Ultraluminous X-ray sources with flat-topped noise and QPO. (arXiv:1904.04905v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Atapin_K/0/1/0/all/0/1">K. Atapin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fabrika_S/0/1/0/all/0/1">S. Fabrika</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Caballero_Garcia_M/0/1/0/all/0/1">M. D. Caballero-Garcia</a>
We analyzed the X-ray power density spectra of five ultraluminous X-ray
sources (ULXs) NGC5408 X-1, NGC6946 X-1, M82 X-1, NGC1313 X-1 and IC342 X-1
that are the only ULXs which display both flat-topped noise (FTN) and
quasi-periodic oscillations (QPO). We studied the QPO frequencies, fractional
root-mean-square (rms) variability, X-ray luminosity and spectral hardness. We
found that the level of FTN is anti-correlated with the QPO frequency. As the
frequency of the QPO and brightness of the sources increase, their fractional
variability decreases. We propose a simple interpretation using the
spherizarion radius, viscosity time and $alpha$-parameter as basic properties
of these systems. The main physical driver of the observed variability is the
mass accretion rate which varies >3 between different observations of the same
source. As the accretion rate decreases the spherization radius reduces and the
FTN plus the QPO move toward higher frequencies resulting in a decrease of the
fractional rms variability. We also propose that in all ULXs when the accretion
rate is low enough (but still super-Eddington) the QPO and FTN disappear.
Assuming that the maximum X-ray luminosity depends only on the black hole (BH)
mass and not on the accretion rate (not considering the effects of either the
inclination of the super-Eddington disc nor geometrical beaming of radiation)
we estimate that all the ULXs have about similar BH masses, with the exception
of M82 X-1, which might be 10 times more massive.
We analyzed the X-ray power density spectra of five ultraluminous X-ray
sources (ULXs) NGC5408 X-1, NGC6946 X-1, M82 X-1, NGC1313 X-1 and IC342 X-1
that are the only ULXs which display both flat-topped noise (FTN) and
quasi-periodic oscillations (QPO). We studied the QPO frequencies, fractional
root-mean-square (rms) variability, X-ray luminosity and spectral hardness. We
found that the level of FTN is anti-correlated with the QPO frequency. As the
frequency of the QPO and brightness of the sources increase, their fractional
variability decreases. We propose a simple interpretation using the
spherizarion radius, viscosity time and $alpha$-parameter as basic properties
of these systems. The main physical driver of the observed variability is the
mass accretion rate which varies >3 between different observations of the same
source. As the accretion rate decreases the spherization radius reduces and the
FTN plus the QPO move toward higher frequencies resulting in a decrease of the
fractional rms variability. We also propose that in all ULXs when the accretion
rate is low enough (but still super-Eddington) the QPO and FTN disappear.
Assuming that the maximum X-ray luminosity depends only on the black hole (BH)
mass and not on the accretion rate (not considering the effects of either the
inclination of the super-Eddington disc nor geometrical beaming of radiation)
we estimate that all the ULXs have about similar BH masses, with the exception
of M82 X-1, which might be 10 times more massive.
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