X-ray emission from the Galactic bulge is systematically studied with the ASCA and RXTE satellites. Contribution from point sources has been subtracted utilizing the imaging capability of ASCA, and scan profiles of RXTE. The spectral study in the wide energy range of 0.7 to 20 keV has revealed a coexistence of three distinct emission components, two of them are thermal (cooler and hotter) and the other non-thermal. The temperature of the cooler component is remarkably constant (0.6 keV), while the hotter temperature (3 keV) is essentially same all over the bulge region. The third non-thermal component is well represented by a power-law spectrum of photon index ~ 1.8, and dominates spectra above the Fe-K line emission.
The Galactic longitudinal and latitudinal surface brightness distribution of the hotter and non-thermal components are strongly correlated, and is almost symmetric with respect to both the Galactic plane and meridional plane, with a scale height of 2 degree and scale length of 7 degree. On the other hand, the surface brightness distribution of lower thermal component shows local asymmetric structures. Although the Galactic ridge X-ray emission (GRXE) has similar spectral properties and similarly broad vertical distribution component, the longitudinal spatial variation has elucidated the distinction between the bulge and ridge, with a steep brightness enhancement at |l| < 10 degree.
The 2--10 keV X-ray luminosity of the bulge emission is about 1 x 1038 erg/s, which is almost comparable to that of the GRXE. Since the scale height of the emission region far exceeds those of supernova remnants and star forming regions, these emission components indicate the presence of truly diffuse hot plasmas and energetic particles in the bulge volume. This in turn suggests that some heating and acceleration mechanisms are working on the Galaxy scale.