The effect of the mass accretion rate on the burst oscillations in 4U 1728-34

We present a comprehensive study of the properties of nearly coherent brightness oscillations in a large sample of type I X-ray bursts observed from the low-mass X-ray binary 4U 1728-34. We have analyzed 547 ks of data of this source obtained with the Rossi X-Ray Timing Explorer over a 3 yr span from 1996 to 1999. The data contain 38 bursts, 16 of which show oscillations. We find no burst oscillations present when the inferred mass accretion rate of the system is lowest. Furthermore, we define a measure of the strength of the oscillations and find that this integrated strength increases with increasing mass accretion rate. This correlation is particularly evident within bursts detected only a few weeks apart and becomes less clear for bursts separated in time by several months to years. The correlation we find for 4U 1728-34 between the burst oscillations and the inferred accretion rate of the system is similar to that found for KS 1731-260 by Muno and coworkers, where the burst oscillations are only present at relatively high mass accretion rates. However, unlike the case for KS 1731-260, we find an anticorrelation in the bursts of 4U 1728-34 between the existence of episodes of photospheric radius expansion and the inferred mass accretion rate of the system. Moreover, we distinguish between burst oscillations present in the rise and in the decay phases of the burst and find that the bursts with oscillations only in the decay phase occur at intermediate accretion rates while those with oscillations in both the rise and the decay phases are concentrated at high inferred mass accretion rates. We discuss these results in the context of the theory of thermonuclear bursts and propose intrinsic differences in the neutron stars in KS 1731-260 and 4U 1728-34 as the origin of the different behaviors.