The diffuse X-ray emission from the thin disk surrounding the Galactic midplane (the so-called Galactic ridge) was measured with RXTE PCA 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 ∼ < 0 • .5 centered roughly on the Galactic mid-plane, and a broad component which 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 non-thermal origins for the diffuse emission. The averaged spectrum from the ridge in the 3−35 keV band can be modelled with a RaymondSmith 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 1 NAS/NRC Research Associate; valinia@milkyway.gsfc.nasa.gov 2 marshall@milkyway.gsfc.nasa.gov -2 -SN explosion rate of less than 5 per century is adequate to power the thermal emission from the ridge. The origin of the emission in the hard X-ray band modelled by a power law remains uncertain. Possible contributions from non-thermal bremsstrahlung of cosmic ray electrons and protons, inverse Compton scattering of energetic electrons from ambient microwave, infrared, and optical photons, non-thermal 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.

We propose a mechanism for the origin of the Galactic ridge X-ray background that naturally explains the properties of the Fe K line, speciÐcally the detection of the centroid line energy below 6.7 keV and the apparent broadness of the line. Motivated by recent evidence of nonthermal components in the spectrum of the Galactic X-ray/c-ray background, we consider a model that is a mixture of thermal plasma components of perhaps supernova origin and nonthermal emission from the interaction of low-energy cosmic-ray electrons (LECRe) with the interstellar medium. The LECRe may be accelerated in supernova explosions or by ambient interstellar plasma turbulence. Atomic collisions of fast electrons produce characteristic nonthermal, narrow X-ray emission lines that can explain the complex Galactic background spectrum. Using the ASCA GIS archival data from the Scutum arm region, we show that a two-temperature thermal plasma model with kT D 0.6 and D2.8 keV, plus a LECRe component models the data satisfactorily. Our analysis rules out a purely nonthermal origin for the emission. It also rules out a signiÐcant contribution from low-energy cosmic-ray ions, because their nonthermal X-ray production would be accompanied by a nuclear c-ray line di †use emission exceeding the upper limits obtained using OSSE, as well as by an excessive Galaxy-wide Be production rate. The proposed model naturally explains the observed complex line features and removes the difficulties associated with previous interpretations of the data which evoked a very hot thermal component (kT D 7 keV).

NGC 4945 is one of the brightest Seyfert galaxies on the sky at 100 keV, but is completely absorbed below 10 keV; its absorption column is probably the largest that still allows a direct view of the nucleus at hard X-ray energies. Our observations of it with the Rossi X-Ray Timing Explorer (RXTE) satellite confirm the large absorption, which for a simple phenomenological fit using an absorber with solar abundances implies a column of 4.5+0.4-0.4x1024 cm(-2). Using a more realistic scenario (requiring Monte Carlo modeling of the scattering), we infer the optical depth to Thomson scattering of approximately 2.4. If such a scattering medium were to subtend a large solid angle from the nucleus, it should smear out any intrinsic hard X-ray variability on timescales shorter than the light-travel time through it. The rapid (with a timescale of approximately 1 day) hard X-ray variability of NGC 4945 discovered by us with RXTE implies that the bulk of the extreme absorption in this object does not originate in a parsec-size, geometrically thick molecular torus. Instead, the optically thick material on parsec scales must be rather geometrically thin, subtending a half-angle less than 10 degrees, and it is likely to be the same disk of material that is responsible for the water maser emission observed in NGC 4945. Local number counts of Seyfert 1 and Seyfert 2 galaxies show a large population of heavily obscured active galactic nuclei (AGNs) which are proposed to make up the cosmic X-ray background (CXRB). However, for this to be the case, the absorption geometry in the context of axially symmetric unification schemes must have the obscuring material subtending a large scale height-contrary to our inferences about NGC 4945-implying that NGC 4945 is not a prototype of obscured AGNs postulated to make up the CXRB. The small solid angle of the absorber, together with the black hole mass (of approximately 1.4x106 M( middle dot in circle)) from megamaser measurements, allows a robust determination of the nuclear luminosity, which in turn implies that the source radiates at approximately 10% of the Eddington limit.

The Galactic background radiation near the Scutum arm was observed simultaneously with RXT E and OSSE in order to determine the spectral shape and the origin of the emission in the hard X-ray/soft c-ray band. The spectrum in the 3 keV to 1 MeV band is well modeled by four components : a highenergy continuum dominating above 500 keV that can be characterized by a power law of photon index D1.6 (an extrapolation from measurements above D1 MeV), a positron annihilation line at 511 keV and positronium continuum, a variable hard X-ray/soft c-ray component that dominates between 10 and 200 keV (with a minimum detected Ñux of D7.7 ] 10~7 photons cm~2 s~1 keV~1 deg~2 at 100 keV averaged over the Ðeld of view of OSSE) and that is well modeled by an exponentially cuto † power law of photon index D0.6 and energy cuto † at D41 keV, and Ðnally a thermal plasma model of solar abundances and temperature D2.6 keV that dominates below 10 keV. We estimate that the contribution of bright discrete sources to the minimum Ñux detected by OSSE was D46% at 60 keV and D20% at 100 keV. The remaining unresolved emission may be interpreted either as truly di †use emission with a hard spectrum (such as that from inverse Compton scattering) or the superposition of discrete sources that have very hard spectra.