DISCOVERY OF A 115 DAY ORBITAL PERIOD IN THE ULTRALUMINOUS X-RAY SOURCE NGC 5408 X-1

We report the detection of a 115 day periodicity in Swift/X-Ray Telescope 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 x 10-4. The fractional modulation amplitude decreases with increasing energy, ranging from 0.13 ± 0.02 above 1 keV to 0.24 ± 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 ∼ 2 x 1040 erg 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.