C. D. Dermer and M. Böttcher
Astrophysical Journal, 643, 1081 (2006)
Recent High Energy Stereoscopic System (HESS) observations show that microquasars in high-mass systems are sources of very high energy gamma-rays. A leptonic model for microquasar gamma-ray emission is developed. Using the head-on approximation for the Compton cross section, we derive expressions to calculate the spectrum of gamma-rays when nonthermal jet electrons Compton-scatter photons of the stellar radiation field that take into account angular effects from the star's orbital motion. 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 derived using delta-function approximations for the cross sections. Calculations are presented for power-law distributions of nonthermal electrons that are assumed to be isotropically distributed in the comoving jet frame. The results are applied to gamma-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; (2) fitting both the HESS data and the EGRET data claimed to be associated with LS 5039 requires a very improbable leptonic model with very hard electron spectra; hadronic models face comparable difficulties. One possibility is that the EGRET and HESS gamma-rays are variable, and the nonsimultaneous data are not representative of the broadband spectrum. Another is that the EGRET gamma-rays originate from another source in the error box that contains LS 5039. A third possibility is that a two-component lepton-hadron model forms the gamma-ray spectrum of LS 5039. Detection of periodic modulation of the TeV emission from LS 5039 would favor a leptonic SSC origin of the emission, whereas a steady power-law spectrum would favor nuclear production from hadrons accelerated by the microquasar jets. The puzzle of the EGRET gamma-rays from LS 5039 will be quickly solved with GLAST.