Ultraluminous X-ray sources with flat-topped noise and QPO

Abstract

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.

15 pages, 7 figures, accepted for publication in MNRAS

References in corpus (19)

• An Ultraluminous X-ray Source Powered by An Accreting Neutron Star
• Super-critically accreting stellar-mass black holes as ultraluminous X-ray sources
• An accreting pulsar with extreme properties drives an ultraluminous x-ray source in NGC 5907
• Discovery of a 0.42-s pulsar in the ultraluminous X-ray source NGC 7793 P13
• Discovery of coherent pulsations from the Ultraluminous X-ray Source NGC 7793 P13
• XMM-Newton observations of the brightest Ultraluminous X-ray sources
• The critical accretion luminosity for magnetized neutron stars
• A spectral-timing model for ULXs in the super-critical regime
• X-ray observations of ultraluminous X-ray sources
• Pulsing ULXs: tip of the iceberg?
• Pulsator-like spectra from Ultraluminous X-ray sources and the search for more ultraluminous pulsars
• Discovery of mHz X-ray Oscillations in a Transient Ultraluminous X-ray Source in M82
• Quasi Periodic Oscillations and Strongly Comptonized X-ray emission from Holmberg IX X-1
• A radiation-hydrodynamic model of accretion columns for ultra-luminous X-ray pulsars
• Discovery of a Quasi-Periodic Oscillation in the Ultraluminous X-ray Source IC 342 X-1: XMM-Newton Results
• Searching for coherent pulsations in ultraluminous X-ray sources
• X-ray variability of SS433: effects of the supercritical accretion disc
• X-ray Variability of SS 433: Evidence for Supercritical Accretion
• Artificial variability in XMM-Newton observations of X-ray sources: M31 as a case study