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We comment on the recent observation of a 115-day modulation in the X-rayflux of the ultraluminous X-ray source (ULX) NGC 5408 X-1, and in particular,the interpretation of this modulation as the orbital period. We suggest thatthis modulation may instead be due to a precessing jet, and is thussuperorbital in nature. Comparing the properties of this ULX with those of theprototypical microquasar SS 433, we argue that NGC 5408 X-1 is very similar toSS 433: a hyper-accreting stellar mass black hole in a shorter-period binary.If the analogy holds, the 115-day modulation is best explained by the stillpoorly-understood physics of inner-disc/jet precession and a longer observingbaseline would be able to reveal an intrinsic phase jitter that is associatedwith such a precession.

Interpretation of the 115 Day Periodic Modulation in the X-ray Flux of NGC 5408 X-1