An unusually massive stellar black hole in the Galaxy

The X-ray source known as GRS1915+105 belongs to a group dubbed ‘microquasars’1,2. These objects are binary systems which sporadically eject matter at speeds that appear superluminal, as is the case for some quasars. GRS1915+105 is also one of only two known binary sources thought to contain a maximally spinning black hole3. Determining the basic parameters of GRS195+105, such as the masses of the components, will help us to understand jet formation in this system, as well as providing links to other objects which exhibit jets. Using X-ray data, indirect methods4,5 have previously been used to infer a variety of masses for the accreting compact object in the range 10–30 solar masses (M⊙). Here we report a direct measurement of the orbital period and mass function of GRS1915+105, which allow us to deduce a mass of 14 ± 4 M⊙ for the black hole. Black holes with masses >5–7 M⊙ challenge the conventional picture of black-hole formation in binary systems6,7,8,9. Based on the mass estimate, we interpret the distinct X-ray variability of GRS1915+105 as arising from instabilities in an accretion disk that is dominated by radiation pressure, and radiating near the Eddington limit (the point where radiation pressure supports matter against gravity). Also, the mass estimate constrains most models which relate observable X-ray properties to the spin of black holes in microquasars.