ASCA observation of X-ray emission from the Galactic ridge

The recent Chandra result confirmed the diffuse origin for the most part of the extended X-ray emission from the Galactic ridge. The ASCA spectrum of the emission shows a hard continuum (kT similar to 10 keV if thermal) and the emission lines from highly-ionized ions of various elements up to iron. The thermal origin hypothesis contains fundamental problems concerning heating and confinement of the plasmas, and the energy source. This paper presents the results of renewed analysis of the ASCA SIS spectra of the ridge emission at three places, l similar to 28.5degrees; 10degrees and near the Galactic Center. All the observed spectra are found to be essentially identical in shape, indicating the same emission mechanism regardless of the brightness (or the volume emissivity). Thermal emission models, either non-equilibrium or equilibrium ionization, cannot self-consistently explain the observed H-lile to He-like K-line intensity ratios of Si, S and Fe. In addition, the He-like and H-like Fe K-lines are significantly broadened, corresponding to a velocity dispersion of a few thousand km s(-1). This is much higher than the thermal or bulk motion velocity for plasmas of kT similar to 10 keV, and provides strong evidence against the thermal origin of the lines. An alternative possibility of line emission by charge exchange interaction of low-energy cosmic-ray heavy ions has been proposed, which gives natural explanation of the observed line broadening. The low-energy cosmic-ray origin is also consistent with the presence of a non-thermal component in the ridge emission, most probably due to low-energy cosmic-ray electrons. Heavy ions must be accelerated simultaneously with electrons.