A celestial gamma-ray foreground due to the albedo of small solar system bodies and a remote probe of the interstellar cosmic ray spectrum

Kavli Affiliate: Seth W. Digel

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| Summary:

We calculate the gamma-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 gamma-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 ~6×10^{-6} cm^{-2} s^{-1} (100-500 MeV), which
corresponds to ~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. Additionally, 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 gamma-ray albedo has to be
taken into account when analyzing weak gamma-ray sources close to the ecliptic,
especially near the Galactic center and for signals at high Galactic latitudes,
such as the extragalactic gamma-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

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