Suzaku observation of the black hole transient 4U1630-472: Discovery of absorption lines

We present the results of six Suzaku observations of the recurrent black hole transient 4U 1630472 during its decline from its most recent outburst in 2006. All observations show the typical high/soft state spectral shape in the 250 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 K 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 Hlike iron K equivalent width remains approximately constant at 30 eV over all the observations, while that of the Helike K 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 2002000 km s ¹, and the size of the plasma as 10 ¹⁰ cm assuming a source distance of 10 kpc.