The recurrent soft X-ray transient Aquila X-1 was observed with Suzaku for a gross duration of 79.9?ks, on 2011 October 21 when the object was in a rising phase of an outburst. During the observation, the source exhibited a clear spectral transition from the hard state to the soft state, on a time scale of ～30?ks. Across the transition, the 0.8–10?keV X-Ray Imaging Spectrometer count rate increased by a factor ～3, that of Hard X-ray Detector PIN (HXD-PIN) in 15–60?keV decreased by a similar factor, and the unabsorbed 0.1–100?keV luminosity increased from 3.5 × 1037?erg?s?1 to 5.1 × 1037?erg?s?1. The broadband spectral shape changed continuously, from a power-law-like one with a high-energy cut-off to a more convex one. Throughout the transition, the 0.8–60?keV spectra were successfully described with a model consisting of a multi-color blackbody and a Comptonized blackbody, which are considered to arise from a standard accretion disk and a closer vicinity of the neutron star, respectively. All the model parameters were confirmed to change continuously, from those typical in the hard state to those typical of the soft state. More specifically, the inner disk radius decreased from 31?km to 18?km, the effects of Comptonization on the blackbody photons weakened, and the electron temperature of Comptonization decreased from 10?keV to 3?keV. The derived parameters imply that the Comptonizing corona shrinks towards the final soft state, and/or the radial infall velocity of the corona decreases. These results reinforce the view that the soft and hard states of Aql?X-1 (and of similar objects) are described by the same “disk plus Comptonized blackbody” model, but with considerably different parameters.
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