Probing the nature of ultraluminous X-ray sources through fast (a few milliseconds) and slow (a few years) X-ray variability
Abstract
The X-ray point sources in nearby galaxies with luminosities exceeding the Eddington limiting value of a 20 solar mass black hole (> 3x10^39 ergs/sec) are referred to as ultraluminous X-ray sources (ULXs). Currently, it is unclear whether these sources are powered by super-Eddington accretion onto stellar-mass black holes (mass range of 3-50 solar mass) or if they are intermediate-mass black holes (IMBHs: mass range of a few 100-1000 solar mass) accreting at sub-Eddington rates. In my thesis, I studied the X-ray quasi-periodic variability and long-term modulations of a sample of variable ULXs. My thesis work consists of two parts: (1) understanding the nature of the mHz quasi-periodic oscillations (QPOs) from ULXs (fast variability) and (2) search for and study the properties of the long X-ray periods from ULXs (slow variability). (1) A sample of ULXs exhibit X-ray QPOs in the frequency range of 10-200 mHz. These QPOs have been argued to be analogous to the type-C low-frequency QPOs of stellar-mass black holes (frequency range of 1-15 Hz) but occurring at longer timescales owing to the presence of IMBHs within these systems. We tested this hypothesis by searching for a correlation between the QPO's centroid frequency and the power-law index of the energy spectrum which is a characteristic property of the type-C QPOs of stellar-mass black holes. Using roughly 1 Mega-second of XMM-Newton data from ULXs NGC 5408 X-1 and M82 X-1 we find that the two quantities are NOT correlated. This suggests that one of the strongest arguments for existence of IMBHs in ULXs may be flawed. I will discuss the implications of this result on the masses of the black holes within these systems. (2) Swift/XRT and RXTE/PCA have monitored a sample of ULXs on timescales of 3-6 years. We analyzed all the archival data of these sources to search for periodicities that might represent the orbital motion of the black hole binary. We detected periodicities in some ULXs. I will discuss the various properties of the detected modulations including their coherence, energy dependence, phase-resolved spectroscopy, time-resolved periodograms that will allow us to directly probe the accretion geometries of these systems.
- Publication:
-
American Astronomical Society Meeting Abstracts #223
- Pub Date:
- January 2014
- Bibcode:
- 2014AAS...22342201D