A Celestial Gamma-Ray Foreground Due to the Albedo of Small Solar System Bodies and a Remote Probe of the Interstellar Cosmic-Ray Spectrum

• Published in: The Astrophysical journal Vol. 681; no. 2; pp. 1708 - 1716
• Main Authors: Moskalenko, Igor V, Porter, Troy A, Digel, Seth W, Michelson, Peter F, Ormes, Jonathan F
• Format: Journal Article
• Language: English
• Published: Chicago, IL IOP Publishing 10.07.2008; University of Chicago Press
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; minor planets, asteroids; Galaxy bulge; Cosmic radiation; gamma rays: theory; Albedo; Planets; Gamma radiation; cosmic rays; Asteroids; Elementary particles; Kuiper belt; Galaxy: bulge; Solar system
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/588425

We calculate the -ray albedo flux from cosmic-ray (CR) interactions with the solid rock and ice in Main Belt asteroids (MBAs), Jovian and Neptunian Trojan asteroids, and Kuiper Belt objects (KBOs) using the Moon as a template. We show that the -ray albedo for the Main Belt, Trojans, and Kuiper Belt strongly depends on the small-body size distribution of each system. Based on an analysis of the Energetic Gamma-Ray Experiment Telescope (EGRET) data we infer that the diffuse emission from the MBAs, Trojans, and KBOs has an integrated flux of less than [image] cm super(-2) s super(-1) (100-500 MeV), which corresponds to [image]12 times the lunar albedo, and may be detectable by the forthcoming Gamma-Ray Large Area Space Telescope (GLAST). If detected by GLAST, it can provide unique direct information about the number of small bodies in each system that is difficult to assess by any other method. In addition, the KBO albedo flux can be used to probe the spectrum of CR nuclei at close-to-interstellar conditions. The orbits of MBAs, Trojans, and KBOs are distributed near the ecliptic, which passes through the Galactic center and high Galactic latitudes. Therefore, the asteroid -ray albedo has to be taken into account when analyzing weak -ray sources close to the ecliptic, especially near the Galactic center, and signals at high Galactic latitudes, such as the extragalactic -ray emission. The asteroid albedo spectrum also exhibits a 511 keV line due to secondary positrons annihilating in the rock. This may be an important and previously unrecognized celestial foreground for the International Gamma-Ray Astrophysics Laboratory (INTEGRAL) observations of the Galactic 511 keV line emission including the direction of the Galactic center.