Investigation of the Galactic Ridge X-ray Emission with ASCA,

Hidehiro Kaneda

Properties of the Galactic ridge X-ray emission (GRXE) have been investigated with ASCA from the viewpoints of both imagery and spectroscopy in the energy range of 0.5-10 keV. The X-ray data from a typical Galactic ridge of the Scutum arm region have first been analyzed intensively, then the on-plane data within the Galactic longitude |l| = 30°, which are mainly obtained by the Galactic plane survey, and the high-latitude data up to b = 5° in the Scutum region have been studied extensively. The observed GRXE spectra are basically of the thermal emission from thin hot plasmas, and the on-plane spectrum obtained at any longitude within ±30° is essentially similar from field to field. Individual K emission lines from helium-like Mg, Si, S, and Fe ions are confirmed in the spectra. This means that GRXE cannot be explained by a single temperature ionization equilibrium plasma model. It cannot however be reproduced even if non-equilibrium ionization model is introduced, thus multiple plasma components are required. The spectra are fairly well fitted by a double temperature non-equilibrium ionization plasma model with temperatures of kT ~ 0.8 keV and ~7 keV. The softer component is found to be in an extremely low degree of ionization state, while the harder component is in a relatively high ionization state though not yet in a full equilibrium. Spectral properties of the two components are seen to depend on the latitude; the most noticeable effect is a rapid decrease in the Fe-K line equivalent width seen in the hard component. The surface brightness of the softer component shows large-scale fluctuation along the Galactic plane in contrast to the smooth distribution of the harder-component one. In addition, the softer-component brightness extends towards significantly higher (~3°) Galactic latitudes than the harder component, although their actual scale heights may be similar at ~100 pc if the difference in their observable depths are taken into account. Based on the results of the fluctuation analysis of the surface brightness and so forth, attempts are made to interpret the two components in terms of a sum of discrete sources, SNRs, or diffuse hot plasmas filling the interstellar space.

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