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 Gamma approximate to 2.6-2.9 and a thermal component with blackbody temperature kT(bb) approximate to 0.12-0.14 keV. These spectral features, and the inferred luminosity approximate to 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 powerlaw component, on timescales of similar to10(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 V(b) approximate to 2.5 mHz, and has a slope approximate to -1 at higher frequencies. A comparison with the power spectra of Cyg X-I and of a sample of other BH candidates and AGN suggests a mass of similar to10(2) M(.). It is also possible that the BH is at the upper end of the stellar-mass class (M similar to 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.