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Abstract

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 |$\sim 20 \,\mathrm{keV}$|⁠. 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 |$\mathrm{K}\alpha$|’s at 7.0 keV and 6.7 keV. The energies of these absorption lines suggest a blue shift with an outflow velocity of |$\sim 1000 \,\mathrm{km} \,\mathrm{s}^{-1}$|⁠. The H-like iron |$\mathrm{K}\alpha$| equivalent width remains approximately constant at |$\sim 30 \,\mathrm{eV}$| over all of the observations, while that of the He-like |$\mathrm{K}\alpha$| 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 |$(1.0 \hbox{--} 0.7) \times 10^{23} \,\mathrm{cm}^{-2}$| and |$(6 \hbox{--} 4) \times 10^4$|⁠, respectively, for a velocity dispersion of |$500 \,\mathrm{km} \,\mathrm{s}^{-1}$|⁠. This in turn constrains the size of the plasma to be |$\sim 10^{10} \,\mathrm{cm}$|⁠, assuming a source distance of 10 kpc.

© 2007 Astronomical Society of Japan
Subject
X-rays: individual accretion, accretion disks
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