X-ray and optical variability of the ultraluminous X-ray source NGC 1313 X-2

We present an analysis of recent XMM-Newton and HST archive data of the ultraluminous X-ray source NGC 1313 X-2. Quasi-simultaneous observations taken with XMM-Newton, HST, and VLT allow us to study both the X-ray light curve and its correlation with the optical emission of the two proposed counterparts of the ultraluminous X-ray source (ULX). At the end of 2003 December the source experienced a short but intense flare, reaching a maximum luminosity of ~ 1040 ergs s-1. At the same time, the optical flux of both the suggested counterparts did not show pronounced variations (<~30%). Assuming that the ULX emission is isotropic and taking X-ray reprocessing into account, the optical data for one of the proposed counterparts are consistent with its being an early-type, main-sequence star of ~10-18 Msolar losing matter through Roche lobe overflow onto a ~120 Msolar black hole at an orbital separation corresponding to a period of ~2 days.