1. Department of Astronomy, University of Padova Department of Physics, University of Padova INAF-Astronomical Observatory of Padova Paola Mucciarelli In collaboration with: Zampieri, Turolla, Treves, Falomo, Chieregato, Kaaret & di Stefano. The UltraLuminous X-ray source NGC 1313 X-2

2. NGC1313 X-2: X-ray properties  NGC1313 is a nearby spiral galaxy hosting two ULXs, NGC1313 X-1 and X-2, and the interacting supernova 1978K.  NGC1313 X-2 is a well known ULX, variable on a timescale of months.  Assuming that at maximum the source radiates at the Eddington limit, a mass of 50 M  is derived for the compact remnant. NGC1313 X-2 NGC1313 X-1 MODEST-6, August 29-31, 2005 ♣The XMM spectrum shows statistical evidence for a MCD component with kT=0.2 keV (Miller et al. 2003,Zampieri et al. 2004) M BH ~ 90f 4 M 

3. NGC1313 X-2: optical counterpart NGC 1313 X-2 optical counterpart was identified in an ESO 3.6m R band image, using the Chandra position with an uncertainty of 0.7  (Zampieri et al., 2004). In archive ESO VLT (BVR) band images the counterpart is resolved in two distinct point-like sources, C1 and C2, separated by 0.7”(Mucciarelli et al., in prep.). Taking into account Galactic absorption: C1: R=23.5, (B-V) 0 =-0.2 (V-R) 0 =-0.2 C2: R=23.4, (B-V) 0 >1.0 (V-R) 0 =0.4 (R-I) 0 =0.2 (from Liu et al. 2005) MODEST-6, August 29-31, 2005 Both object are nor an AGN (absence of redshifted emission lines and very high f X /f R ) neither foreground stars. C1: BO-O9 main sequence star of 20M  in NGC 1313 C2: G supergiant of about 10M  in NGC 1313

4. Conclusions MODEST-6, August 29-31, 2005 Two optical counterparts are detected (on VLT images) inside the Chandra error box of the ULX NGC 1313 X-2. Irrespectively of wich of the two objects is the actual counterpart, NGC 1313 X-2 appears to be a high mass X-ray binary with a massive donor star. At the present stage, there seems to be no compelling argument to rule out any of the two object C1 and C2 as possible counterpart. Follow up optical spectroscopy under optimal seeing conditions and with the slit along the line of conjunction C1-C2 is crucial to disentangle the emission from the two objects. Theoretical modelling may also help to distinguish the counterpart (Patruno & Zampieri, in prep.)