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T. E. Strohmayer (NASA/GSFC)
We report on Rossi X-ray Timing Explorer (RXTE) observations of the LMXB 4U 1820-30 which reveal the longest X-ray burst ever observed from this source. On September 9th, 1999 UT a powerful, long duration X-ray flare was observed during RXTE pointed observations of 4U 1820-30. This object is a known LMXB burster with the shortest known orbital period (\approx 11 min.), but prior to these observations only a single type I X-ray burst from it had been observed with RXTE. The flare begins in the decaying tail of a more typical (\approx 20 s duration) type I X-ray burst which may have triggered the larger event. The event starts with an initial rise but after a few seconds the flux drops sharply down to background level for a few seconds before increasing again. This behavior, reminiscent of so called 'fast transient with a precursor' events, is consistent with a powerful episode of photospheric radius expansion, and strongly suggests the event is thermonuclear in origin. The flux is still decaying 10,000 seconds after the initial rise when our observations ended. The long duration of the flare suggests a source of thermonuclear energy released at significantly greater column depth than the more common type I bursts from this source and could be caused by ignition of the 'ashes' left behind by the more frequent helium flashes. We will present detailed spectral and timing measurements during the flare.