Abstract for Proposal 088351

Measuring the properties of the long-duration transient 4U 0836-429

Few low-mass X-ray binaries exhibit thermonuclear (type-I) X-ray bursts with consistent lightcurves and regular recurrence times. Those that do are of high priority to observers, due to their utility in probing thermonuclear processes. We propose target-of-opportunity observations of the long-interval transient 4U 0836-429, in order to comprehensively measure its observational properties. Our highest priority is to make measurements of the burst properties over a range of accretion rates, in order to unambiguously constrain the properties of the accreted fuel as well as the neutron star mass and radius. In the event that the source enters the ``soft'' spectral state, we may expect to observe radius-expansion bursts, from which we can constrain the source distance.

Details on Observing Strategy and Trigger Criteria
Detection of thermonuclear bursts or new transient activity (indicated by persistent intensity sustained above 1E-9 erg/cm^2/s, equivalent to approx. 30 mCrab) by the instruments onboard INTEGRAL or Swift, or the MAXI instrument onboard ISS. Since the target is only 0.3~deg away from the X-ray pulsar GS 0834-430, the two objects are confused by the automatic MAXI light-curve generation software. As a result, MAXI will trigger on activity from either source, but we will use followup observations (e.g. with Swift) to confirm activity from our target prior to triggering the XMM-Newton observations.