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We report the detection of a 115 day periodicity in SWIFT/XRT monitoring datafrom the ultraluminous X-ray source (ULX) NGC 5408 X-1. Our ongoing campaignsamples its X-ray flux approximately twice weekly and has now achieved atemporal baseline of ~485 days. Periodogram analysis reveals a significantperiodicity with a period of 115.5 +- 4 days. The modulation is detected with asignificance of 3.2 e-4. The fractional modulation amplitude decreases withincreasing energy, ranging from 0.13 above 1 keV to 0.24 below 1 keV. The shapeof the profile evolves as well, becoming less sharply peaked at higherenergies. The periodogram analysis is consistent with a periodic process,however, continued monitoring is required to confirm the coherent nature of themodulation. Spectral analysis indicates that NGC 5408 X-1 can reach 0.3 - 10keV luminosities of ~2 e40 ergs/s. We suggest that, like the 62 day period ofthe ULX in M82 (X41.4+60), the periodicity detected in NGC 5408 X-1 representsthe orbital period of the black hole binary containing the ULX. If this is truethen the secondary can only be a giant or supergiant star.

Discovery of a 115 Day Orbital Period in the Ultraluminous X-ray Source NGC 5408 X-1