2014ApJ...786....6L

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 {dot}M - Σ 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, {dot}M_crit, corresponding to the thermal instability threshold, is significantly increased in the presence of disk winds. For α = 0.01 and B_φ= 10B_p, the disk is quite stable even for a very weak initial poloidal magnetic field [β_p,O ∼ 2000, [β_p = (P_gas + P_rad)/(β_p²/8π)]. However, when B_φ= B_p or B_φ= 0.1B_p, a somewhat stronger (but still weak) field (β_p, 0_∼ 200 or β_p, 0_∼ 20) is required to make the disk stable. Nevertheless, despite the great increase of {dot}M_crit, the luminosity threshold, corresponding to instability, remains almost constant or decreases slowly with increasing {dot}M_crit 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.