Reverberation mapping of the disc wind in ultraluminous X-ray source NGC 5408 X-1

Abstract

Recent observational evidences suggested that ultraluminous X-ray sources (ULXs) which are in supercritical accretion rate might release the outflowing wind and change the geometry to slim disc. However, the exact structure of the disc is still unclear. In this work, we develop a simple reverberation model to analyse the X-ray time lags of the ULX NGC 5408 X-1, in order to determine the geometry of the disc and the outflowing wind. The data of NGC 5408 X-1 from the XMM-Newton archive are binned into three groups based on the instrument count rates, i.e. low, medium and high count rates. The lag profiles are then extracted from each group. Fitting the models to the lag data suggests that the wind-launching radius tends to increase with the count rates, which is in agreement with the super-Eddington framework that prefers a stellar mass black hole (sMBH). The wind-launching radius for the sMBH is constrained to be in the order of ∼ 105rg. We also report a degeneracy between the black hole mass and the wind launching radius. A more realistic geometry and model might be required to explain the time lags in NGC 5408 X-1. © Published under licence by IOP Publishing Ltd.