  The Neutron Star Interior Composition Explorer (NICER) on the International
Space Station (ISS) observed strong photospheric expansion of the neutron star
in 4U 1820-30 during a Type I X-ray burst. A thermonuclear helium flash in the
star's envelope powered a burst that reached the Eddington limit. Radiation
pressure pushed the photosphere out to ~200 km, while the blackbody temperature
dropped to 0.45 keV. Previous observations of similar bursts were performed
with instruments that are sensitive only above 3 keV, and the burst signal was
weak at low temperatures. NICER's 0.2-12 keV passband enables the first
complete detailed observation of strong expansion bursts. The strong expansion
lasted only 0.6 s, and was followed by moderate expansion with a 20 km apparent
radius, before the photosphere finally settled back down at 3 s after the burst
onset. In addition to thermal emission from the neutron star, the NICER spectra
reveal a second component that is well fit by optically thick Comptonization.
During the strong expansion, this component is six times brighter than prior to
the burst, and it accounts for 71% of the flux. In the moderate expansion
phase, the Comptonization flux drops, while the thermal component brightens,
and the total flux remains constant at the Eddington limit. We speculate that
the thermal emission is reprocessed in the accretion environment to form the
Comptonization component, and that changes in the covering fraction of the star
explain the evolution of the relative contributions to the total flux.
