In some x-ray binary systems matter is transferred from one star to the other via the donor star's stellar wind. The stellar wind, like the Sun's solar wind, is a steady stream of particles that blasts away from the star's surface and out into space. But when another star is present – in this case a neutron star – the latter captures some of the wind material. The gas forms a bow shock around the compact star, somewhat like the wake of a boat in water.