Browsing by Author "Chakravorty, Susmita"

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    AGN disk emission through X-ray and UV absorbers
    (2011-01-29) Chakravorty, Susmita; Lee, Julia
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    Dielectronic recombination and stability of warm gas in active galactic nuclei
    (2008-09) Chakravorty, Susmita; Kembhavi, A.K.
    Understanding the thermal equilibrium (stability) curve may offer insights into the nature of the warm absorbers often found in active galactic nuclei. Its shape is determined by factors such as the spectrum of the ionizing continuum and the chemical composition of the gas. We find that the stability curves obtained under the same set of the above-mentioned physical factors, but using recently derived dielectronic recombination rates, give significantly different results, especially in the regions corresponding towarmabsorbers, leading to different physical predictions. Using the current rates we find a larger probability of having a thermally stable warmabsorber at 105 Kthan previous predictions and also a greater possibility for itsmultiphase 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 these because of a lack of available data.
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    Properties of warm absorbers in active galaxies: a systematic stability curve analysis
    (2008-10-14) Chakravorty, Susmita; Kembhavi, A.K.
    Signatures of warm absorbers are seen in soft X-ray spectra of about half of all Seyfert1 galaxies observed and in some quasars and blazars. We use the thermal equilibrium curve to study the influence of the shape of the ionizing continuum, density and the chemical composition of the absorbing gas on the existence and nature of the warm absorbers. We describe circumstances in which a stable warm absorber can exist as a multiphase medium or one with continuous variation in pressure. In particular we find the following results: i) the warm absorber exists only if the spectral index of the X-ray power-law ionizing continuum α > 0.2 and has a multiphase nature if α ∼ 0.8, which interestingly is the spectral index formost of the observed Seyfert 1 galaxies; ii) thermal and ionization states of highly dense warm absorbers are sensitive to their density if the ionizing continuum is sufficiently soft, i.e. dominated by the ultraviolet iii) absorbing gas with super Solar metallicity is more likely to have a multiphase nature; iv) the nature of the warm absorber is significantly influenced by the absence of iron and associated elements which are produced in the later stages of star formation history in supernovae of type Ia.