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D. W. Fox, M. P. Muno, E. H. Morgan (MIT)
We have performed in-burst phase connection of the burst pulsations detected in type I X-ray bursts from the low-mass X-ray binaries KS~1731-260 and Aql~X-1. The train of pulsations in each of the bursts examined (two from KS~1731-260 and one from Aql~X-1) is greater than nine seconds in length and -- accounting for the systematic drift in frequency observed in all cases -- completely coherent for \gtrsim5000 cycles. Phase-connecting the pulsations allows us to characterize the frequency drift in a model-independent fashion: we find that the frequency evolution, in each case, is consistent with a \tau ~3s exponential relaxation to an asymptotic frequency. Exponential fits to the frequency evolution during the two bursts from KS~1731-260, although different in specific details, are remarkably similar; in particular, the stability of the asymptotic pulse frequency over the 2.6 year interval between the bursts sets a 2\sigma upper limit of |\dot{P}| < 3e-14 on the time-average period derivative for these oscillations. We suggest that our results further support the interpretation of the burst pulsations as reflecting the neutron star spin period in these systems.
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