Suzaku Discovery of Iron Absorption Lines in Outburst Spectra of the X-Ray Transient 4U 1630—472

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 2–50 keV 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 ~ 20 \,keV. The disk temperature decreases from 1.4 keV to 1.2 keV 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 7.0 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 of 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. 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) × 10²³ \,cm⁻² and (6 -- 4) × 10⁴, respectively, for a velocity dispersion of 500 \,km \,s⁻¹. This in turn constrains the size of the plasma to be ~ 10¹⁰ \,cm, assuming a source distance of 10 kpc.