Janosz Dewberry

PhD candidate
University of Cambridge Department of Applied Maths and Theoretical Physics

About

I am a PhD student studying astrophysical fluid dynamics at Cambridge University, in the Department of Applied Mathematics and Theoretical Physics. I work with Dr. Henrik Latter and Professor Gordon Ogilvie to explore the dynamics of oscillations and instabilities in astrophysical accretion disks. Accretion disks are both ubiquitous in astrophysics, and essential to many of the phenomenon observed in our universe. Appearing around young stars, compact objects like black holes, and even Saturn, astrophysical disks provide a host of interesting problems for theoreticians to consider. I focus in particular on magnetized flows, using magnetohydrodynamic models and simulations to investigate the effects of magnetic fields on accretion disk dynamics.

My research

My current research focuses on diskoseismic oscillations in astrophysical accretion disks. Much of the previous work in this area has been purely hydrodynamic. In contrast, I focus on the modification of diskoseismic modes by magnetic fields, which are almost certain to play large role in driving the dynamics of most accretion disks.

Learn more about my research here:

Trapped inertial waves

So-called `High-frequency quasi-periodic oscillations' (HFQPOs) observed in the emission from black hole binary systems provide a potential probe into the structure of strongly curved space-time, but are still poorly understood. One explanation offered is that the oscillations are cause by diskoseismic trapped inertial waves (r-modes) confined by relativistic effects to a `self-trapping region.' In the un-magnetized, hydrodynamic theory, r-modes provide an attractive model: their frequencies can be directly related to the mass and spin of the central black hole, and they are readily excited by warping or eccentric deformations in the disk.

An illustrative contour-plot showing the pressure perturbation (in radius and height above/below the disk) associated with a trapped inertial wave in the inner regions of a black hole accretion disk (with spin parameter a=0.5).

Trapped inertial waves' natures are changed by magnetic fields, however, which provide restoring forces through magnetic tension and pressure. In my PhD I have worked to precisely quantify the effects of magnetic fields with different geometries on r-modes through a semi-analytical approach, while also running numerical simulations to investigate the resonant excitation of r-modes through coupling with very eccentric streamlines in the flow.

An exagerated illustration of a magnetohydrodynamic r-mode's distortion of a magnetic field threading the disk purely perpendicularly.

Magnetorotational instability

The magnetorotational instability (MRI) provides a widely accepted explanation for turbulent accretion in astrophysical disks. However, although studied extensively in local simulations, numerical capabilities have only recently allowed for the dynamics of MRI turbulence to be explored on a global scale. I am interested in how the MRI might drive, damp or otherwise interact with other waves and instabilities theorized to occur on large scales in astrophysical accretion flows.

A slice showing the radial magnetic field perturbation associated with a large-scale MRI mode in the linear stage of growth.

Selected publications

Contact

  • Email

    jwd43(at)cam.ac.uk
  • Address

    Churchill College, Storey's Way
    Cambridge, CAM, CB3 0DS
    UK