Broad Components in Optical Emission Lines from the Ultra-Luminous X-ray
  Source NGC 5408 X-1
  High-resolution optical spectra of the ultraluminous X-ray source NGC 5408
X-1 show a broad component with a width of ~750 km/s in the HeII and Hbeta
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 HeII line. The broad component likely originates in the ULX system itself,
probably in the accretion disk. The central wavelength of the broad HeII line
is shifted by 252 \pm 47 km/s between the two observations. If this shift
represents motion of the compact object, then its mass is less than ~1800
M_sun.
