2009ApJ...697..950K

We obtained optical spectra of the counterpart of the ultraluminous X-ray source NGC 5408 X-1 using the FORS spectrograph on the Very Large Telescope. The spectra show strong high-excitation emission lines, He II λ4686 and [Ne V] λ3426, indicative of X-ray photoionization. Using the measured X-ray spectrum as input to a photoionization model, we calculated the relation between the He II and X-ray luminosities and found that the He II flux implies a lower bound on the X-ray luminosity of 3x10³⁹ erg/s. The [Ne V] flux requires a similar X-ray luminosity. After subtraction of the nebular emission, the continuum appears to have a power-law form with a spectral slope of -2.0^+0.1_–0.2. This is similar to low-mass X-ray binaries where the optical spectra are dominated by reprocessing of X-rays in the outer accretion disk. In one observation, the continuum, He II λ4686, and [Ne V] λ3426 fluxes are about 30% lower than in the other five observations. This implies that part of the line emission originates within 1 lt-day of the compact object. Fitting the optical continuum emission and archival X-ray data to an irradiated disk model, we find that (6.5±0.7)x10^–3 of the total bolometric luminosity is thermalized in the outer accretion disk. This is consistent with values found for stellar-mass X-ray binaries and larger than expected in models of super-Eddington accretion flows. We find no evidence for absorption lines that would permit measurement of the radial velocity of the companion star.