Suzaku observations of low-mass X-ray binaries at high mass-accretion rates

To investigate physical states of "Z" sources, which are a subclass of low-mass X-ray binaries consisting of a weakly-magnetized neutron star and a companion star, two Z sources (LMC X-2 and GX 349+2) were observed by the Suzaku satellite. Since the interstellar absorption toward LMC X-2 is relatively low, its X-ray signal was detected down to a similar to 0.4 keV energy band. From horizontal branch through normal branch to flaring branch, the soft count rate below I keV was observed to increase continuously, while the hard one above 6 keV decreased at the normal branch. This result confirms the existence of the additional soft component with temperature of sub keV, proposed by [4, 5], where luminous LMXB spectra have been shown to consist of three optically-thick components; the softest one from a retreated disk, the hardest one from the NS surface, and an additional intermediate component presumably from outflows caused by the increased radiation pressure. A broad-band energy spectrum of GX 349+2 in flaring branch was also well represented by the above three-component model.