Astronomy. Revealing the gamma-ray jet in a black hole binary.

One of the most important advances in high-energy astrophysics over the past decade has been the detailed understanding that has emerged 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 X-rays, but 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 relativistic jets which inject energy into the environment around the binary system and which give the objects their alternative name of ‘microquasars’[3,4]. As that name suggests, X-ray binaries may be offering us an opportunity to study analogs of the much more powerful, but much more distant, relativistic outflows associated with the supermassive black holes at the centers of active galaxies, and to watch them evolve over timescales that are compatible with a human lifetime. But direct measurements of the properties of the jets in X-ray binaries have been rather elusive to date. In 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.