Several observational pieces of evidence confirmed the existence of a relativistically blurred reflection component in AGNs. In particular, X-ray spectroscopy has proven to be a powerful tool to identify the reflection features in AGNs X-ray spectra, allowing us to probe the innermost regions of AGNs (e.g. Fabian et al. 2009). Additionally, this technique may provide estimates of black hole spins (e.g. Risaliti et al. 2013; Marinucci et al. 2014; Walton et al. 2014). This was achieved thanks to the high-quality spectra provided by XMM–Newton (Jansen et al. 2001) and NuSTAR (Harrison et al. 2013) in the 0.3–80 keV range. The first observational X-ray feature associated with relativistic effects was the anomalous shape of the aforementioned iron K α line detected in type-1 AGNs, where the observer has a direct view of the central engine through the polar direction of the system (e.g. Matsuoka et al. 1990; Pounds et al. 1990). The shape of the iron line, with an extended red wing spanning over several keV was soon associated with special and general relativistic effects blurring the signal (e.g. Tanaka et al. 1995; Iwasawa et al. 1996; Nandra et al. 1997). By fitting the observed iron line with relativistic models, it became possible to determine the spin of the black hole (BH), its mass and inclination (see e.g. Miller 2007, for a review). Yet, an alternative interpretation, based on partial covering absorption, has been proposed in order to explain the apparent red wing of the Fe line and the spectral curvature at hard X-rays (e.g. Miller, Turner & Reeves 2008, 2009). The two scenarios have different advantages: on the one hand, blurred reflection is able to explain the spectral and timing properties of accreting systems for a wide range of BH mass. On the other hand, the Compton-thin to Compton-thick (and vice versa) rapid transitions that are observed in “changing-look” AGNs (Matt, Guainazzi & Maiolino 2003) is suggestive that partial covering should be also taken into consideration. In addition, several occultation events, associated with both Compton-thin and Compton-thick clouds in the broad-line region (BLR), have been reported in AGNs (e.g. Risaliti et al. 2007, ; Nardini & Risaliti 2011; Sanfrutos et al. 2013; Torricelli-Ciamponi et al. 2014). Markowitz, Krumpe & Nikutta (2014) estimated the probability to observe an X-ray eclipse (of any duration between 0.2 d and 16 yr) in a given source to be in in the ranges 0.003–0.166 and 0.039–0.571, for type I and type II AGNs, respectively. By studying X-ray eclipses, it might be possible to constrain the importance of partial obscuration, together with the geometry and location of the distant obscuring clouds. This is particularly relevant since obscuration events from BLR clouds do not affect only the AGNs light curves but they show also a strong impact on their spectroscopic and polarimetric properties. Risaliti et al. (2011) investigated the effects of successive eclipses of the receding and approaching parts of the accretion disc on the shape of the iron line. In fact, due to special and general relativistic effects, obscuring various parts of the disc will result in a variability in the profile of the observed emission line, which provides a new probe of the innermost regions of the disc. In a similar fashion, Marin & Dovčiak (2015) explored the effects of such events on the polarimetric signal. The authors showed that eclipses induce a variability in the polarization signal due to the covering of different parts of the disc emitting a non-uniformly polarized light, mainly due to relativistic effects.
Citation: Pounds et al. 1990
Citation function: Background