2004A&A...423..955S

We have studied an ultra-luminous X-ray source (ULX) in the dwarf galaxy NGC5408 with a series of XMM-Newton observations, between 2001 July and 2003 January. We find that its X-ray spectrum is best fitted with a power law of photon index Γ≃2.6-2.9 and a thermal component with blackbody temperature kT_bb≃0.12-0.14keV. These spectral features, and the inferred luminosity ≃10⁴⁰erg/s in the 0.3-12keV band, are typical of bright ULXs in nearby dwarf galaxies. The blackbody plus power-law model is a significantly better fit than either a simple power law or a broken power law (although the latter model is also acceptable at some epochs). Doppler-boosted emission from a relativistic jet is not required, although we cannot rule out this scenario. Our preliminary timing analysis shows flaring behaviour which we interpret as variability in the power-law component, on timescales of ∼10²s. The hard component is suppressed during the dips, while the soft thermal component is consistent with being constant. The power density spectrum is flat at low frequencies, has a break at ν_b≃2.5mHz, and has a slope ≃-1 at higher frequencies. A comparison with the power spectra of Cyg X-1 and of a sample of other BH candidates and AGN suggests a mass of ∼10²M_☉. It is also possible that the BH is at the upper end of the stellar-mass class (M∼50 M_☉), in a phase of moderately super-Eddington accretion. The formation of such a massive BH via normal stellar evolution may have been favoured by the very metal-poor environment of NGC 5408.