  The transient atoll source 4U 1608-52 had been extensively observed by the
Rossi X-ray Timing Explorer RXTE during its 1998 outburst. We analyse its X-ray
spectra as a function of inferred accretion rate from both the 1998 outburst
and from the non-outburst 1996 and 1998 data. We can fit all the spectra by a
model in which seed photons from the neutron star surface are Comptonized in a
boundary layer. The Comptonized emission illuminates the accretion disc
surface, producing an ionized, relativistically broadened reflection signature,
while the direct emission from the accretion disc can also be seen. The
evolution of the source can be explained if the main parameter driving the
spectral evolution is the average mass accretion rate, which determines the
truncation radius of the inner accretion disc. At low mass accretion rates, in
the island state, the disc truncates before reaching the neutron star surface
and the inner accretion flow/boundary layer is mostly optically thin. The disc
emission is at too low a temperature to be observed in the RXTE spectra, but
some of the seed photons from the neutron star can be seen directly through the
mostly optically thin boundary layer. At higher mass accretion rates, in the
banana state, the disc moves in and the boundary layer becomes much more
optically thick so its temperature drops. The disc can then be seen directly,
but the seed photons from the neutron star surface cannot as they are buried
beneath the increasingly optically thick boundary layer.
