Revealing the gamma-ray jet in a black hole binary

An important advance in high-energy astrophysics over the past decade has been the detailed understanding of the nature of binary stellar systems in which material from an ordinary star is being accreted on to a compact object, either a neutron star or a black hole (1, 2). These systems are called x-ray binaries because emission from the accretion disk and associated material appears very bright in the x-ray regime. In recent years, it has become clear that a substantial amount of the energy generated as matter is accreted can be ejected in the form of high-velocity jets that inject energy into the environment around the binary system (3, 4). Because of similarities in their structure, these x-ray binaries (or microquasars) may offer us insights into the much more powerful, but much more distant, outflows associated with the supermassive black holes at the centers of active galaxies such as quasars. Moreover, their smaller size allows us to watch them evolve over time scales that are compatible with a human lifetime. Direct measurements of the properties of the jets in x-ray binaries have been rather elusive, however. On page 438 of this issue, Laurent et al. (5) show that gamma-ray polarization measurements can be used to make a convincing case for a jet origin for some of the gamma-ray emission observed from the prototypical x-ray binary, Cygnus X-1.