Complex Spectra of the Galactic Ridge X‐Rays Observed withASCA

Abstract

X-ray spectra of the Galactic ridge emission in the Scutum arm region have been obtained with ASCA GIS and SIS in the energy range 0.7-10 keV. The observed spectra are basically of thermal emission from thin hot plasmas, and individual K emission lines from helium-like Mg, Si, S, and Fe ions are confirmed in both the GIS and SIS spectra. This means that the Galactic ridge X-ray emission cannot be explained by a single-temperature ionization-equilibrium plasma model. It cannot, however, be reproduced even if a nonequilibrium ionization model is introduced; thus multiple plasma components are required. The GIS spectra are fairly well fitted by a double-temperature nonequilibrium ionization plasma model with temperatures of kT ~ 0.8 keV and kT ~ 7 keV. The softer component is found to be in an extremely low ionization state, with net ~ 109 cm-3 s, while the harder component is in a relatively high ionization state, though not yet in a full equilibrium. The GRXE properties obtained with the GIS are carefully reexamined by the highly resolved spectral-line features with the SIS. The soft and hard components are absorbed by equivalent hydrogen columns of 0.7 × 1022 cm-2 and 4.6 × 1022 cm-2, respectively. The surface brightness of the soft and hard components at b ~ 0° are estimated to be 1.9 × 10-6 and 5.3 × 10-7 ergs cm-2 s-1 sr-1 respectively, both in the 0.5-10 keV band. The surface brightness of the softer component extends toward significantly higher (~2°) Galactic latitudes than the harder component, although their actual scale heights may be similar at ~100 pc if the differences in their observable depths are taken into account. 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. Attempts are made to interpret the two components in terms of diffuse hot plasmas filling the interstellar space.