Timing variability of Vela X-1 during a bright flare

Abstract

We present an in-depth analysis of the temporal behaviour of the HMXB Vela X-1 which exhibited strongly varying flux levels during an observation by XMM-Newton. During the 100 ks observation the source went from being in a highly absorbed initial state, to one of increased activity which was followed by a giant X-ray flare, before finally reaching a settled state with low absorption. The lower absorption state allows the normally absorbed pulse profile below 1 keV to be studied. Vela X-1 also showed multiple changes in the structure of the pulse profile including a phase lag with the harder photons leading the softer ones. A change in the pulse period, significant at approximately the 2σ level, was observed at the beginning of the powerful flare and is likely related to the large accreted mass. For comparison purposes, an earlier XMM-Newton observation in a lower active state is used to put the variations in pulse profile and phase lag in context.