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The ultracompact X-ray binary 4U 1820-30 is well known for its ~170-dsuperorbital modulation in X-ray flux and spectrum, and the exclusiveness ofbursting behavior to the low hard 'island' state. In May-June 2009, there wasan exceptionally long 51-d low state. This state was well covered by X-rayobservations and 12 bursts were detected, 9 with the high-throughput RXTE. Weinvestigate the character of these X-ray bursts and find an interesting changein their photospheric expansion behavior. At the lowest inferred mass accretionrates, this expansion becomes very large in 4 bursts and reaches the so-calledsuperexpansion regime. We speculate that this is due to the geometry of theinner accretion flow being spherical and a decreasing accretion rate: when theflow geometry nearest to the neutron star is spherical and the accretion rateis low, the ram pressure of the accretion disk may become too low to counteractthat of the photospheric expansion. In effect, this may provide a novel meansto probe the accretion flow. Additionally, we observe a peculiar effect: thewell-known cessation of X-ray bursts in the high state is too quick to beconsistent with a transition to stable helium burning. We suggest analternative explanation, that the cessation is due to the introduction of anon-nuclear heat source in the neutron star ocean.

Superexpansion as a possible probe of accretion in 4U 1820-30