Superbursts at near-Eddington mass accretion rates

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.