Korea | Sydney | 4U 1636-536
A busy few weeks, with a trip to Seoul to attend the 7th Pacific Rim Conference on Stellar Astrophysics. I gave a talk (PDF file) on accretion-powered millisecond pulsars, highlighting the recent observations of the latest discovery, HETE J1900.1-2455. Returned via Sydney where I visited a collaborator at U. Sydney and caught up with friends and family.
In the meantime, the long awaited paper on the radius-expansion bursts from 4U 1636-536 was finally accepted. In a study of all the bursts observed by RXTE, we found a bimodal distribution of peak radius-expansion burst fluxes, separated by a factor of 1.7. This is exactly what you would expect if the fainter bursts reach the Eddington limit in an atmosphere containing hydrogen at approximately solar abundances, while the brighter bursts instead occur in a pure helium environment. It remains a mystery exactly how the accreted hydrogen is eliminated from the atmosphere in the brighter radius-expansion bursts.
Read the paper