Discovery of a bow shock around Vela X-1

We report the discovery of a symmetric bow shock around the well-known high-mass X-ray binary (HMXB) Vela X-l. Wind bow shocks are a ubiquitous phenomenon around OB-runaway stars, but now such a structure is found around an HMXB. The presence of a bow shock indicates that the system has a high (supersonic) velocity with respect to the interstellar medium. From the symmetry of the bow shock, the direction of motion and, moreover, the origin and age of the system can be derived. Our observation supports Blaauw's scenario for the formation of an OB-runaway star by the supernova explosion of the binary companion.