  The ultracompact X-ray binary 4U 1820-30 is well known for its ~170-d
superorbital modulation in X-ray flux and spectrum, and the exclusiveness of
bursting behavior to the low hard 'island' state. In May-June 2009, there was
an exceptionally long 51-d low state. This state was well covered by X-ray
observations and 12 bursts were detected, 9 with the high-throughput RXTE. We
investigate the character of these X-ray bursts and find an interesting change
in their photospheric expansion behavior. At the lowest inferred mass accretion
rates, this expansion becomes very large in 4 bursts and reaches the so-called
superexpansion regime. We speculate that this is due to the geometry of the
inner accretion flow being spherical and a decreasing accretion rate: when the
flow geometry nearest to the neutron star is spherical and the accretion rate
is low, the ram pressure of the accretion disk may become too low to counteract
that of the photospheric expansion. In effect, this may provide a novel means
to probe the accretion flow. Additionally, we observe a peculiar effect: the
well-known cessation of X-ray bursts in the high state is too quick to be
consistent with a transition to stable helium burning. We suggest an
alternative explanation, that the cessation is due to the introduction of a
non-nuclear heat source in the neutron star ocean.
