Sam Gralla

I am Associate Professor of Physics at the University of Arizona. I am broadly interested in the physics of strong gravitational and electromagnetic fields and the associated astrophysics of black holes and neutron stars.  Strong fields give rise to theoretical simplification and novel behavior, allowing clean study of important problems.  I use analytic and semi-analytic approaches to identify and/or characterize physical mechanisms of relevance to astronomy.  I also maintain an active research program in pure relativity and bring this “spacetime” perspective to the astronomical problems I study.   I thrive on cross-disciplinary collaboration and am thrilled to join this Simons Collaboration.

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My astrophysical research in the last several years has been divided into two main areas.  First, I have studied the plasma magnetospheres of neutron stars and black holes in the strong-field limit.  When there is sufficient plasma to screen out electric fields, the plasma becomes “force-free”, whereas without screening it becomes “Aristotelian”.  These different simplifying regimes have similar theoretical underpinnings but wildly different observational implications; both may be relevant in real magnetospheres.  Currently, I am keen to find perturbative expansions around these extreme regimes, with unknown coefficients that can be determined by comparison with Particle-In-Cell simulations, hoping that the resulting effective theories can aid in the construction of global magnetosphere models.

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My other area of recent astrophysical research has been black hole imaging, i.e., horizon-scale interferometry of emission from near supermassive black holes.  I have focused on identifying properties of black hole images that are universal (independent of the details of the emitting plasma) and finding methods to extract the associated information about the underlying spacetime from noisy interferometric data.  I believe that successfully disentangling the “gravity” from the “astrophysics” is essential to harnessing the power of this exciting new observational tool.  I am enthusiastic to continue this challenge in the context of the Simons Collaboration.

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Five recent relevant references

 

Cai, Y., Gralla, S. and Paschalidis, V., 2023, "Dynamics of ultrarelativistic charged particles with strong radiation reaction. I. Aristotelian equilibrium state" (accepted to PRD)

 

Lockhart W., and Gralla, S. 2022, "How narrow is the M87* ring? II. A new geometric model", MNRAS 517, 2463-2470 

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Gralla S. and Lupsasca, A. 2020, "Lensing by Kerr Black Holes." Phys. Rev. D 101, 044032

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Gralla S. and Iqbal, N. 2019, "Effective Field Theory of Force-Free Electrodynamics" Phys. Rev. D 99, 105004

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Lockhart W., Gralla, S., Psaltis D. and Ozel, F., 2019,  "X-ray Light Curves from Realistic Polar Cap Models: Inclined Pulsar Magnetospheres and Multipole Fields", MNRAS 490, 1774-1783

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