Constraints on the cosmic rays in the Small Magellanic Cloud.

We show that recent γ-ray observations of the Small Magellanic Cloud with EGRET rule out a universal cosmic ray flux only at energies below ≈ 10 GeV, while the observed diffuse X-ray and γ-ray background radiations have already ruled out, by more than three orders of magnitude, a universal extragalactic cosmic ray flux identical to that observed in the local solar neighborhood at energies below 106 GeV. Submitted to Physical Review Letters Comments ⋆ Permanent Address: Technion, Israel Inst. of Tech., Haifa 32000 ISRAEL In a recent letter [1] Sreekumar et al. reported an upper limit (95% confidence) of Fγ(E > 100 MeV) < 0.5× 10 −7 photons cm−2s−1 for the high energy γray flux from the Small Magellanic Cloud (SMC), while the expected flux that was calculated by Sreekumar and Fichtel [2] , assuming that the cosmic ray flux is universal and identical to that observed in the local solar neighborhood, is Fγ(E > 100 MeV) ≈ 2.4× 10 −7 photons cm−2s−1. From this discrepancy Sreekumar et al. concluded [1] that the bulk of the cosmic ray energy density is not universal and must be galactic in origin. In this comment we show that the observations of Sreekumar et al. rule out a universal cosmic ray flux only at energies below ≈ 10 GeV, while the observed diffuse X-ray and γ-ray background radiations have already ruled out (by more than three orders of magnitude) a universal extragalactic cosmic ray flux identical to that observed in the local solar neighborhood at energies below 106 GeV. The production of high energy γ-rays by cosmic rays in the SMC is dominated by the reaction pp → π0X → 2γX of cosmic ray protons on interstellar hydrogen, by bremsstrahlung from cosmic ray electrons in the interstellar gas and by inverse Compton scattering of cosmic ray electrons on infrared, visible and ultraviolet SMC photons. We limit our discussion to these three processes. The product of the production cross section and π0 multiplicity beyond threshold (at 279.6 MeV) is a slowly increasing function of incident energy. Folding this energy dependence with the differential cosmic ray power law spectrum, dFp/dE ∼ E −2.75, one finds [3] that the bulk (> 90%) of the π0’s and γ-rays with Eγ > 100 MeV are produced by cosmic ray protons of energy below 10 GeV. The γ-ray flux from the SMC due to bremsstrahlung from cosmic ray electrons in the interstellar gas is proportional to [4] Fe(> Eγ)/Eγ , where Fe(> Eγ) is the integral flux of electrons with energy above Eγ . If the high energy cosmic ray electron flux is universal and identical to its flux in the local solar neighborhood [5] , dFe/dE ≈ 1.16× 10 −2(E/GeV)−2.6cm−2s−1sr−1GeV−1 , then less than 2% of the γ-rays with Eγ > 100 MeV are due to bremsstrahlung from electrons with