| As most luminous objects in the universe, most AGNs are strong X-ray emitters. The X-ray radiation from AGNs is believed from the innermost region of the nuclei, and X-ray observations are efficient and essential to study the stueture and physical properties of AGNs. This thesis focuses on two issues:the broad Fe line of AGNs, and X-ray unobscured type2AGNs.The Fe Ka fluorescent emission line in AGN is believed from the accretion disk by reflecting the illuminating hard power-law X-ray continuum produced by hot corona region above the disk. The lines show asymmetric and complex profile at around6.4keV as a consequence of the relativistic Doppler shift and strong gravitational redshift. B(?)ad Fe line have been detected in many AGNs, and provide an important tracer for probing the physical conditions and geometrical distribution of matter in the vicinity of supermassive black holes. In a few sources the red wings of the Fe Ka emission line appear too broad to be produced from accretion disk surrounding Schwarzschild black holes, and fast rotating Kerr black holes are required, for which the accretion disk could extend further to the horizon and stronger gravitational redshift could be produced. Therefore, the Fe Ka line profile enables astronomers to measure the spin of SMBHs, which is otherwise extremely difficult or even impossible.However, there are strong debates on the nature of broad Fe line, an alternative scenario is that the observed broad and skewed line profile is artificial due to continuum curvature produced by more complex absorption or the partially covered by ionized absorber. These two scenarios could not be statistically distinguished since both provide equall-good fit to the observed spectra. Meanwhile, type2AGNs provide a different insight into the nature of broad Fe Ka emission. While the ionized absorption, which plays the key role in the alternative scenario to the broad Fe Ka line, is common in type1AGNs, X-ray absorption in type2sources is dominated by cold matter. Till now, broad Fe K.a lines were detected only in a few type2AGNs, neither of the Fe Ka line red wing in these type2AGNs is broad enough to demand a rotating SMBF1.We present the detection of an ultra strong and broad Fe Ka line in the XMM-Newton spectra of IRAS00521-7054(EW=1.3±0.3keV in the rest frame), with the red wing extending down to3keV. The line could be b fitted with a Laor diskline model or kerrconv*reflionx model, which yielding a SMBH spin parameter of0.97. This would be the first measurement of SMBH in a type2AGN.Theoretical calculations have shown that reflection from a constant density slab illuminated with a power-law spectrum could produce the Fe Ka line with EW no larger than800cV. The line EW in IRAS00521-7054is1.3keV. Similar large line EWs were only detected in low-flux states of MCG-06-30-15and in1H0707-495. One possible explanation is the light bending effect, that the X-ray continuum source located very close to the central SMBH could yield much weaker observed flux in continuum, but almost constant reflection component.We have also tried alternative models to fit the spectra. A single warm absorber, a partially covering warm absorber, or a partial covering cold absorber model can be easily ruled out because of the much larger reduced X2-A dual cold absorber model still gives a worserx2than the laor line model (△χ2=9), and the best-fit power-law slope appears considerably steep (F=2.8±0.1, greater than that of99.7%of AGNs). We conclude that the broad Fe Ka line is not likely produced by complex absorption, but need follow-up observations with higher spectral resolution and’or larger sensitive energy bandpass to securely rule out the alternative model.In the unification model, type1and type2AGNs are believed to be intrinsically the same but viewed from different orientations and the absence of broad emission lines in type2AGNs is simply due to the obscuration by an optically thick dusty torus from our line of sight. The detection of polarized broad lines and hidden broad line in the near-infrared spectra give convincing evidences supporting the unified model.However, a fraction of type2AGNs were reported to show no or very low X-ray absorption, insufficient to explain the disappearance of broad line region, thus challenging the unification model. However, it’s possibly due to the difficulty in measuring the X-ray obscuration. On one hand, the intrinsic X-ray spectrum of an AGN is usually much more complex than a simple pow-law model, and one should also consider the soft X-ray excess, thermal absorption, partially covered absorption, scattering component, reflecting component, broad Fe Ka line and so on.On the other hand, the X-ray contribution from host galaxies isn’t negligible, even for Chandra and XMM-Ncwton observations. Many unabsorbed type2AGNs are indeed proved to be severely obscured by higher spatial resolution observations. We present a systematic study of the contamination from the X-ray emission in the host galaxy to the measurements of nuclei X-ray obscurations in type2AGNs. The[OIV]25.89μm is believed a good indicator of the intrinsic luminosity of AGNs, while [NeII]/[OIV] can represent the relative intensity of the activity of host galaxies and AGNs. We comapred12X-ray unabsorbcd and29X-ray absorbed type2AGNs, and found the former have significantly higher [NelI]/[OV] ratio, indicating that their X-ray unabsorbed nature are more likely caused by X-ray contamination from the host galaxies. We found no significant difference in [Nell] luminosity between two samples, but lower [O1V] luminosity in unabsorbed ones, indicating that lower luminosity AGNs are more likely polluted by host galaxies. Besides, we propose a new method to select efficiently those type2AGNs contaminated seriously by host galaixes, that is to compare the observed flux in2-10keV band and the value estimated from their observed star formation. |
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