ULX Ngc 5408 X-1: : Coupled Spectral-timing Behavior Similar To Accreting Galactic Bhs And Agn ?
Abstract
The ultraluminous X-ray source (ULX) NGC 5408 X-1 is unique. It is one of the only few ULXs known to show quasi-periodic X-ray variability. The qualitative nature of its X-ray variability is consistent with stellar mass black holes in the Steep Power-law (SPL) state; where the power spectral density rises below a certain frequency, then flattens and often shows a quasi periodic oscillation (QPO) on the rising portion. However, the characteristic frequencies seen in X-1 are scaled down by a factor of ∼ 100. Further, the soft X-ray excess (0.2 - 2.0 keV) present in the X-ray spectrum can be modeled with a disk black body, perhaps suggesting the presence of a cool disk (∼ 0.15 keV). Simple scaling of the characteristic frequencies noted in the power spectra, and scaling of the disk temperature imply a mass of ∼ > few 100 M ⊙ for the black hole. Hence, X-1 is a strong candidate for an intermediate mass black hole (100 - a few 1000 M⊙). However, this mass estimate is sensitive to the assumption that X-1 is in the SPL state, and is further complicated by the fact that the disk temperature scaling is applicable for systems whose total flux is dominated by emission from the disk. We present results from 700 ksecs of XMM Newton observations of X-1, including 420 ksecs of new exposure, which further explore the timing and spectral properties of X-1 and allow additional tests of the assumption that X-1 accretes like an SPL state black hole. We also present our results in the context of previously established "variability plane" correlations for both Galactic black holes and AGN. We discuss the implications of our results on mass estimates for X-1.
- Publication:
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AAS/High Energy Astrophysics Division #12
- Pub Date:
- September 2011
- Bibcode:
- 2011HEAD...12.1322P