Astrophysics of Galactic Cosmic Rays and Gamma Rays

Cosmic rays are high-energy particles produced in supernova remnants and other objects in the Galaxy. They constitute a substantial part of the total energy budget of the Galaxy. These particles (protons, nuclei, electrons ...) can be observed directly from balloons and spacecraft, and indirectly via the gamma-rays and other radiation produced by their interactions in the interstellar medium. The gamma-rays are observed by space observatories such as NASA's COMPTON Gamma-Ray Observatory, ESA's INTEGRAL, and in the future NASA's GLAST Gamma-Ray Observatory.

The cosmic-rays emerging from their sources are referred to as primary cosmic-rays, while those produced by nuclear reactions during their passage though the interstellar gas are known as secondary cosmic-rays. Secondary cosmic-rays include elements like Boron (which are rare in normal matter), and antimatter in the form of positrons and antiprotons. By studying these secondary particles it is possible to deduce the way in which the cosmic-rays move in the Galaxy from their sources until they escape into intergalactic space. While direct observations are restricted to the environment near the solar system, gamma-rays give information on cosmic-rays throughout the Galaxy, shedding light on their origin and propagation.

A long-term program to exploit these complementary types of observations is underway. It uses a sophisticated cosmic-ray propagation program to model spectra of particles and gamma-rays for comparison with observations.

The model is based on many types of astronomical input, such as surveys of atomic hydrogen, molecular hydrogen (traced by the CO molecule), stellar populations, the distribution of supernova remnants, interstellar isotopic abundances. The physics of cosmic-ray propagation by processes such as diffusion and energy losses is implemented. In addition physics input in the form of cross-sections for the production of cosmic-ray secondary particles and gamma-rays is an essential part of the program. The model software is publicly available and has a substantial user community. It has been used to interpret data from a variety of missions..

The galprop project is a collaboration between Andrew Strong (MPE), Igor Moskalenko (NASA GSFC) and Olaf Reimer (Bochum), with other contributors as well. Detailed information can be found at

Cosmic ray propagation/diffuse gamma rays project
and literature, presentations at
A. Strong's home page
I. Moskalenko's home page

Andrew Strong, Nov 18 2004