2018A&A...617A.118S

We present the analysis of the restframe optical-to-UV spectrum of APM 08279+5255, a well-known lensed broad absorption line (BAL) quasar at z=3.911. The spectroscopic data were taken with the optical DOLoRes and near-IR NICS instruments at TNG, and include the previously unexplored range between CIII] λ1910 and [OIII] λλ4959,5007. We have investigated the possible presence of multiple BALs by computing "balnicity" and absorption indexes (i.e., BI, BI₀, and AI) for the transitions SiIVλ1400, CIVλ1549, AlIIIλ1860, and MgIIλ2800. No clear evidence for the presence of absorption features is found in addition to the already known, prominent BAL associated to CIV, which supports a high-ionization BAL classification for APM 08279+5255. We also studied the properties of the [OIII], Hβ, and MgII emission lines. We find that [OIII] is intrinsically weak (F_[OIII]/F_Hβ≤0.04), as it is typically found in luminous quasars with a strongly blueshifted CIV emission line (∼2500km/s for APM 08279+5255). We computed the single-epoch black hole mass based on MgII and Hβ broad emission lines, finding M_BH=(2/3)x10¹⁰µ^–1M_☉, with the magnification factor µ that can vary between 4 and 100 according to CO and restframe UV-to-mid-IR imaging respectively. Using a MgII equivalent width (EW)-to-Eddington ratio relation, the EW_MgII∼27Å measured for APM 08279+5255 translates into an Eddington ratio of ∼0.4, which is more consistent with µ=4. This magnification factor also provides a value of M_BH that is consistent with recent reverberation-mapping measurements derived from CIV and SiIV.