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

TL;DR

The paper discusses whether the 115-day X-ray flux modulation in NGC 5408 X-1 is due to orbital motion or jet precession, proposing the latter as a superorbital phenomenon similar to SS 433.

Contribution

It challenges the orbital period interpretation by suggesting jet precession as the cause, drawing parallels with SS 433 to explain the modulation.

Findings

The 115-day modulation may be due to jet precession, not orbital motion.

NGC 5408 X-1 is similar to SS 433 in its accretion and jet properties.

Long-term observations could reveal phase jitter indicative of precession.

Abstract

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.