  INTEGRAL detected on August 31, 2004, an unusual thermonuclear X-ray burst
from the low-mass X-ray binary GX 3+1. Its duration was 30 min, which is
between the normal burst durations for this source (<~10 s) and the superburst
observed in 1998 (several hours). We see emission up to 30 keV energy during
the first few seconds of the burst where the bolometric peak luminosity
approaches the Eddington limit. This peculiar burst is characterized by two
distinct phases: an initial short spike of ~6 s consistent with being similar
to a normal type I X-ray burst, followed by a remarkable extended decay of
cooling emission. We discuss three alternative schemes to explain its twofold
nature: 1) unstable burning of a hydrogen/helium layer involving an unusually
large amount of hydrogen, 2) pure helium ignition at an unusually large depth
(unlikely in the present case), and 3) limited carbon burning at an unusually
shallow depth triggered by unstable helium ignition. Though none of these
provide a satisfactory description of this uncommon event, the former one seems
the most probable.
