Pulse phase-resolved X-ray spectroscopy of the transient high-mass X-ray binary V0332 + 53

We present an analysis of the EXOSAT observations of the transient binary X-ray pulsar, V0332 + 53, obtained during a series of flares in the 1983–4 outburst. For many of these observations, the data were taken in a mode which provided high time resolution and energy resolution simultaneously. This meant that detailed pulse phase-resolved X-ray spectroscopy of the 4.37-s pulsar was possible for the first time. We consider the data in two sets, namely those showing a double peak in the pulse profile and those showing a single peak only. The spectra of both data sets can be adequately fitted in terms of a power law plus exponential cut-off model, and show significant changes in some of the spectral parameters as a function of pulse phase. We interpret our results in terms of a system in which our line of sight is not close to the magnetic axis of the neutron star and in which contributions from two active regions are seen to vary as the outburst evolves. We also investigate one particular data set which was distinct from any of the other observations. This data set had a large modulation in the flux level which is attributable to excess power at low frequencies. This represents a further novel property of V0332 + 53, which is already known to exhibit both pulsations and short-time-scale flicker.