  Models for superbursts from neutron stars involving carbon shell flashes
predict that the mass accretion rate should be anywhere in excess of one tenth
of the Eddington limit. Yet, superbursts have so far only been detected in
systems for which the accretion rate is limited between 0.1 and 0.25 times that
limit. The question arises whether this is a selection effect or an intrinsic
property. Therefore, we have undertaken a systematic study of data from the
BeppoSAX Wide Field Cameras on the luminous source GX 17+2, comprising 10 Msec
of effective observing time on superbursts. GX 17+2 contains a neutron star
with regular Type-I X-ray bursts and accretes matter within a few tens of
percents of the Eddington limit. We find four hours-long flares which
reasonably match superburst characteristics. Two show a sudden rise (i.e.,
faster than 10 s), and two show a smooth decay combined with spectral
softening. The implied superburst recurrence time, carbon ignition column and
quenching time for ordinary bursts are close to the predicted values. However,
the flare decay time, fluence and the implied energy production of (2-4) x
10^17 erg/g are larger than expected from current theory.
