X-ray flares from the ultra-luminous X-ray source in NGC 5408

We have studied an ultra-luminous X-ray source (ULX) in the dwarf galaxy NGC 5408 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 kTbb ≈ 0.12−0.14 keV. These spectral features, and the inferred luminosity ≈ 10 40 erg s −1 in the 0.3−12 keV 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 ∼10 2 s. 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 νb ≈ 2.5 mHz, 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 ∼10 2 M� . 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.