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 $\Gamma \approx 2.6{-}2.9$ and a thermal component with blackbody temperature $kT_{\rm bb} \approx 0.12{-}0.14$ keV. These spectral features, and the inferred luminosity $\approx 10^{40}$ 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 ~ 102 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 $
u_{\rm b} \approx 2.5$ mHz, and has a slope $\approx -1$ 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 ~ $10^2~M_{\odot}$. It is also possible that the BH is at the upper end of the stellar-mass class ( $M \sim 50~M_{\odot}$), 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.