2007PASJ...59S.185K

We present the results of six Suzaku observations of the recurrent black hole transient 4U 1630-472 during its decline from outburst from February 8 to March 23 in 2006. All observations show the typical high/soft state spectral shape in the 2-50keV band, roughly described by an optically thick-disk spectrum in the soft energy band plus a weak power-law tail that becomes dominant only above ∼20keV. The disk temperature decreases from 1.4keV to 1.2keV as the flux decreases by a factor of 2, consistent with a constant radius, as expected for disk-dominated spectra. All of the observations reveal significant absorption lines from highly ionized (H-like and He-like) iron Kα's at 7.0keV and 6.7keV. The energies of these absorption lines suggest a blue shift with an outflow velocity of ∼1000 km/s. The H-like iron Kα equivalent width remains approximately constant at ∼30 eV over all of the observations, while that of the He-like Kα line increases from 7eV to 20eV. Thus, the ionization state of the material decreases, as expected from the decline in flux. By fitting the profile with Voigt functions (curve of growth) together with detailed photo-ionization calculations, the total absorbing column, and the ionization parameter were estimated to be (1.0-0.7) x 10²³/cm2 and (6-4) x 10⁴, respectively, for a velocity dispersion of 500 km/s. This in turn constrains the size of the plasma to be ∼10¹⁰ cm, assuming a source distance of 10kpc.