1020018

Thermonuclear bursts in systems accreting pure helium reveal the structure of the neutron star (NS) interiors. In the absence of heating from steady H-burning between bursts the burst recurrence times and energetics are much more sensitive to the thermal properties and processes in the NS crust and core. Interestingly burst ignition models predict significantly longer burst recurrence times than observed for such sources. One way to resolve this discrepancy is fractional covering of the accreted fuel on the NS surface and recent analysis of a large sample of bursts from 4U 172834 appears to lend support to this hypothesis. However a large variation in the properties of bursts observed at similar accretion rates suggests that other processes perhaps including steady He-burning or incomplete burning may also be important.We propose to observe 4U 172834 with INTEGRAL and XMM-Newton simultaneously in order to measure the burst recurrence time and the persistent X-ray flux (and hence the accretion rate) in the broadest possible band to distinguish between these possibilities. The precise accretion rate measured over a wide multi-instrument energy band is crucial as it will enable detection of additional soft (or hard) spectral components outside the sensitivity band of individual detectors. The results of this study will provide stringent constraints for modeling of burst ignition conditions. [truncated! Please see actual data for full text]