The ultraluminous X-ray source NGC 1313 X-2

Its optical counterpart and environment

¹ Observatoire Astronomique de Strasbourg, 11 rue de l'Université, 67000 Strasbourg, France e-mail: grise@astro.u-strasbg.fr
² Mullard Space Science Laboratory (UCL), Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
³ Department of Physics and Astronomy, Rice University, 6100 South Main, MS108, Houston, TX 77251-1892, USA
⁴ AngloAustralian Observatory, PO Box 296, Epping, NSW 1710, Australia
⁵ Astrophysical Institute, Department of Physics and Astronomy, Ohio University, Clippinger Hall 251B, Athens, OH 45701-2979, USA

Received: 11 February 2008
Accepted: 18 May 2008

Abstract

NGC 1313 X-2 is one of the brightest ultraluminous X-ray sources in the sky, at both X-ray and optical wavelengths; therefore, quite a few studies of available ESO VLT and HST data have appeared in the literature. Here, we present our analysis of VLT/FORS1 and HST/ACS photometric data, confirming the identification of the B ~ 23 mag blue optical counterpart. We show that the system is part of a poor cluster with an age of 20 Myr, leading to an upper mass limit of some 12  for the mass donor. We attribute the different results with respect to earlier studies to the use of isochrones in the F435W and F555W HST/ACS photometric system that appear to be incompatible with the corresponding Johnson B and V isochrones. The counterpart exhibits significant photometric variability of about 0.2 mag amplitude, both between the two HST observations and during the one month of monitoring with the VLT. This includes variability within one night and suggests that the light is dominated by the accretion disk in the system and not by the mass donor.