Eddington-limited X-ray bursts as distance indicators. II. Possible compositional effects in bursts from 4U 1636-536

We analyzed 123 thermonuclear (type I) X-ray bursts observed by the Rossi X-Ray Timing Explorer (RXTE) from the low-mass X-ray binary 4U 1636-536. All but two of the 40 radius expansion bursts in this sample reached peak fluxes normally distributed about a mean of 6.4 × 10^-8 ergs cm ^-2 s^-1, with a standard deviation of 7.6%. The remaining two radius-expansion bursts reached peak fluxes a factor of 1.69 ± 0.13 lower than this mean value; as a consequence, the overall variation in the peak flux of the radius-expansion bursts was a factor of ≈2. This variation is comparable to the range of the Eddington limit between material with solar H fraction (X = 0.7) and pure He. Such a variation may arise if, for the bright radius-expansion bursts, most of the accreted H is either eliminated by steady hot CNO burning or expelled in a radiatively driven wind. However, steady burning cannot exhaust the accreted H for solar composition material within the typical ≈2 hr burst recurrence time, nor can it result in sufficient elemental stratification to allow selective ejection of the H only. An additional stratification mechanism appears to be required to separate the accreted elements and thus allow preferential ejection of the hydrogen. We found no evidence for a gap in the peak flux distribution between the radius-expansion and non-radius-expansion bursts, previously observed in smaller samples. Assuming that the faint radius-expansion bursts reached the Eddington limit for H-rich material (X ≈ 0.7), and the brighter bursts the limit for pure He (X = 0), we estimate the distance to 4U 1636-536 (for a canonical neutron star with M_NS = 1.4 M_⊙, R_NS = 10 km) to be 6.0 ± 0.5 kpc, or for M_NS = 2 M_⊙ at most 7.1 kpc.