X-ray variability and energy spectra from NGC 5408 X-1 with XMM-Newton

The notion of source states characterizing the X-ray emission from black hole binaries has revealed to be a very useful tool to disentangle the complex spectral and aperiodic phenomenology displayed by those classes of accreting objects. We seek to use the same tools for ultraluminous X-ray (ULX) sources. We analysed the data from the longest observations obtained from the ULX source in NGC 5408 (NGC 5408 X-1) taken by XMM-Newton. We performed a study of the timing and spectral properties of the source. In accordance with previous studies on similar sources, the intrinsic energy spectra of the source are well described by a cold accretion disc emission plus a curved high-energy emission component. We studied the X-ray variability of the source and found an anti-correlation between the root-mean-square variability in the 0.0001-0.2 Hz and intensity, similar to that observed in black hole binaries during the hard states. We discuss the physical processes responsible for the X-ray features observed and suggest that NGC 5408 X-1 harbours a black hole accreting in an unusual bright hard intermediate state.