  Powerful Type I X-ray burst with strong radial expansion was observed from
the low mass X-ray binary 4U 1820-30 with Rossi X-ray Timing Explorer on May 2,
  1997. We investigate closely the flux profile during the burst expansion
stage. Applying a semi-analytical model we are able to uncover the behavior of
a photospheric radius and to simulate the evolution of neutron star
(NS)-accretion disk system. The bottom flux L_{bot} is a few times the
Eddington limit L_{Edd} for outer layers, because the electron cross-section is
a few times less than the Thomson cross-section at such a high temperatures.
The surplus of energy flux with respect to the Eddington, $L_{bot}-L_{Edd}$,
goes into the potential energy of the expanded envelope. As cooling of the
burning zone starts the surplus decreases and thus the envelope shrinks while
the emergent photon flux stays the same $L=L_{Edd}$. At a certain moment the NS
low-hemisphere, previously screened by the disk, becomes visible to the
observer. Consequently, the flux detected by the observer increases. Indeed, we
observe to the paradoxical situation when the burning zone cools, but the
apparent flux increases because of the NS-accretion disk geometry. We
demonstrate a strong observational evidence of NS-accretion disk occultation in
the behavior of the observed bolometric flux. We estimate the anisotropy due to
geometry and find that the system should have a high inclination angle.
Finally, we apply an analytical model of X-ray spectral formation in the
neutron star atmosphere during burst decay stage to infer the neutron star (NS)
mass-radius relation.
