"Warm-absorbers" in Seyfert 1 Galaxies

Many Active Galactic Nuclei (AGN) are believed to contain substantial volumes of ionized gas close to their central regions. This gas is primarily observed by the deep absorption features which it imprints on the underlying continuum. In cases in which this gas is photoionized, it is often refered to as a "warm-absorber".
The term "warm absorber" was first introduced by Halpern (1984) as a shorthand to denote the fact that the electron temperature (e.g. at the illuminated face of the material; typically <10⁵K) is much lower than in the case of collisionally-ionized ("thermal") gas with a similar level of ionization.

[Image: Schematic X-ray spectrum of a Seyfert 1 galaxy]
Early studies of this ionized gas (including those performed by JCA members) were severely limited by the relatively poor spectral resolution of the detectors on board the ROSAT and ASCA satellites.
The only detectable features were the bound-free absorption "edges", primarily those due to He- and H-like oxygen.
The depths of the features varied from object to object, presumeably due to different amounts of gas that 'happen' to lie along the line of sight. However the consensus was that the physical conditions within the gas is the result of photo-ionization by the intense radiation field of the nucleus.

More recent observations, using the far superior spectral resolution of the Chandra X-ray Observatory and XMM-Newton, have started to reveal the true complexity of the ionized gas.
Resonant absorption lines are how detectable, revealing the kinematics of the absorbing material. It is invariably found to be outflowing from the central engine at velocities of few x 10² km s^-1. Furthermore the gas is found to have a velocity dispersion (without which the resonant lines would not be detectable using the instruments on board the Chandra X-ray Observatory and XMM-Newton) of few x 10² km s^-1.
These measurements are beginning to provide constraints on the physical conditions within the absorbing material, its location and its origin/fate.

[Image: CXO/HETGS spectrum of NGC3783 (Kaspi et al 2002)]

It is hoped that additional spectroscopic observations using the Chandra X-ray Observatory and XMM-Newton (and the high-resolution instruments onboard future satellites) will further our knowledge of this gas.

Links

Student Oriented	Research Oriented
The Basic Atomic Physics of Warm Absorbers Warm Absorbers in AGN - brief overview Warm Absorbers in Quasars The Link between the "X-ray Warm Absorbers" and the Absorption Systems seen in the Ultra-violet band	PIG Lovers Monthly Photoionization Codes

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