Ultra hot gas in the Galaxy: possible origins and implications

A remarkable enhancement of the 6.7 keV line emission and associated thermal continuum emission from the Galactic ridge toward the Galactic center discovered recently by Ginga satellite opens new opportunity of exploring high‐energy phenomena in the Galaxy. This emission is thought to originate in a hot optically thin gas, and the central enhancement seems to be associated with an ultra hot (T≊108 K), rarefied [n∼(3−6)⋅10−2 cm−3] interior of the 200‐pc expanding molecular ring in the Galactic center. Assuming that this interior is a superbubble resulting from an adiabatic explosion, we analyze the dynamics of the explosive phenomenon. Some implications of the presence of a cold (molecular) and a warm (radio emitting) gas between us and the ultra hot superbubble, such as fluorescence of iron lines in the cold gas and scattering of iron lines in warm (hot) gas are discussed.