Discovery of Quasi-periodic X-ray Dips from the ULX NGC 5408 X-1: Implications for the Accretion Geometry
Abstract
The nature of ultraluminous X-ray sources (ULXs) remains mysterious. It is not clear whether they are simply scaled-up accreting binaries, i.e., intermediate-mass black holes (mass range of a few 100-1000 solar masses) in binary orbits around companion stars accreting at a sub-Eddington rate or if they are stellar-mass black holes accreting and emitting via a super-Eddington mechanism. Perhaps the ULX population comprises both categories. We present evidence from an archetypal ULX, NGC 5408 X-1 (X-ray luminosity > 10^40 ergs/sec), that the source accretes in a geometry similar to that of Roche lobe overflow binaries. Using the approximately 3.5 years of Swift/X-ray monitoring data of the source we detect two distinct phenomena: (1) a quasi-sinusoidal, energy-dependent (modulation amplitude decreases with increasing energy upto 8 keV) modulation of the X-ray flux with a period of 112 days and (2) quasi-periodic, energy-independent dips in the X-ray intensity that recur on average every 243 days and with a variance of 23 days, suggesting a moderately high inclination. These two modulations (including their phase separation) appear to be consistent with the predicted variations for Roche-lobe accreting binaries with low-mass ratios, q = M(donor)/M(accretor). The smooth component can be produced by absorption/obscuration by two spatially distinct bulges along the outer rim of the accretion disk (produced by the tidal effects of the massive companion star) while the sharp dips are possibly produced by absorption due to clumps of material produced by the accretion stream - disk impact. We present an idealized accretion geometry of ULX NGC 5408 X-1 that is consistent with these modulations.
- Publication:
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AAS/High Energy Astrophysics Division #13
- Pub Date:
- April 2013
- Bibcode:
- 2013HEAD...1340205R