Quasi-Periodic Variability in NGC 5408 X-1

Details Quasi-Periodic Variability in NGC 5408 X-1 Quasi-Periodic Variability in NGC 5408 X-1

2007-01-12

arxiv.org/abs/astro-ph/0701390v1

Astrophys.J.660:580-586,2007

Astronomy and Astrophysics

Astrophysics

23 pages, 7 figures. Accepted for publication in the Astrophysical Journal

Scientific paper

10.1086/512723

We report the discovery with XMM-Newton of quasiperiodic variability in the 0.2 - 10 keV X-ray flux from the ultraluminous X-ray source NGC 5408 X-1. The average power spectrum of all EPIC-pn data reveals a strong 20 mHz QPO with an average amplitude (rms) of 9%, and a coherence of Q~6. In a 33 ksec time interval when the 20 mHz QPO is strongest we also find evidence for a 2nd QPO peak at 15 mHz. This is the first indication for a close pair of QPOs in a ULX source. Interestingly, the frequency ratio of this QPO pair is inconsistent with 3:2 at the 3sigma level, but is consistent with a 4:3 ratio. A powerlaw noise component with slope near 1.5 is also present below 0.1 Hz with evidence for a break to a flatter slope at about 3 mHz. The source shows substantial broadband variability, with a total amplitude (rms) of about 30% in the 0.1 - 100 mHz frequency band, and there is strong energy dependence to the variability. We discuss the implications of these findings in the context of models for ULXs, and their implications for the object's mass.

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