Optical emission of the ultraluminous X-ray source NGC 5408 X-1: donor
  star or irradiated accretion disk?
  We obtained 3 epochs of simultaneous Hubble Space Telescope (HST)/Wide Field
Camera 3 and Chandra observations of the ultraluminous X-ray source NGC 5408
X-1. The counterpart of the X-ray source is seen in all HST filters, from the
UV through the NIR, and for the first time, we resolve the optical nebula
around the ULX. We identified a small OB association near the ULX that may be
the birthplace of the system. The stellar association is young, ~ 5 Myr,
contains massive stars up to 40 solar masses, and is thus similar to
associations seen near other ULXs, albeit younger. The UV/Optical/NIR spectral
energy distribution (SED) of the ULX counterpart is consistent with that of a
B0I supergiant star. We are also able to fit the whole SED from the X-rays to
the NIR with an irradiated disk model. The three epochs of data show only
marginal variability and thus, we cannot firmly conclude on the nature of the
optical emission.
