  We present results of a detailed study of X-ray flaring in the Z-track source
Sco X-1 in a highly super-Eddington state made using high quality Rossi-XTE
data from the PCA and HEXTE instruments. The emission model successfully used
to explain the dipping LMXB, and other classes of LMXB in recent years, was
applied to study the physical evolution along the Z-track which remains a major
problem. This model consists of blackbody emission from the neutron star plus
Comptonized emission from an extended accretion disk corona. As found in
earlier work, major changes take place in the neutron star blackbody emission
with kT increasing in flaring, and the blackbody radius R_BB increasing
substantially to a maximum value of 9.4 +/- 0.6 km, consistent with the radius
of the neutron star, after which R_BB decreases. Thus this result is a
measurement of neutron star radius. The behaviour of Sco X-1 in flaring is
compared with our previous results for the strong flaring that takes place in
the bright dipping, flaring LMXB X 1624-490. Remarkably, during movement along
the Normal Branch towards the apex with the Flaring Branch, the luminosities of
both spectral components decrease, suggesting the possibility that Mdot may
decrease on the Normal Branch, contrary to the widely-held view that Mdot
increases monotonically along the Z-track. During flaring, we detect for the
first time an increase of the Comptonization cut-off energy which may suggest
heating of the ADC plasma by the neutron star flare. The energy of a broad
Gaussian line at 6.4 keV does not change, but the intensity of the line
increases in flaring suggesting either an increase in ADC size in flaring or
the effects of irradiation by the neutron star.
