Dielectronic recombination and stability of warm gas in AGN

Susmita Chakravorty (Astronomy Department, Harvard University, Cambridge, MA, USA)

High resolution ultraviolet and X-ray spectra show that material outflow occur from the close neighbourhoods of super-massive black holes in active galactic nuclei (AGN). It is important to understand the structure and the thermal and ionisation states of the
outflowing material because they play significant roles in the mass-energy budget of the AGN.

The absorption features seen in the high-resolution soft  X-ray spectra is attributed to gas which is conventionally termed as the warm absorber (WA) and often the thermal equilibrium (stability) curve is used as a theoretical tool to offer insights into the nature of the WA. The shape of the stability curve is determined by factors like the spectral energy distribution of the ionizing flux and the chemical composition of the absorbing gas. We find that the stability curves obtained under the same set of assumptions for the prevalent physical conditions in the AGN environment, but using recently derived dielectronic recombination rates, give significantly different results from what is predicted with older atomic data. The variations in phase space region of the stability curves corresponding to warm absorbers, lead to different physical predictions. Using the current rates we find a larger probability of having thermally stable warm absorber at 105 K than previous predictions and also a greater possibility for its multiphase nature. The results obtained with the current dielectronic recombination rate coefficients are more reliable because the warm absorber models along the stability curve have computed coefficient values, whereas previous calculations relied on guessed averages  for the same, due to lack of available data.

Paper presented at the 17th International Conference on Atomic Processes in Plasmas, Quuen’s University Belfast, 19-22 July 2011.

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