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