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We report results from long-term (approximately 1240 days) X-ray (0.3-8.0keV) monitoring of the ultraluminous X-ray source NGC 5408 X-1 with theSwift/X-Ray Telescope. Here we expand on earlier work by Strohmayer (2009) whoused only a part of the present data set. Our primary results are: (1) thediscovery of sharp, quasi-periodic, energy-independent dips in the X-rayintensity that recur on average every 243 days, (2) the detection of anenergy-dependent (variability amplitude decreases with increasing energy),quasi-sinusoidal X-ray modulation with a period of 112.6+-4 days the amplitudeof which weakens during the second half of the light curve and (3) spectralevidence for an increase in photoelectric absorption during the last continuoussegment of the data. We interpret the X-ray modulations within the context ofbinary motion in analogy to that seen in high-inclination accreting X-raybinaries. If correct, this implies that NGC 5408 X-1 is in a binary with anorbital period of 243+-23 days, in contrast to the 115.5 day quasi-sinusoidalperiod previously reported by Strohmayer (2009). We discuss the overall X-raymodulation within the framework of accretion via Roche lobe overflow of thedonor star. In addition, if the X-ray modulation is caused by verticallystructured obscuring material in the accretion disk, this would imply a highvalue for the inclination of the orbit. A comparison with estimates fromaccreting X-ray binaries suggests an inclination > 70 degrees. We note that, inprinciple, a precessing accretion disk could also produce the observed X-raymodulations.

Evidence for Quasi-periodic X-Ray Dips from an Ultraluminous X-Ray Source: Implications for the Binary Motion