NGC 3783 was observed for approximately 210ks by Suzaku and in this time showed significant spectral and flux variability at both short (20ks) and long (100ks) time scales. The full observation is found to consist of approximately six "spectral periods" where the behaviour of the soft (0.3-1.0keV) and hard (2-10keV) bands are somewhat distinct. Using a variety of methods we find that the strong warm absorber present in this source does not change on these time scales, confirming that the broad-band variability is intrinsic to the central source. The time resolved difference-spectra are well modelled with an absorbed powerlaw below 10keV, but show an additional hard excess at ~20keV in the latter stages of the observation. This suggests that, in addition to the variable powerlaw, there is a further variable component that varies with time but not monotonically with flux. We show that a likely interpretation is that this further component is associated with variations in the reflection fraction or possibly ionization state of the accretion disk a few gravitational radii from the black hole.
Active Galactic Nuclei (AGN) are commonly found to display persistent X-ray spectral and flux variability in a variety of time-scales (e.g. McHardy 1989, Green et al. 1993). Studies of this broadband variability usually suggest that at least part is due to strong gravitational effects close to the central black hole. Time-resolved spectral analyses of such sources are thus an important tool in the study of the inner regions of the accretion flow around supermassive black holes, providing information on both the geometry and physical evolution of the inner accretion disk and on intrinsic physical parameters such as black hole mass and spin.
It has been shown for a number of Seyfert galaxies, most notably MCG-6-30-15 (see e.g. Fabian & Vaughan 2003), that the spectrum above a few keV becomes harder at lower X-ray fluxes. At all flux levels, however, the X-ray spectrum of such systems usually consists of a primary continuum which is accurately approximated as a powerlaw, together with a broad Fe-Kα line and cold reflection features (Nandra et al. 1997). In some cases it is also accompanied by highly ionized warm absorbers and a “soft excess” (Halpern 1984;Reynolds 1997). The most common interpretation for the spectra of such AGN is that the primary X-ray emission irradiates the underlying optically thick material in the innermost part of the accretion disk, resulting in “reflection signatures” consisting of fluorescent and recombination emission lines as well as absorption features. The most prominent signature is the broad, skewed Fe-Kα line (see e.g. Miller 2007 for a recent review of relativistically broadened lines). Furthermore, the soft excess is a natural consequence of reflection from the innermost regions around a black hole for a wide range of ionisation states.
Recent spectral variability studies of Seyfert galaxies such as MCG-6-30-15 (Miniutti et al. 2007), 1H0707-495 (Fabian et al. 2009;Zoghbi et al. 2010) and NGC 4051 (Uttley et al. 2004;Terashima et al. 2008) have found different explanations for the origin of the variability -sometimes in the same object. Amongst the possibilities are (i) the spectrum consists of a soft component (powerlaw) with constant spectral slope and variable flux, together with a constant hardcomponent associated with strong reflection from the innermost accretion disk (e.g MCG-6-30-15; Vaughan et al. 2003;Taylor et al. 2003;Miniutti et al. 2007); (ii) the soft component consists of a pivoting powerlaw with the photon index increasing with the logarithm of the flux, together with a constant reflection component (e.g. NGC 4051; Uttley et al. 2004); and (iii) a pivoting powerlaw together with a constant, partially-covered, neutral (cold) reflection (e.g. NGC 4051; Terashima et al. 2008). Suzaku provides an ideal opportunity to discern between these various distinct interpretations due to its broad-band spectral coverage and good signal-to-noise in the full 0.5-50 keV range as well as large effective area below ∼ 10 keV.
NGC 3783 is a bright, nearby Seyfert 1 galaxy exhibiting prominent broad emission lines and strong X-ray absorption features. Based on a total of approximately 900 ks of Chandra observation, various authors concluded that this source contains three zones of ionized absorption covering a large range of ionization (Kaspi et al. 2002;Netzer et al. 2003;Krongold et al. 2003Krongold et al. , 2005)). Analyses of the grating data showed little, if any, evidence for changes in the optical depths of the absorbers. Similar conclusions have also been made using XMM-Newton RGS data (Blustin et al. 2002). We observed NGC 3783 for approximately 340 ks with Suzaku as part of the Suzaku AGN Spin Survey Key Project. A detailed analysis of the time averaged spectrum is presented in Brenneman et al. (2011) (hereafter Paper 1). For the first time in NGC 3783, the dimensionless spin parameter for the central black hole was estimated to be > 0.88 at the 99 per cent level of confidence. This was based on the self-consistent modelling of the full reflection features, whilst simultaneously accounting for absorption due to the warm absorber in the line of sight. In this paper we present an examination of the X-ray spectrum and spectral variability of NGC 3783 using this same long Suzaku observation with the goal of better understanding the accretion disk flow geometry and evolution of this system. By usage of a variety of techniques we show that the spectral and flux variation seen in NGC 3783 are likely due to a combination of a varying powerlaw (with constant index) as well as a hard-reflection component. Furthermore, we find that the ionization and reflection fraction in the innermost region of the accretion disk are also likely to be varying.
We start in § 2 with specifics of the observation and data reduction procedure. This is then followed by a detailed analysis of the spectral variability ( § 3) using a combination of flux-flux relations ( § 3.2), flux-resolved ( § 3.3)
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