2011ApJ...728L...5C

High-resolution optical spectra of the ultra-luminous X-ray source (ULX) NGC 5408 X-1 show a broad component with a width of ∼750 km/s in the He II and Hβ lines in addition to the narrow component observed in these lines and [O III]. Reanalysis of moderate-resolution spectra shows a similar broad component in the He II line. The broad component likely originates in the ULX system itself, probably in the accretion disk. The central wavelength of the broad He II line is shifted by 252±47 km/s between the two observations. If this shift represents motion of the compact object, then its mass is less than ∼1800 M_☉.