A Search for X-ray Reverberation in the Ultraluminous X-ray Source NGC 5408 X-1

We propose simultaneous optical (VLT/FORS2) and Xray (XMMNewton) observationsof the ultraluminous Xray source (ULX) NGC 5408 X1 (hereafter X1) in order tosearch for the signatures of Xray reprocessing in its accretion disk. Ourproposed observations will enable the first sensitive search for Xray opticalcorrelations in a ULX and will address key questions regarding the nature of theaccretor in X1. For example, detection of an Xray optical correlation, asignature of reprocessing, will establish that the optical flux is largelyproduced in the accretion disk and not the companion star. Measurement of Xrayoptical time lags can lead to a mass estimate via the techniques ofreverberation mapping. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]