A superburst from GX 3+1

I found one long X-ray flare from the X-ray burster GX 3+1 in almost 6 years of observations with the RXTE All Sky Monitor (ASM). The event had a peak flux of about 1.1 Crab (1.5-12 keV), lasted between 4.4 and 16.2 hours and exhibited a fluence of more than about 5x10^{41} erg for a source distance of 5 kpc. During the exponential-like decay, with an exponential decay time of 1.6 hours, spectral softening is seen. The total ASM effective exposure time on GX 3+1 is estimated to be around a year. The flare bears all the characteristics of the recently discovered so-called superbursts in other X-ray burst sources.