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-massX-ray binary 4U 1636-53. The characteristics of these flares (i.e., decay timescales, spectral softening, fluences) are very similar to the very long type-IX-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-raybursts. This would make this source the fifth to exhibit this phenomenon andthe first one for which multiple bursts have been found. Interestingly, allfive sources accrete at approximately 10% of the Eddington mass accretion rate.Although a chance coincidence or a selection effect cannot be ruled-out atpresent, this correlation is suggestive and might indicate that only at anarrow range of mass accretion rate such very long type-I X-ray bursts canoccur.Comment: Accepted for publication in ApJ Letter