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2. THE WARM ABSORBER

At least 50% of the low resolution X-ray spectra of type 1 (optically unobscured) AGNs show the presence of a broad absorption feature at ~ 0.7 - 0.8 keV, ascribed to absorption edges of OVIII and OVII due to a "warm", ionized gas along our line of sight [Reynolds (1997), Crenshaw & Kraemer (1999)]. However, the advent of grating X-ray spectroscopy with Chandra and XMM improved dramatically our understanding of warm absorbers. In particular, the recent, high resolution X-ray spectra revealed that the trough at ~ 0.7 - 0.8 keV is actually the blend of various absorption lines and absorption edges. The 900 ks Chandra HETGS spectrum of NGC 3783 is the high resolution spectrum with the highest quality currently available, and it has been extensively used to investigate the properties of the warm absorber [Kaspi et al. (2002), Netzer et al. (2003), Krongold et al. (2003)]. It was found that, at least in the case of NGC 3783, the warm absorber has a column density of the order of a few times 1022 cm-2, it is outflowing with velocities ranging from a few to several 100 km s-1 and it is located between ~ 0.2 pc and ~ 3 pc from the ionizing, nuclear source. According to [Netzer et al. (2003)] and [Krongold et al. (2003)] matching the several absorption lines observed in the spectrum of NGC 3783 requires the presence of two or three different phases of the absorbing medium, at different temperatures and different ionization stage, but in pressure equilibrium. However, [Gonçalves et al. (2006)] pointed out that multi-temperature components arise naturally in a single medium as a consequence of the stratification of the ionization structure of each cloud exposed to the nuclear source. In particular, [Gonçalves et al. (2006)] could fit the various absorption lines observed in NGC 3783 with a single medium, without the need for different, separate components of the absorbing medium.

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