Accretion geometry of the Low-Mass X-Ray Binary Aquila X-1 in the soft and hard states

Soki Sakurai, Shin'Ya Yamada, Shunsuke Torii, Hirofumi Noda, Kazuhiro Nakazawa, Kazuo Makishima, Hiromitsu Takahashi

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30 Citations (Scopus)


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 previous studies, the present study utilized two out of 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 × 10-9 erg s-1 cm-2, together with typical characteristics of the soft state of this type of object. The spectrum was successfully explained by an optically thick disk emission plus a Comptonized blackbody component. Although these results are in general agreementwith 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 × 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 the form of an optically-thick and geometrically-thin disk down to a radius of 21 ± 4 km from the neutron star, and then turn into an optically thin nearly spherical hot flow.

Original languageEnglish
Pages (from-to)72
Number of pages1
JournalPublications of the Astronomical Society of Japan
Issue number4
Publication statusPublished - 2012 Aug 25
Externally publishedYes


  • Accretion, accretion disks - stars
  • Binaries
  • Neutron - X-rays

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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