Probing the Distribution and Environment of Ultra Hot Gas by Fluorescence and Scattering of Iron Lines

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In an earlier work(1) , we have shown that an ultra hot (T ~ 10(8) K), optically thin gas in the central 200 pc of the Galaxy could be a superbubble resulting from an adiabatic explosion. Further progress in understanding the origin of this ultra hot gas could be reached by using iron lines, which contain information about the physical state of the gas. We present a detailed analysis of the ionization balance in the plasma and give a convenient analytical approximation for the equivalent width of the iron lines; it depends both on the temperature and iron abundance, which in principle can be separately determined if combined with additional data. The presence of both cold (molecular) gas and warm/hot (10(4) K/ 10(6) K) gases in the environment of the ultra hot gas makes it possible to investigate, in addition to the direct study of iron line emission, two other processes: (i) fluorescence of iron lines in the cold gas and (ii) scattering of iron lines by warm/hot gas. The equivalent width of the fluorescent K_alpha -line is found to be $EW~ 5* 10(-3}(f/0.1)({1/3)) (tau_0 /mu )Y_Feepsilon_K exp (epsilon_K /10 KeV), where \tau_0 is the optical depth of the source assumed to be a flat layer, \mu=\cos \theta (\theta is the angle with the normal to the layer), and f is the filling factor. The ratio of EWs for fluorescent line and H/He-like iron lines could be as large as \sim 10% for \tau_0/\mu\sim 1, which might be the case. The ratio of intensities of the scattered and direct components of iron lines is expected to be of the order of \tau_0ln \tau_0^{-1/2}, i.e. might be significant. The shape of the scattered line would bring information on the plasma electron temperature. Ozernoy, L., Titarchuk, L., and Ramaty, R. 1992, in {sl Back to the Galaxy} (eds. S.S. Holt, etal) AIP Conf. Proc. (in press)

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