摘要：We present the analysis of the restframe optical-to-UV spectrum of APM 08279+5255, a well-known lensed broad absorption line (BAL) quasar atz= 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 C III]λ1910 and [O III]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 Si IVλ1400, C IVλ1549, Al IIIλ1860, and Mg IIλ2800. 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 studied the properties of the [O III], Hβ, and Mg IIemission lines. We find that [O III] is intrinsically weak (F[OIII]∕FHβ≲ 0.04), as it is typically found in luminous quasars with a strongly blueshifted C IVemission line (~2500 km s−1for APM 08279+5255). We computed the single-epoch black hole mass based on Mg IIand Hβbroad emission lines, findingMBH= (2 ÷ 3) × 1010μ−1M⊙, with the magnification factorμthat can vary between 4 and 100 according to CO and restframe UV-to-mid-IR imaging respectively. Using a Mg IIequivalent width (EW)-to-Eddington ratio relation, the EWMgII~ 27 Å measured for APM 08279+5255 translates into an Eddington ratio of ~0.4, which is more consistent withμ= 4. This magnification factor also provides a value ofMBHthat is consistent with recent reverberation-mapping measurements derived from C IVand Si IV.
