Gamma Rays from Compton Scattering in the Jets of Microquasars: Application to LS 5039

Recent High Energy Stereoscopic System (HESS) observations show that microquasars in high-mass systems are sources of very high energy γ-rays. A leptonic jet model for microquasar γ-ray emission is developed. Using the head-on approximation for the Compton cross section and taking into account angular effects from the star's orbital motion, we derive expressions to calculate the spectrum of γ-rays when nonthermal jet electrons Compton scatter photons of the stellar radiation field. The spectrum of Compton scattered accretion disk radiation is also derived by approximating the accretion disk as a point source of radiation located behind the jet. Numerical results are compared with simpler expressions obtained using δ-function approximations for the cross sections, from which beaming factors are derived. Calculations are presented for power-law distributions of nonthermal electrons that are assumed to be isotropically distributed in the comoving jet frame and applied to γ-ray observations of LS 5039. We conclude that (1) the TeV emission measured with HESS cannot result only from Compton scattered stellar radiation (CSSR), but could be synchrotron self-Compton (SSC) emission or a combination of CSSR and SSC; and (2) fitting both the HESS data and the EGRET data claimed to be associated with LS 5039 requires a very improbable leptonic model with a very hard electron energy distribution. Because the γ-rays would be variable in a leptonic jet model, the data sets are unlikely to be representative of a simultaneously measured γ-ray spectrum. We therefore attribute EGRET γ-rays primarily to CSSR emission and HESS γ-rays to SSC emission. Detection of periodic modulation of the TeV emission from LS 5039 would favor a leptonic SSC or cascade hadron origin of the emission in the inner jet, whereas stochastic variability alone would support a more extended leptonic model. The puzzle of the EGRET γ-rays from LS 5039 will be quickly solved with GLAST.

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