Suzaku studies of luminosity-dependent changes in the low-mass X-ray binary Aquila X-1

The neutron-star low-mass X-ray binary Aquila X-1 was observed by Suzaku 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 ∼1037 erg s−1 to ∼1034 erg s−1, by ∼3 orders of magnitude. To investigate luminositydependent changes in the accretion geometry, five of the seven data sets with a typical exposure of ∼18 ks each were analyzed; the other two were utilized in a previous work (Sakurai et al. 2012, PASJ, 64, 72). The source was detected up to 100 keV in the second to the fourth observations, to 40 keV in the fifth, and to 10 keV on the last two occasions. All spectra were reproduced successfully by a Comptonized blackbody model with relatively high ( 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 ( 0.8) optical depth. The estimated radius of the blackbody emission, including seed photons for the Comptonization, was 10 ± 2 km at a 0.8–100 keV luminosity of 2.4 × 1036 erg s−1 (the second to the fourth observations). In contrast, it decreased to 7 ± 1 km and further to 3 ± 1 km, at a luminosity of (4.8−5.2) × 1035 erg s−1 (the fifth observation) and ∼2 × 1034 erg s−1 (the sixth and seventh), respectively, regardless of the above model ambiguity. This can be taken as evidence for the emergence of a weak magnetosphere from the neutron star.