  To investigate the physics of mass accretion onto weakly-magnetized neutron
stars, 95 archival RXTE datasets of an atoll source 4U 1608-522, acquired over
1996-2004 in so-called upper-banana state, were analyzed. The object meantime
exhibited 3-30 keV luminosity in the range of <~ 10^35 - 4 x 10^37 erg s^-1,
assuming a distance of 3.6 kpc. The 3-30 keV PCA spectra, produced one from
each dataset, were represented successfully with a combination of a soft and a
hard component, of which the presence was revealed in a model-independent
manner by studying spectral variations among the observations. The soft
component is expressed by so-called multi-color disk model with a temperature
of ~1.8 keV, and is attributed to the emission from an optically-thick standard
accretion disk. The hard component is a blackbody emission with a temperature
of ~2.7 keV, thought to be emitted from the neutron-star surface. As the total
luminosity increases, a continuous decrease was observed in the ratio of the
blackbody luminosity to that of the disk component. This property suggests that
the matter flowing through the accretion disk gradually becomes difficult to
reach the neutron-star surface, presumably forming outflows driven by the
increased radiation pressure. On time scales of hours to days, the overall
source variability was found to be controlled by two independent variables; the
mass accretion rate, and the innermost disk radius which changes both
physically and artificially.
