The physical processes that should be taken into account when modeling young type-Ia supernova remnants (SNRs) with ages of several hundred years in which forward (into the interstellar medium) and reverse (into the ejection) shocks propagate are analyzed. It is shown that K_alpha lines from species produced by inner-shell collisional ionization should be included when modeling an X-ray emission from SNR. Parameters of the shocked ejecta are found from fitting the model spectrum to the observed one. The application of the method to Tycho SNR using the classical parametric model w7 for the explosion enables us to well reproduce the observed X-ray spectra and radial brightness profiles of the remnant.

Time-dependent thermal X-ray spectra are calculated from physically plausible conditions around GRB. It is shown that account for time-dependent ionization processes strongly affects the observed spectra of hot rarefied plasma. These calculations may provide an alternative explanation to the observed X-ray lines of early GRBs afterglows (such as GRB 011211). Our technique will allow one to obtain independent constraints on the GRB collimation angle and on the clumpiness of circumstellar matter.

We compare the radial distributions of known localized gamma-ray bursts (GRBs) relative to the centers of their host galaxies with the distributions of known objects in nearby galaxies (supernovae of various types, X-ray binaries), the hypothetical dark-matter profiles, and the distribution of luminous matter in galaxies in the model of an exponential disk. By comparing the moments of empirical distributions, we show that the radial distribution of GRBs in galaxies differs significantly from that of other sources (including supernovae Ib/c). The exponential-disk profile was found to be in best agreement with the radial distribution of GRBs.

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