2016MNRAS.462..960S

We calculate the minimal amount of large-scale poloidal magnetic field that has to thread the inner, radiation-over-gas pressure dominated region of a thin disc for its thermal stability. Such a net field amplifies the magnetization of the saturated turbulent state and makes it locally stable. For a 10 M_☉ black hole the minimal magnetic flux is 10²⁴{dot}M/{dot}M_Edd^20/21, G/cm2. This amount is compared with the amount of uniform magnetic flux that can be provided by the companion star - estimated to be in the range 10²²-10²⁴ G cm². If accretion rate is large enough, the companion is not able to provide the required amount and such a system, if still sub-Eddington, must be thermally unstable. The peculiar variability of GRS 1915+105, an X-ray binary with the exceptionally high BH mass and near-Eddington luminosity, may result from the shortage of large-scale poloidal field of uniform polarity.