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

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, $F_{0.64-32.2} \leq 6\times10^{-17}~{\rm erg\,cm^{-2}\,s^{-1}}$, while simultaneous UV and optical observations reveal a rather blue spectrum extending to 650 Ain 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 $N_{\rm H} \geq 3.4\times10^{25}\; {\rm cm^{-2}}$. Such a large column has a Thomson optical depth to electron scattering ${\rm \tau_{Th} \geq 23}$, 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, ${\rm \tau_{Ly}\,\geq\,2\times 10^{8}}$, conflicting with the source detection below 912 A. 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.