Measurement of the Galactic X‐Ray/Gamma‐Ray Background Radiation: Contribution of Discrete Sources

Abstract

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.

On the Origin of the Iron K Line in the Spectrum of the Galactic X‐Ray Background

“…This is because the purely thermal models require a very hot gas (kT D 7 keV) that is uniformly produced and conÐned in the Galactic disk (a very contrived scenario). On the other hand, the presence of a nonthermal component for the emission is demonstrated by hard X-ray/soft c-ray observations of the Galactic background (e.g., Yamasaki et al 1997 ;Valinia & Marshall 1998 ;VKM00 ;Boggs et al 1999). The LECRe model along with a shock heated plasma component (both of which may have been created in supernovae explosions) provide a plausible explanation for the di †use Galactic background emission.…”

“…We also calculated the di †use ARF for a more realistic distribution of the emission along the Galactic plane. We assumed that the spatial distribution of emission was Gaussian in latitude with a 4¡ FWHM (as measured by Valinia & Marshall 1998) and constant in longitude. We found an insigniÐcant di †erence in the best-Ðt model parameters with the two di †erent ARFs.…”

“…is assumed that the broadband di †use emission has a spatial Gaussian distribution of which is consistent FWHM [ 0¡ .5, with the interpretation that the hard X-ray/soft c-ray emission is conÐned to a thin disk centered roughly on the [1¡ Galactic midplane. Indeed, from the RXT E survey of the Galactic plane, Valinia & Marshall (1998) Ðnd two spatial components for the di †use emission : a thin disk of full width centered roughly on the Galactic midplane and [0¡ .5 a broad component of Gaussian distribution with FWHM of about 4¡. Moreover, this is consistent with the observation that the Fe K line is either extremely weak or not detected at all in ASCA measurements of the Galactic plane di †use emission at high latitudes, l [ o 1¡ o (K97).…”

“…Below 10 keV, the spectrum is rich in emission lines from Fe, Mg, Si, and S (e.g., Kaneda et al 1997, hereafter K97). Above 10 keV, a power-law component is clearly detected (e.g., Valinia & Marshall 1998) with an exponential cuto † near 40 keV (Valinia, Kinzer, & Marshall 2000, hereafter VKM00).…”

Long-term melatonin administration attenuates low-LET γ-radiation-induced lymphatic tissue injury during the reproductively active and inactive phases of Indian palm squirrels (Funambulus pennanti)

“…Radiation-induced injury due to free radicals and reactive oxygen intermediates (ROI) could impair specific immune responses. Besides ultraviolet (UV) radiation, a common environmental stressor that influences animals [1] and humans [2], X-rays and c-rays are readily available from natural and artificial sources [3][4][5]. The earth and all living things are constantly bombarded by the background radiation present in soil, water, earth and vegetation [6,7]; one of the major contributors to background radiation is c-radiation [8,9].…”