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

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.

arXiv comment5 pages, 1 figure, Accepted for publication in ApJ

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• Optical emission of the ultraluminous X-ray source NGC 5408 X-1: donor star or irradiated accretion disk?
• A long-term X-ray monitoring of the ultraluminous X-ray source NGC 5408 X-1 with Swift reveals the presence of dips but no orbital period
• An accreting stellar binary model for active periodic fast radio bursts
• Comparing Spectral Models for ULXs with NGC 4517 ULX1
• Outbursts of the intermediate-mass black hole HLX-1: a wind instability scenario
• Broad Components in Optical Emission Lines from the Ultra-Luminous X-ray Source NGC 5408 X-1
• Evidence for Quasi-periodic X-Ray Dips from an Ultraluminous X-Ray Source: Implications for the Binary Motion
• Towards a dynamical mass of the ultraluminous X-ray source NGC 5408 X-1
• Long-term X-ray Variability of Ultraluminous X-ray Sources
• Temporal evolution of long-timescale periodicities in ULX NGC 5408 X-1
• Investigating the nature of the ultraluminous X-ray sources in the galaxy NGC 925
• Confirming the 115.5-day periodicity in the X-ray light curve of ULX NGC 5408 X-1
• X-ray reverberation models of the disc wind in ultraluminous X-ray source NGC 5408 X-1
• Monitoring the Very-Long-Term Variability of X-ray Sources in the Giant Elliptical Galaxy M87
• The black hole in NGC 1313 X-2