Recurrent Very Long Type I X-Ray Bursts in the Low-Mass X-Ray Binary 4U 1636-53

Two flares with a duration of several hours are reported for the low-mass X-ray binary 4U 1636-53. The characteristics of these flares (i.e., decay timescales, spectral softening, fluences) are very similar to those of the very long type I X-ray bursts recently found in several other low-mass X-ray binaries, suggesting that the flares in 4U 1636-53 are also very long type I X-ray bursts. This would make this source the fifth to exhibit this phenomenon and the first one for which multiple bursts have been found. Interestingly, all five sources accrete at approximately 10% of the Eddington mass accretion rate. Although a chance coincidence or a selection effect cannot be ruled out at present, this correlation is suggestive and might indicate that such very long type I X-ray bursts can occur only at a narrow range of mass accretion rates.