A cosmic-ray explanation of the galactic ridge of cosmic X-rays

Recent observations¹ with the Exosat X-ray satellite have confirmed the existence of an apparently diffuse galactic ridge of emission extending over the inner Galaxy (l = 320° -40°) and confined to about +/-2° in galactic latitude. On the basis of measurements² carried out in the galactic plane at l~=55°, we have proposed previously³ that the diffuse galactic X-ray flux known at that time could be understood in terms of the synchrotron radiation of cosmic-ray electrons in the galactic magnetic field. The more extensive Exosat measurements fit nicely into this picture and the X-ray-producing electrons can now be readily understood as being secondary to the ultra-high-energy protons, which are themselves generated in galactic objects as epitomized by the recently discovered^4,5 Cygnus X-3. We show here that, on this model, the number of Cyg X-3-like objects present in the Galaxy at any one time, averaged over the past 10³ yr or so, is ~=30.