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A new accretion disc instability and measuring the inner torque of a relativistic disc

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In the first part of the talk I will introduce a disc instability caused by a variable alpha parameter. The alpha parameter is expected from theory and simulations to depend on the detailed plasma properties, in particular, the magnetic Prandtl number. I will show that a variable alpha can lead to a disc instability resulting in limit cycle behaviour and flaring (analogous to the hydrogen ionisation instability in dwarf novae). This new instability is likely to be important in explaining the complex outbursts of X-ray binary systems. In the second part of the talk I will show that the functional form of the high frequency tail of a relativistic Novikov-Thorne disc spectrum depends on the size of the inner torque on the disc. Measuring the inner torque acting on accretion discs is important in understanding the physics in the plunging region and relativistic jet production. I demonstrate this difference numerically using a ray-tracing code utilising adaptive mesh refinement.

This talk is part of the DAMTP Astro Mondays series.

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