A multi-epoch study of the C IV absorption variability in the broad absorption line quasar APM 08279+5255 F.G. Saturni a, * , D. Trevese a , F. Vagnetti b , M. Perna a a Dipartimento di Fisica, Universita` di Roma “La Sapienza”, p.le A. Moro 5, I-00185 Rome, Italy b Dipartimento di Fisica, Universita` di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, I-00133 Rome, Italy Abstract Broad absorption line (BAL) variability potentially represents a powerful tool to investigate the physical nature and the structure of gas outflows in active galactic nuclei. Most existing BAL variability studies rely on observations taken at a few epochs for samples of tens of BAL QSOs. In this study we present the first “monitoring” of a single object, APM 08279+5255, which has been observed more than 20 times since 2003. All available spectra from the literature have also been analysed, including two high resolution spectra, extending the time interval from 1998 to 2012. A relative stability of the shape of the absorption profile is found. At the same time significant variations of the equivalent width are observed. A correlation of the BAL equivalent width with the QSO luminosity is found for the first time. These results suggest that changes in the ionisation state of the gas are causing opacity changes. Ó 2013 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Galaxies: active; Quasars: general; Quasars: absorption lines; Quasars: individual: APM 08279+5255 1. Introduction Broad absorption line (BAL) quasars are objects that show broad, deep troughs in their UV and optical spectra (velocity spreads over 2000 km/s), arising from line absorp- tion in gas outflowing with velocities of up to ’ 0:1c (for a complete introductory review on BAL quasars, see e.g. Weymann and Foltz, 1983). This kind of troughs is present in about 20% of all quasars, and the most prominent BALs correspond to the C IV k1549 line (Gibson et al., 2008). Since their discovery, two main mechanisms to explain the BAL origin in quasars have been considered: (i) BAL quasars are normal objects viewed along a particular line of sight, with absorption arising from an accretion disk wind (Elvis, 2000); (ii) BAL quasars are youthful objects, still surrounded by gas and dust in which the absorption takes place (Farrah et al., 2007). BAL outflows have a complex structure. The covering factor of the absorbers can vary strongly across the absorp- tion trough as a function of the outflow velocity. As a result, the absorption is often saturated although the resid- ual flux is not zero. Moreover, spectropolarimetric studies have found that the emission at the bottom of deep troughs is highly polarized and likely scattered around the absorber (Gibson et al., 2008). Therefore BAL variability potentially represents a pow- erful tool to investigate the physical nature and the struc- ture of gas outflows in active galactic nuclei (AGNs), that can, in turn, provide informations on the AGN feedback on the evolution of the host galaxy. Most existing BAL variability studies rely, however, on observations taken at few epochs for samples of tens of BAL QSOs (see e.g. Capellupo et al., 2011). In this study we focus our attention on the first spectro-photometric “monitoring” of a single object, the BAL quasar APM 08279+5255, which has been observed with the 1.8 m Copernico telescope at the Asiago Observatory (Italy) more than 20 times since 2003. Collect- ing also all the available spectra from the literature, we 0273-1177/$36.00 Ó 2013 COSPAR. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.asr.2013.11.007 * Corresponding author. Tel.: +39 3485879740. E-mail addresses: saturnfg@roma1.infn.it, francesco.gabriele.satur- ni@uniroma1.it (F.G. Saturni). www.elsevier.com/locate/asr Available online at www.sciencedirect.com ScienceDirect Advances in Space Research xxx (2013) xxx–xxx Please cite this article in press as: Saturni, F.G., et al. A multi-epoch study of the C IV absorption variability in the broad absorption line quasar APM 08279+5255. J. Adv. Space Res. (2013), http://dx.doi.org/10.1016/j.asr.2013.11.007