RXTE measurement of the diffuse X-ray emission from the galactic ridge: Implications for the energetics of the interstellar medium

The diffuse X-ray emission from the thin disk surrounding the Galactic midp lane (the so-called Galactic ridge) was measured with the Rossi X-Ray Timin g Explorer proportional counter array in order to determine the spatial ext ent, spectral nature, and origin of the emission. Spatial examination of th e diffuse emission in the central 30 degrees of the plane in Galactic longi tude reveals the presence of two components: a thin disk of full width less than or similar to 0.degrees 5 centered roughly on the Galactic midplane a nd a broad component that can be approximated as a Gaussian distribution wi th FWHM of about 4 degrees. Assuming an average distance of 16 kpc to the e dge of the Galaxy, a scale height of about 70 pc and 500 pc is derived for the thin and broad disk components, respectively. Spectral examination of t he emission clearly reveals the presence of a hard power-law tail above 10 keV and an emission line from He-like iron, indicating both thermal and pos sibly nonthermal origins for the diffuse emission. The averaged spectrum fr om the ridge in the 3-35 keV band can be modeled with a Raymond-Smith plasm a component of temperature similar to 2-3 keV and a power-law component of photon index similar to 1.8. Based on this finding, we argue that the tempe rature of the hot phase of the interstellar medium (ISM) is less than the p reviously reported values of 5-15 keV. Motivated by the similarities betwee n the characteristics of the thermal component of the Galactic ridge emissi on in our model and the thermal emission from supernova remnants (SNRs), we discuss the origin of the thermal emission in terms of a population of SNR s residing in the Galactic disk. We find that a supernova explosion rate of less than 5 century(-1) is adequate to power the thermal emission from the ridge. The origin of the emission in the hard X-ray band modeled by a powe r law remains uncertain. Possible contributions from nonthermal bremsstrahl ung of cosmic-ray electrons and protons; inverse Compton scattering of ener getic electrons from ambient microwave, infrared and optical photons; nonth ermal emission from SNRs; and emission from discrete X-ray sources are disc ussed. We speculate that bremsstrahlung of accelerated electrons and proton s in SNR sites can play a significant role in producing the hard tail of th e spectrum. Moreover, their collisional losses can play a major role in the ionization of the ISM.