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 \approx 485 days .
Periodogram analysis reveals a significant periodicity with a period of 115.5 \pm 4 days .
The modulation is detected with a significance of 3.2 \times 10 ^ { -4 } .
The fractional modulation amplitude decreases with increasing energy , ranging from 0.13 \pm 0.02 above 1 keV to 0.24 \pm 0.02 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 \approx 2 \times 10 ^ { 40 } ergs s ^ { -1 } .
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 .