The neutron-star Low-Mass X-ray Binary Aquila X-1 was observed by Suzaku for seven times , from 2007 September 28 to October 30 .
The observations successfully traced an outburst decay in which the source luminosity decreased almost monotonically from \sim 10 ^ { 37 } erg s ^ { -1 } to \sim 10 ^ { 34 } erg s ^ { -1 } , by \sim 3 orders of magnitude .
To investigate luminosity-dependent changes in the accretion geometry , five of the seven data sets with a typical exposure of \sim 18 ks each were analyzed ; the other two were utilized in a previous work \citep Sakurai2012 .
The source was detected up to 100 keV in the 2nd to the 4th observations , to 40 keV in the 5th , and to 10 keV on the last two occasions .
All spectra were reproduced successfully by Comptonized blackbody model with relatively high ( \gtrsim 2.0 ) optical depths , plus an additional softer optically-thick component .
The faintest three spectra were reproduced alternatively by a single Comptonized blackbody model with a relatively low ( \lesssim 0.8 ) optical depth .
The estimated radius of the blackbody emission , including seed photons for the Comptonization , was 10 \pm 2 km at a 0.8–100 keV luminosity of 2.4 \times 10 ^ { 36 } erg s ^ { -1 } ( the 2nd to the 4th observations ) .
In contrast , it decreased to 7 \pm 1 km and further to 3 \pm 1 km , at a luminosity of ( 4.8 - 5.2 ) \times 10 ^ { 35 } erg s ^ { -1 } ( the 5th observation ) and \sim 2 \times 10 ^ { 34 } erg s ^ { -1 } ( the 6th and 7th ) , respectively , regardless of the above model ambiguity .
This can be taken as evidence for the emergence of a weak magnetosphere of the neutron star .