2009/03/30 by Philip Kaaret, Stephane Corbel, Stéphane Corbel · 2 citations
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysical Phenomena and Observations #Astrophysics and Star Formation Studies #astro-ph.CO #astro-ph.HE
paper · pdf · doi:10.1088/0004-637x/697/1/950
published as Astrophys.J.697:950-956,2009 · accepted by ApJ, 8 pages
arxiv created 2009/03/30 · openalex publication_date 2009/05/05 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/31
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 3 × 10 39 erg s −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 +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) × 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.