Interpretation of the 115 Day Periodic Modulation in the X-ray Flux of
  NGC 5408 X-1
  We comment on the recent observation of a 115-day modulation in the X-ray
flux 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 that
this modulation may instead be due to a precessing jet, and is thus
superorbital in nature. Comparing the properties of this ULX with those of the
prototypical microquasar SS 433, we argue that NGC 5408 X-1 is very similar to
SS 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 still
poorly-understood physics of inner-disc/jet precession and a longer observing
baseline would be able to reveal an intrinsic phase jitter that is associated
with such a precession.
