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We performed simultaneous observations with Suzaku and NuSTAR of the Galactic black hole binary 4U 1630−47 in the high/soft state (HSS) during the 2015 outburst. To compare our results with those observed in the HSS at lower luminosities, we re-analyze the Suzaku data taken during the 2006 outburst. The continuum can be well explained by thermal disk emission and a hard power-law tail. All spectra show strong iron-K absorption line features, suggesting that a disk wind is always developed in the HSS. We find that the degree of ionization of the wind dramatically increased at the brightest epoch in 2015, when the continuum became harder. Detailed XSTAR simulations show that this cannot be explained solely by an increase of the photoionization flux. Instead, we show that the observed behavior in the HSS is consistent with a theory of thermally-driven disk winds, where the column density and the ionization parameter of the disk wind are proportional to the luminosity and the Compton temperature, respectively.

Evolution of Thermally Driven Disk Wind in the Black Hole Binary 4U 1630–47 Observed with Suzaku and NuSTAR