Understanding the Very High-Energy Emission from Microquasars

Microquasars are X-ray binaries with relativistic jets. These jets are powerful energy carriers, thought to be fed by accretion, which produce non-thermal emission at different energy bands. To date, several Galactic sources showing extended radio emission, among them at least one confirmed microquasar, Cygnus X-1, have been detected in the TeV range. All of them show complex patterns of spectral and temporal behavior. In this work, we discuss the physics behind the very high-energy emission in microquasars. In concrete, we focus on the microquasar Cygnus X-1, and also in the other two TeV binaries with detected extended outflows, LS 5039 and LS I +61 303, pointing out relevant aspects of the complex phenomena occurring in them. We conclude that the TeV emission is likely of leptonic origin, although hadrons cannot be discarded. In addition, efficient electromagnetic cascades can hardly develop since even relatively low magnetic fields suppress them. Also, the modeling of the radiation from some of the detected sources points to them as either extremely efficient accelerators, and/or having the TeV emitter at a distance from the compact object of about 10e12 cm. Finally, we point out that the role of a massive and hot stellar companion, due to its strong photon field and wind, cannot be neglected when trying to understand the behavior of microquasars at high and very high energies. The complexity of microquasars precludes straightforward generalizations to a whole population, and are better studied presently in a source by source base.