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We report on an approximately twelve hour long X-ray flare from the low-massX-ray binary KS 1731-260. The flare has a rise time of less than 13 min anddeclines exponentially with a decay time of 2.7 hours. The flare emission iswell described by black-body radiation with peak temperature of 2.4 keV. Thetotal energy release from the event is 10^{42} erg (for an assumed distance of7 kpc). The flare has all the characteristics of thermo-nuclear X-ray bursts(so-called type I X-ray bursts), except for its very long duration andtherefore large energy release (factor of 1500-4000 longer and 250-425 moreenergy than normal type I X-ray bursts from this source). The flare is precededby a short and weak X-ray burst, possibly of type I. Days to weeks before theflare, type I X-ray bursts were seen at a rate of ~3 per day. However, afterthe flare type I X-ray bursting ceased for at least a month, suggesting thatthe superburst affected the type I bursting behaviour. The persistent emissionis not significantly different during the non-bursting period. We compare thecharacteristics of this event with similar long X-ray flares, so-calledsuperbursts, seen in other sources (4U 1735-44, 4U 1820-30, 4U 1636-53, SerX-1, GX 3+1). The event seen from KS 1731-260 is the longest reported so far.We discuss two possible mechanisms that might cause these superbursts, unstablecarbon burning (as proposed recently) and electron capture by protons withsubsequent capture of the resulting neutrons by heavy nuclei.Comment: 10 pages, 4 figures, accepted for publication in A&

A half-a-day long thermonuclear X-ray burst from KS 1731-260