A Photoionized Nebula Surrounding and Variable Optical Continuum Emission from the Ultraluminous X-Ray Source in NGC 5408

Abstract

We obtained optical spectra of the counterpart of the ultraluminous X-ray source NGC 5408 X-1 using the FORS spectrograph on the VLT. The spectra show strong high excitation emission lines, He{\sc ii} $\lambda$4686 and [Ne{\sc V}] $\lambda$3426, indicative of X-ray photoionization. Using the measured X-ray spectrum as input to a photoionization model, we calculated the relation between the He{\sc ii} and X-ray luminosities and found that the He{\sc ii} flux implies a lower bound on the X-ray luminosity of $3\times 10^{39}\mathrm{erg}{\mathrm{s}}^{-1}$. The [Ne{\sc 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 $-2.0_{-0.2}^{+0.1}$. 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{\sc ii} $\lambda$4686, and [Ne{\sc V}] $\lambda$3426 fluxes are about 30% lower than in the other five observations. This implies that part of the line emission originates within one light-day of the compact object. Fitting the optical continuum emission and archival X-ray data to an irradiated disk model, we find that $(6.5\pm 0.7)\times 10^{-3}$ 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.

accepted by ApJ, 8 pages

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• X-ray Properties of Black-Hole Binaries
• Super-critically accreting stellar-mass black holes as ultraluminous X-ray sources
• Reprocessing of X-rays in the outer accretion disc of the black hole binary XTE J1817--330

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