Periodic Thermonuclear X‐Ray Bursts from GS 1826−24 and the Fuel Composition as a Function of Accretion Rate

Abstract

We analyze 24 type I X-ray bursts from GS 1826À24 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 AE 0:13 hr. The persistent intensity of GS 1826À24 increased by 36% between 1997 and 2000, by which time the burst interval had decreased to 4:10 AE 0:08 hr. In 2002 July the recurrence time was shorter again, at 3:56 AE 0:03 hr. The bursts within each epoch had remarkably identical light curves over the full %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 À1:05AE0:02 (assuming that ˙ M is linearly proportional to the X-ray flux), so that the ratio between the integrated persistent and burst fluxes was inversely correlated with ˙ M . The average value of was 41:7 AE 1:6. Both the -value and the long burst durations indicate that the hydrogen is burning during the burst via the rapid-proton (rp) process. The variation in with ˙ M implies that hydrogen is burning stably between bursts, requiring solar metallicity (Z $ 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 $ 0:001) reproduce the observed trends in recurrence time and fluence but are ruled out by the variation in . 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 .