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A Multi-epoch Timing and Spectral Study of the Ultraluminous X-Ray Source NGC 5408 X-1 with XMM-Newton

We present results of new XMM-Newton observations of the ultraluminous X-ray source (ULX) NGC 5408 X-1, one of the few ULXs to show quasi-periodic oscillations (QPO). We detect QPOs in each of four new (~ 100 ks) pointings, expanding the range of frequencies observed from 10 - 40 mHz. We compare our results with the timing and spectral correlations seen in stellar-mass black hole systems, and find that the qualitative nature of the timing and spectral behavior of NGC 5408 X-1 is similar to systems in the steep power-law state exhibiting Type-C QPOs. However, in order for this analogy to quantitatively hold we must only be seeing the so-called saturated portion of the QPO frequency - photon index (or disk flux) relation. Assuming this to be the case, we place a lower limit on the mass of NGC 5408 X-1 of > 800 solar masses. Alternatively, the QPO frequency is largely independent of the spectral parameters, in which case a close analogy with the Type-C QPOs in stellar systems is problematic. Measurement of the source's timing properties over a wider range of energy spectral index is needed to definitively resolve this ambiguity. We searched all the available data for both a broad Fe emission line as well as high frequency QPO analogs (0.1 - 1 Hz), but detected neither. We place upper limits on the equivalent width of any Fe emission feature in the 6 - 7 keV band, and of the amplitude (rms) of a high frequency QPO analog of approximately 10 eV and approximately 4%, respectively.