0 keV), it is probably a foreground (0.29 ≤ z ≤ 0.46) cluster of galaxies.