Suzaku discovery of absorption lines from the black hole transient 4U1630-472

We present the results of six Suzaku observations of the recurrent black hole transient 4U 1630-472 during its decline from its most recent outburst in 2006. All observations show the typical high/soft state spectral shape in the 2-50 keV band, roughly described by an optically thick disk spectrum in the soft energy band plus a weak power-law tail. The disk temperature decreases from 1.4 keV to 1.2 keV as the flux decreases by a factor 2, consistent with a constant radius as expected for disk-dominated spectra. All the observations reveal significant absorption lines from highly ionized (H-like and He-like) iron KQ at 7.0 keV and 6.7 keV. The energies of these absorption lines suggest a blue shift with an outflow velocity of ∼ 1000 km s^-1. The H-like iron Kα equivalent width remains approximately constant at ∼ 30 eV over all the observations, while that of the He-like Kα line increases from 7 eV to 20 eV. Thus the ionization state of the material decreases, as expected from the decline in flux. The data constrain the velocity dispersion of the absorber to 200-2000 km s^-1, and the size of the plasma as ∼ 10 ¹⁰ cm assuming a source distance of 10 kpc.