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 midplane (the so-called Galactic ridge) was measured with the Rossi X-Ray Timing Explorer proportional counter array in order to determine the spatial extent, spectral nature, and origin of the emission. Spatial examination of the diffuse emission in the central 30° of the plane in Galactic longitude reveals the presence of two components: a thin disk of full width ≲05 centered roughly on the Galactic midplane and a broad component that can be approximated as a Gaussian distribution with FWHM of about 4°. Assuming an average distance of 16 kpc to the edge 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 the 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 possibly nonthermal origins for the diffuse emission. The averaged spectrum from the ridge in the 3-35 keV band can be modeled with a Raymond-Smith plasma component of temperature ~2-3 keV and a power-law component of photon index ~1.8. Based on this finding, we argue that the temperature of the hot phase of the interstellar medium (ISM) is less than the previously reported values of 5-15 keV. Motivated by the similarities between the characteristics of the thermal component of the Galactic ridge emission 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 SNRs 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 power law remains uncertain. Possible contributions from nonthermal bremsstrahlung of cosmic-ray electrons and protons; inverse Compton scattering of energetic electrons from ambient microwave, infrared, and optical photons; nonthermal emission from SNRs; and emission from discrete X-ray sources are discussed. We speculate that bremsstrahlung of accelerated electrons and protons in SNR sites can play a significant role in producing the hard tail of the spectrum. Moreover, their collisional losses can play a major role in the ionization of the ISM.