Diffuse Cosmic X-rays observed from Balloons

Abstract

THREE recent observations have given evidence for the existence of a steep spectrum for diffuse cosmic X-rays at energies above 20 to 30 keV (refs. 1 and 2) or above about 40 keV (unpublished work of D. A. Schwartz, H. S. Hudson and L. E. Peterson). Balloon flights by Bleeker et al.¹ and by Rangan et al.² give α≃2.5, and the OSO-III results given by Schwartz et al. give α≃3.0. The implication is that there is a change in the spectral index of about one over a short energy range (the so-called break in the spectrum). We have had some difficulty reconciling these observations with the data of our rocket flight which give a spectral index of 1.95±0.1 in the energy range 25 to 200 keV, a result not very different from that observed from 1 to 10 keV. Nor is there any evidence for a break within our energy interval. Our data were given in a paper by D. Brini, F. Fuligni and E. Horstman-Moretti at the IAU meeting on non-solar X- and gamma ray astronomy in Rome (1969). The results presented at Rome gave 1.8 for the spectral index, but a more refined analysis to be published later gives 1.95 ± 0.10.