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Description
We present the analysis of the rest-frame optical-to-UV spectrum of APM 08279+5255, a well-known lensed broad absorption line (BAL) quasar at z=3.911. The spectroscopic data are taken with the optical DOLoRes and near-IR NICS instruments at TNG, and include the previously unexplored range between CIII] {lambda}1910 and [OIII] {lambda}{lambda}4959,5007. We investigate the possible presence of multiple BALs by computing "balnicity" and absorption indexes (i.e. BI, BI_0_ and AI) for the transitions SiIV {lambda}1400, CIV {lambda}1549, AlIII {lambda}1860 and MgII {lambda}2800. No clear evidence for the presence of absorption features is found in addition to the already known, prominent BAL associated to C IV, which supports a high-ionization BAL classification for APM 08279+5255. We also study the properties of the [OIII], H{beta} and MgII emission lines. We find that [OIII] is intrinsically weak (F_[OIII]_/F_H{beta}_<~0.04), as it is typically found in luminous quasars with a strongly blueshifted CIV emission line (2500km/s for APM 08279+5255). We compute the single-epoch black hole mass based on MgII and H{beta} broad emission lines, finding M_BH_=(2/3)*10^10^{mu}^-1^M_{sun}_, with the magnification factor {mu} that can vary between 4 and 100 according to CO and rest-frame UV-to-mid-IR imaging respectively. Using a MgII equivalent width (EW)-to-Eddington ratio relation, the EW_MgII_~27{AA} measured for APM 08279+5255 translates into an Eddington ratio of ~0.4, which is more consistent with {mu}=4. This magnification factor also provides a value of M_BH_ that is consistent with recent reverberation-mapping measurements derived from CIV and SiIV.
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