Photodisintegration of Ultra–High‐Energy Cosmic Rays: A New Determination

We present the results of a new calculation of the photodisintegration ofultrahigh energy cosmic ray (UHCR) nuclei in intergalactic space. The criticalinteractions for energy loss and photodisintegration of UHCR nuclei occur withphotons of the 2.73K cosmic background radiation (CBR) and with photons of theinfrared background radiation (IBR). We have reexamined this problem making useof a new determination of the IBR based on empirical data, primarily from IRASgalaxies, consistent with direct measurements and upper limits from TeV gamma-ray observations. We have also improved the calculation by including thespecific threshold energies for the various photodisintegration interactions inour Monte Carlo calculation. With the new smaller IBR flux, the steepness ofthe Wien side of the now relatively more important CBR makes their inclusionessential for more accurate results. Our results indicate a significantincrease in the propagation time of UHCR nuclei of a given energy over previousresults. We discuss the possible significance of this for UHCR origin theory.Comment: 16 pages with 4 figures, final version with minor revisions in text and figure captions, Astrophysical Journal, vol. 512, in press 20 Feb. 199