The neutron-star Low-Mass X-ray Binary Aquila X-1 was observed seven times in total with the Suzaku X-ray observatory from September 28 to October 30 in 2007 , in the decaying phase of an outburst .
In order to constrain the flux-dependent accretion geometry of this source over wider energy bands than employed in most of previous works , the present study utilized two out of the seven data sets .
The 0.8–31 keV spectrum on September 28 , taken with the XIS and 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 typical characteristics of the soft state of this type of objects .
The spectrum was successfully explained by an optically-thick disk emission plus a Comptonized blackbody component .
Although these results are in general agreement with previous studies , the significance of a hard tail recently reported using the same data was inconclusive in our analysis .
The spectrum acquired on October 9 for 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 was successfully explained by the same two continuum components but with rather different parameters including much stronger thermal Comptonization , of which the seed photon source was identified with blackbody emission from the neutron-star surface .
As a result , the accretion flow in the hard state is inferred to take a form of an optically-thick and geometrically-thin disk down to a radius of 21 \pm 4 km from the neutron star , and then turn into an optically-thin nearly-spherical hot flow .