A PHOTOIONIZED NEBULA SURROUNDING AND VARIABLE OPTICAL CONTINUUM EMISSION FROM THE ULTRALUMINOUS X-RAY SOURCE IN NGC 5408
- AIM-Unite Mixte de Recherche CEA-CNRS and Universite Paris VII-UMR 7158, CEA Saclay, Service d'Astrophysique, F-491191 Gif sur Yvette (France)
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 {lambda}4686 and [Ne 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 II and X-ray luminosities and found that the He II flux implies a lower bound on the X-ray luminosity of 3 x 10{sup 39} erg s{sup -1}. 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{sup +0.1} {sub -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 {lambda}4686, and [Ne V] {lambda}3426 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) x 10{sup -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.
- OSTI ID:
- 21300545
- Journal Information:
- Astrophysical Journal, Vol. 697, Issue 1; Other Information: DOI: 10.1088/0004-637X/697/1/950; Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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