We present the results of a multi-epoch observational campaign on the mini-broad absorption line quasar (mini-BAL QSO) PG 1126-041 performed with XMM-Newton from 2004 to 2009. Time-resolved X-ray spectroscopy and simultaneous UV and X-ray photometry were performed on the most complete set of observations and on the deepest X-ray exposure of a mini-BAL QSO to date. Complex X-ray spectral variability, found on time scales of both months and hours, is best reproduced by means of variable and massive ionized absorbers along the line of sight. In the highest signal-to-noise observation, highly-ionized X-ray absorbing material outflowing much faster than the UV absorbing one is detected. This highly-ionized absorber is found to be variable on very short time scales of a few hours.
Accretion disk winds are among the most promising physical mechanisms able to link the small and the large-scale phenomena in active galactic nuclei (AGN), to shed light on the physics of accretion/ejection around supermassive black holes (SMBHs), and to help understanding the impact of the AGN phase on the host galaxy evolution (e.g. Silk & Rees 1998;Proga & Kallman 2004;Di Matteo et al. 2005;Booth & Schaye 2009).
Such winds are currently directly observed, as blueshifted and broadened absorption lines in the UV and X-ray spectra of a substantial fraction of AGN. In the UV band we observe broad absorption lines (BALs, FWHM > 2000 km s -1 ), mini-broad absorption lines (mini-BALs, 500 km s -1 < FWHM < 2000 km s -1 ), and narrow absorption lines (NALs, FWHM < 500 km s -1 ) in about 30% of AGN, and their intrinsic fraction is estimated to be as high as 50% (Knigge et al. 2008;Ganguly & Brotherton 2008;Allen et al. 2011). These absorbers can be outflowing with speeds as high as ∼ 0.2c. In the X-ray band, we observe lower-velocity (100-1000 km s -1 ) ionized gas (the so-called “warm absorber”) in 50% of AGN (Piconcelli et al. 2005;McKernan et al. 2007). Xray high-velocity absorbers outflowing with speeds up to ∼ 0.3c (called ultra-fast outflows, UFOs) are observed in about 30-40% of local AGN (Tombesi et al. 2010). X-ray BALs, outflowing with speeds up to 0.7c, have been also observed in a handful of AGN (Chartas et al. 2002(Chartas et al. , 2009;;Lanzuisi et al. 2011). These observational results indicate that outflowing matter is a key ingredient of the inner regions of AGN, and that understanding the launching and structure of these winds is fundamental for constructing complete models of the central engine.
Here we present the main results of a multi-epoch XMM-Newton observational campaign on PG 1126-041, a low-redshift (z = 0.06) AGN that shows UV mini-BALs blueshifted by 2000-5000 km s -1 in the C IV, N V, and Si IV species (Wang et al. 1999). An early ROSAT observation suggested the presence of ionized absorption as well as possible short-term variability (Wang et al. 1999;Komossa & Meerschweinchen 2000). The new XMM-Newton campaign provided the largest dataset (a total of four pointings: one archival in 2004, two in 2008, one in 2009) and the deepest X-ray exposure (133 ks in the 2009 observation) ever on a mini-BAL QSO. Our analysis confirmed the presence of massive ionized absorbers along the line of sight, and revealed complex spectral variability on time scales of both months and hours. We focus here on the variability of the X-ray absorbers; details on the X-ray analysis can be found in Giustini et al. (2011).
The mini-BAL QSO PG 1126-041 shows complex and variable X-ray spectral properties. The average 0.2-10 keV spectra as seen by the EPIC-pn instrument aboard XMM-Newton during the four pointed observations are shown in the left panel of Fig. 1: most of the spectral variability on time scales of months occurs at E < 6 keV. The long June 2009 observation (92 ks of contiguous net exposure time) has been split in four consecutive time intervals of 21, 30, 24, and 17 ks, where flux variations were evident. Spectra extracted from each time slice are plotted in the right panel of Fig. 1: strong spectral variability on time scales of hours affects only the E > 1.5 keV part of the spectrum. It is evident that two distinct components are affecting the appearance and the variations of the X-ray spectrum of PG 1126-041. In all epochs, the spectral shape is peculiar and obviously deviates from a simple power-law. A broad absorption feature is evident in all the four observations at E ∼ 0.6 -1.5 keV, i.e. the energy range where X-ray warm absorbers mostly affect the spectral shape of AGN. Indeed a high-column density, moderately-ionized absorber is detected in every observation of PG 1126-041. The absorber column density ranges from a minimum of N m.i. = 3.2 +0.7 -0.4 ×10 22 cm -2 (Dec. 2008 observation), to a maximum of N m.i. = 1.5 ± 0.2 × 10 23 cm -2 (June 2009 observation; errors at 3σ confidence level). Given the low spectral resolution of the EPIC-pn instrument, the gas ionization state is poorly constrained: log ξ m.i. = 1.55 ± 0.15 erg cm s -1 including the systematic uncertainties of the model parameters, therefore its possible variations on time scales of months can not be tracked. As a consequence of the X-ray absorption variability, the observed optical-to-X-ray spectral index is highly variable, going from α ox = -1.7 in the Dec. 2008 observation, to α ox = -2 in the June 2009 observation.
A highly-ionized absorber is clearly detected in the iron K band of PG 1126-041 during the long June 2009 observation. In particular, two deep absorption features at rest frame energy E ∼ 7 and ∼ 7.4 keV are identified with Fe XXV Heα and Fe XXVI Lyα transitions, blueshifted by 0.055c (see Fig. 4 of Giustini et al. 2011). The bestfit parameters for the highly-ionized outflowing absorber detected in the aver
This content is AI-processed based on open access ArXiv data.