Radius-expansion bursts from an enigmatic, episodic burster: 4U 1735-44

It has been predicted that strong (a few hundred eV EW) photoionisation edges might be present in the X- ray spectra at the peak of radius-expansion thermonuclear bursts. We propose to search for such features in bursts from 4U 1735-44, which at times exhibits the most-frequent radius-expansion bursts of any source (approximately once per hour). This observation, to take place during its anticipated (low persistent flux) bursting state, will detect up to 30 bursts, also allowing a detailed comparison of their recurrence time and energetics with numerical ignition models, as well as a high signal-to-noise low-flux spectrum. Detection of discrete features from the burst spectra may lead to constraints on the compactness and hence the