Testing the rotating hotspot model using X-ray burst oscillations from 4U 1636-536

Precise and accurate measurements of neutron star masses and radii would provide valuable information about the still uncertain properties of cold matter at supranuclear densities. One promising approach to making such measurements involves an analysis of the X-ray flux oscillations often seen during thermonuclear (type 1) X-ray bursts. These oscillations are almost certainly produced by emission from hotter regions on the stellar surface modulated by the rotation of the star. One consequence of the rotation is that the oscillation should appear earlier at higher photon energies than at lower energies. Ford found compelling evidence for such a hard lead in the tail oscillations of one type 1 burst from Aql X-1. Subsequently, Muno, Ozel & Chakrabarty analysed oscillations in the tails of type 1 bursts observed using the Rossi X-ray Timing Explorer. They found significant evidence for variation of the oscillation phase with energy in 13 of the 51 oscillation trains they analysed and an apparent linear trend of the phase with energy in six of nine average oscillation profiles produced by folding the energy-resolved oscillation waveforms from five stars and then averaging them in groups. In four of these nine averaged energy-resolved profiles, the oscillation appeared to arrive earlier at lower energies than at higher energies. Such a trend is inconsistent with a simple rotating hotspot model of the burst oscillations and, if confirmed, would mean that this model cannot be used to constrain the masses and radii of these stars and would raise questions about its applicability to other stars. We have therefore re-analysed individually the oscillations observed in the tails of the four type 1 bursts from 4U 1636-536 that, when averaged, provided the strongest evidence for a soft lead in the analysis by Muno et al. We have also analysed the oscillation observed during the superburst from this star. We find that the data from these five bursts, treated both individually and jointly, are fully consistent with a rotating hotspot model. Unfortunately, the uncertainties in these data are too large to provide interesting constraints on the mass and radius of this star.