Periodic thermonuclear X-ray bursts from GS 1826-24 and the fuel composition as a function of accretion rate

We analyze 24 type I X-ray bursts from GS 1826 - 24 observed by the Rossi X-Ray Timing Explorer between 1997 November and 2002 July. The bursts observed between 1997 and 1998 were consistent with a stable recurrence time of 5.74 +/- 0.13 hr. The persistent intensity of GS 1826 - 24 increased by 36% between 1997 and 2000, by which time the burst interval had decreased to 4.10 +/- 0.08 hr. In 2002 July the recurrence time was shorter again, at 3.56 +/- 0.03 hr. The bursts within each epoch had remarkably identical light curves over the full approximate to 150 s burst duration; both the initial decay timescale from the peak and the burst fluence increased slightly with the rise in persistent flux. The decrease in the burst recurrence time was proportional to (M) over dot (-1.05 +/- 0.02) (assuming that. (M) over dot is linearly proportional to the X-ray flux), so that the ratio alpha between the integrated persistent and burst fluxes was inversely correlated with. (M) over dot. The average value of alpha was 41.7 +/- 1.6. Both the alpha-value and the long burst durations indicate that the hydrogen is burning during the burst via the rapid-proton (rp) process. The variation in alpha with (M) over dot implies that hydrogen is burning stably between bursts, requiring solar metallicity (Z similar to 0.02) in the accreted layer. We show that solar metallicity ignition models naturally reproduce the observed burst energies but do not match the observed variations in recurrence time and burst fluence. Low-metallicity models (Z similar to 0.001) reproduce the observed trends in recurrence time and fluence but are ruled out by the variation in alpha. We discuss possible explanations, including extra heating between bursts or that the fraction of the neutron star covered by the accreted fuel increases with (M) over dot.