The absence of thermal instability in the high/soft state of black hole X-ray binaries, in disagreement with the standard thin disk theory, has been a long-standing riddle for theoretical astronomers. We have tried to resolve this question by studying the thermal stability of a thin disk with magnetically driven winds in the plane. It is found that disk winds can greatly decrease the disk temperature and thus help the disk become more stable at a given accretion rate. The critical accretion rate, , corresponding to the thermal instability threshold, is significantly increased in the presence of disk winds. For α = 0.01 and B = 10B p, the disk is quite stable even for a very weak initial poloidal magnetic field []. However, when B = B p or B = 0.1B p, a somewhat stronger (but still weak) field (βp, 0 ~ 200 or βp, 0 ~ 20) is required to make the disk stable. Nevertheless, despite the great increase of , the luminosity threshold, corresponding to instability, remains almost constant or decreases slowly with increasing due to decreased gas temperature. The advection and diffusion timescales of the large-scale magnetic field threading the disk are also investigated in this work. We find that the advection timescale can be smaller than the diffusion timescale in a disk with winds, because the disk winds take away most of the gravitational energy released in the disk, resulting in the decrease of the magnetic diffusivity η and the increase of the diffusion timescale.
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