0820022

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 properties (the burst recurrence times versus the accretion rate) 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. Our recent analysis of one such source 4U 1728-34 confirms this discrepancy suggesting the fractional covering of the accreted fuel on the NS surface as a possible solution. However a large scatter in our data indicates that other processes such as steady burning of He and fuel storage and mixing due to shear instabilities may also be important. To distinguish between these possibilities the precise accretion rate measurements in a wide multi-instrument energy band are crucial as they will enable to detect additional both soft and hard spectral components outside the sensitivity band of individual detectors. We propose to observe 4U~1728-34 with INTEGRAL and XMM-Newton simultaneously in order to measure the burst recurrence time and X-ray flux (and hence the accretion rate) in a much broader band than previous observations. The results of this study will have profound consequences for facilitating future burst ignition modelling. [truncated! Please see actual data for full text]