Magnetic flux stabilizing thin accretion discs

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-circle dot black hole the minimal magnetic flux is 1024((M) over dot/(M) over dot(Edd))(20/21) G cm(2). 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 10(22) - 10(24) G cm(2). 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.