 Microquasar

A microquasar is an accreting stellar binary whose compact object launches relativistic jets, providing a small-scale analogue of a jet-producing active galactic nucleus. The name refers to the analogy between accretion and outflow, not to a miniature galaxy. The accretor may be a black hole or a neutron star.

A revealing observation is a sequence of high-resolution radio images, showing emitting knots moving away from the compact central source. In GRS 1915+105, such observations demonstrated strikingly rapid apparent motions. If a knot moves at speed $\beta c$ at angle $\theta$ toward the observer, light-travel delays giveFor the illustrative values $\beta =0.9$ and $\theta =30^{\circ }$, this is approximately $2.04$: apparent superluminal motion despite a locally subluminal speed. The receding jet has a different arrival-time factor. Comparing both sides and the source distance therefore constrains geometry, rather than measuring a speed directly from one image.^[ref]