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The Ionized Gas and Nuclear Environment in NGC 3783We present a detailed analysis of the 900 ks spectrum of NGC3783 obtained by Chandra in 2000-2001 (Kaspi et al. 2002). We split the data in various ways to look for time dependent and luminosity dependent spectral variations. This analysis, the measured equivalent widths of a large number of X-ray lines, and our photoionization calculations, lead us to the following conclusions: 1) NGC 3783 fluctuated in luminosity, by a factor N 1.5, during individual 170 ks observations. The fluctuations were not associated with significant spectral variations. 2) On a longer time scale, of 20-120 days, we discovered two very different spectral shapes that are noted the high state and the low state spectra. The observed changes between the two can be described as the appearance and disappearance of a soft continuum component. The spectral variations are not related, in a simple way, to the brightening or the fading of the short wavelength continuum, as observed in other objects. NGC3783 seems to be the first AGN to show this unusual behavior. 3) The appearance of the soft continuum component is consistent with beeing the only spectral variation and there is no need to invoke changes in the absorber s opacity. In particular, all absorption lines with reliable measurements show the same equivalent width, within the observational uncertainties, during high and low states. 4) Photoionization model calculations show that a combination of three ionization components, each split into two kinematic components, explain very well the intensity of almost all absorption lines and the bound-free absorption. The components span a large range of ionization and a total column of about 3 x 10(exp 22) per square centimeter Moreover, all components are thermally stable and are situated on the vertical branch of the stability curve.. This means that they are in pressure equilibrium and perhaps occupy the same volume of space. This is the first detection of such a multi-component equilibrium gas in AGN. 5) The only real discrepancy between the model and the observations is the wavelength of the iron M-shell UTA feature. This is most likely due to an underestimation of the dielectronic recombination O VI and discuss its possible origin. 6) The lower limit on the distance of the absorbing gas in NGC3783 is between 0.2 and 3.2 pc, depending of the specific ionization component. The constant pressure assumption imposes an upper limit of about 25 pc on the distance of the least ionized gas from the central sourec.
Document ID
20030102151
Acquisition Source
Goddard Space Flight Center
Document Type
Preprint (Draft being sent to journal)
Authors
Netzer, Hagai (Tel-Aviv Univ., Ramat-Aviv Tel-Aviv, Israel)
Kaspi, Shai (Tel-Aviv Univ., Ramat-Aviv Tel-Aviv, Israel)
Behar, Ehud (Technion - Israel Inst. of Tech. Haifa, Israel)
Brandt, W. N. (Pennsylvania State Univ. University Park, PA, United States)
Chelouche, Doron (Tel-Aviv Univ., Ramat-Aviv Tel-Aviv, Israel)
George, Ian M. (NASA Goddard Space Flight Center Greenbelt, MD, United States)
Crenshaw, D. Michael (Georgia State Univ. Atlanta, GA, United States)
Gabel, Jack R. (NASA Goddard Space Flight Center Greenbelt, MD, United States)
Hamann, Frederick W. (Florida Univ. Gainesville, FL, United States)
George, Steven B. (NASA Goddard Space Flight Center Greenbelt, MD, United States)
Date Acquired
September 7, 2013
Publication Date
January 1, 2003
Subject Category
Astrophysics
Distribution Limits
Public
Copyright
Public Use Permitted.

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