Abstract The majority of X-ray burst sources do not display a burst rate that increases with luminosity as expected, but this is seen in the two clocked bursters XB 1323-619 and GS 1826-24. We present a detailed investigation of these two sources which in the case of the first source, spans 18 years. Based on measurements of the burst rate, X-ray luminosity, the α-parameter and the two time constants generally present in the burst decays, we demonstrate the importance of the rp nuclear burning process. A detailed comparison with theory shows that although the burst rate in each source agrees well with the theoretical value, there is a difference of more than a factor of 5 in the burst rate at a given luminosity between the sources. We show that the main reason for this is that the two sources have substantially different emitting areas on the neutron star in non-burst emission, a factor often neglected. Variation of this area may explain the inverse relation of burst rate with luminosity in the majority of burst sources.

Keywords physical data and processes: accretion: accretion disks stars: neutron stars: individual: XB 1323-619 GS 1826-24 X-rays: binaries

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