Spectral evolution of Sco X-1 observed with MAXI

We present here the results of spectral analyses of Sco X-1 with the MAXI observation from 2010 January to 2012 December. The MAXI has two types of detector: GSC (gas proportional counter) and SSC (X-ray CCD). The SSC covers the energy range from 0.7keV to 8keV and the GSC covers from 2keV to 30keV. Since each has a fan beam collimator, it observes almost the entire sky at every 1.5 hour according to the orbit of the International Space Station. The on-source time is relatively short about one minute or less, therefore, there is almost no pile-up problem even for bright sources like Sco X-1. We obtain the Hardness-Intensity Diagram and confirm its Z-track on it. Then we extract 11 X-ray spectra from selected regions along the Z-track: 2 regions in the horizontal branch (HB), 3 regions in the normal branch (NB) and 6 regions in the flaring branch (FB). When we fit the data by using a standard model, a multi-color disk + a comptonized blackbody, we find that the inner radius of the accretion disk is quite large (> 40km) and that the radius of the neutron star (NS) is also large (15-30km). In the LMXB-NS system in high-soft sate like ScoX-1, the accretion disk in the standard model usually extends very close to the NS surface. Therefore, we add an extra component and find a blackbody to the standard model yielding an improved fit with reasonable parameter values. When Sco X-1 brightens along FB, the third component and the NS become hot while the accretion disk stays almost constant of its size and temperature. The third component is considered to occupy between the NS and the accretion disk, partly occulting the NS. It may be related to the outflow reported in other sources.