OPTICAL EMISSION OF THE ULTRALUMINOUS X-RAY SOURCE NGC 5408 X-1: DONOR STAR OR IRRADIATED ACCRETION DISK?

Abstract

We obtained 3 epochs of simultaneous Hubble Space Telescope (HST)/Wide Field\nCamera 3 and Chandra observations of the ultraluminous X-ray source NGC 5408\nX-1. The counterpart of the X-ray source is seen in all HST filters, from the\nUV through the NIR, and for the first time, we resolve the optical nebula\naround the ULX. We identified a small OB association near the ULX that may be\nthe birthplace of the system. The stellar association is young, ~ 5 Myr,\ncontains massive stars up to 40 solar masses, and is thus similar to\nassociations seen near other ULXs, albeit younger. The UV/Optical/NIR spectral\nenergy distribution (SED) of the ULX counterpart is consistent with that of a\nB0I supergiant star. We are also able to fit the whole SED from the X-rays to\nthe NIR with an irradiated disk model. The three epochs of data show only\nmarginal variability and thus, we cannot firmly conclude on the nature of the\noptical emission.\n