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The neutron-star Low-Mass X-ray Binary Aquila X-1 was observed seven times intotal with the Suzaku X-ray observatory from September 28 to October 30 in2007, in the decaying phase of an outburst. In order to constrain theflux-dependent accretion geometry of this source over wider energy bands thanemployed in most of previous works, the present study utilized two out of theseven data sets. The 0.8-31 keV spectrum on September 28, taken with the XISand HXD-PIN for an exposure of 13.8 ks, shows an absorbed 0.8-31 keV flux of$3.6\times 10^{-9}$ erg s$^{-1}$ cm$^{-2}$, together with typicalcharacteristics of the soft state of this type of objects. The spectrum wassuccessfully explained by an optically-thick disk emission plus a Comptonizedblackbody component. Although these results are in general agreement withprevious studies, the significance of a hard tail recently reported using thesame data was inconclusive in our analysis. The spectrum acquired on October 9for an exposure of 19.7 ks was detected over a 0.8-100 keV band with the XIS,HXD-PIN, and HXD-GSO, at an absorbed flux of $8.5\times 10^{-10}$ erg s$^{-1}$cm$^{-2}$ (in 0.8-100 keV). It shows characteristics of the hard state, and wassuccessfully explained by the same two continuum components but with ratherdifferent parameters including much stronger thermal Comptonization, of whichthe seed photon source was identified with blackbody emission from theneutron-star surface. As a result, the accretion flow in the hard state isinferred to take a form of an optically-thick and geometrically-thin disk downto a radius of $21\pm 4$ km from the neutron star, and then turn into anoptically-thin nearly-spherical hot flow.

Accretion Geometry of the Low-Mass X-ray Binary Aquila X-1 in the Soft and Hard States