Soki Sakurai, Shin'ya Yamada, Shunsuke Torii, Hirofumi Noda, Kazuhiro Nakazawa, Kazuo Makishima
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.
View original:
http://arxiv.org/abs/1201.5891
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