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Black Hole Accretion Disc Variability And The Requirements For Disc Truncation

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posted on 31.07.2019, 11:23 by Charles Field
The unstable nature that is observed in most X-ray Binaries (XRBs), leads to the disc transitioning between different luminosity states. In the high-soft state, the accretion disc is thought to extend to the inner most stable circular orbit (ISCO), however in the low-hard state the disc is thought to truncate and be replaced by an advection dominated accretion flow. Attempts to identify the point of transition between these two disc geometries has produced conflicting results. In the first part of this thesis, I analyse close to a decade of XMM-Newton observations of the transient XRB system GX 339􀀀4. I outline a new approach to the disc truncation debate, whereby I assume the inner disc radius is fixed at the ISCO, in both the hard and soft states. I highlight a new mass constraint upon the XRB system and present a novel count-rate slicing technique for the spectral analysis. I find only limited evidence to suggest that a fixed inner disc radius is an inadequate assumption, when allowing the emissivity value q and colour correction factor fcol to vary. In the final part of this thesis, I consider the steady state XRB LMC X􀀀3, which entered a 3 month period of quiescence in 2012. This period of quiescence was deeper than that previously observed in its 40 year observational history. I investigated this drop into quiescence using a 1D disc instability model, where I examined the variability in the secondary accretion rate required to replicate the observed light-curve. The modelled time to enter quiescence was found to be over an order of magnitude greater than that observed. This appears to result from inadequate cooling in the disc, which may be resolved by solving the vertical disc equations.



Wynn, Graham; Vaughan, Simon

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Department of Physics and Astronomy

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University of Leicester

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