On the predictive power of the minimum energy condition. 2: Fractional calorimeter behaviour in the diffuse high energy gamma emission of spiral galaxies

Details On the predictive power of the minimum energy condition. 2: Fractional calorimeter behaviour in the diffuse high energy gamma emission of spiral galaxies On the predictive power of the minimum energy condition. 2: Fractional calorimeter behaviour in the diffuse high energy gamma emission of spiral galaxies

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...287..453p&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 287, no. 2, p. 453-462

Astronomy and Astrophysics

Astronomy

20

Calorimeters, Emission Spectra, Galactic Cosmic Rays, Gamma Ray Astronomy, Gamma Ray Spectra, Spiral Galaxies, Bremsstrahlung, Energetic Particles, Energy Spectra, Galactic Radio Waves, Radio Spectra

Scientific paper

In this paper we investigate the high energy gamma ray emission from spiral galaxies. The calculations are based on equilibrium spectra for cosmic ray protons and electrons, respectively, which have been derived in an earlier paper (Pohl 1993a). There, the cosmic ray particles are assumed to undergo simultaneously transport by diffusion, escape, and energy losses by ionization, inelastic scattering, bremsstrahlung, adiabatic cooling and radiative losses. In the thick target case a fractional calorimeter behavior occurs both for leptonic and for hadronic gamma ray emission: the resultant gamma ray flux depends solely on the injection rate of cosmic rays and on a fraction factor. This fraction factor is in fact a combination of two: the first is the fraction of cosmic rays which meet the interaction targets like protons or thermal gas. The second is the fraction of the gamma ray producing loss mechanism to the total losses in the gas disk. Once reliable gamma ray and radio spectra of spiral galaxies are obtained these calorimeter fractions may help to gain information about the physical state of the interstellar medium in these objects, especially on the proton-to-electron ratio in cosmic rays. The integrated radio spectra of spiral galaxies tell us whether these systems form a thick target for cosmic rays or not. With the minimum energy consumption for the magnetic field strength we are then able to predict explicitely the gamma ray flux from these objects in a broad energy range. The hitherto promising candidates M 31 and M 82 will not be detected by EGRET, since their integrated flux is les than 2 x 10-8ph./sq cm/sec. It appears that our Galaxy is the only object apart from Large Magellanic Cloud (LMC) which we observe with sufficient accuracy to base clues on the gamma ray emission. Since via the calorimeter effects spiral galaxies can regulate themselves, the minimum energy condition has a predictive power which is much more precise than earlier estimated. Therefore we may be able to test the validity of the minimum energy assumption on the basis of gamma ray observations.

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