Rakshak Adhikari

Rakshak Adhikari
Force Free Electrodynamics

Astrophysical jets are powerful outflows of ionized matter expelled by rotating compact objects (black holes and neutron stars) along their axis of rotation. These jets are characterized by their high speeds and strong emission in different wavelengths. However, the mechanism of formation and ejection of these jets is not fully understood. Over the course of last several decades, various mechanisms have been formulated to describe these phenomenon. One of them is the Blandford-Znajek mechanism. In this mechanism, electromagnetic fields with vanishing Lorentz force can efficiently extract the rotational energy of a black hole and power these relativistic jets. This theoretical framework is called force-free electrodynamics.

BH Fig. An actively accreting Black Hole.
Credit: Aurore Simonnet and NASA’s Goddard Space Flight Center

Observations show that certain black holes, especially supermassive ones residing at the center of galaxies1, exhibit accretion disk in their equatorial plane. These discs contain magnetized plasma which can harbor powerful electromagnetic fields. In absence of Lorentz Force, these powerful electromagnetic fields are described by the equations of Force-Free Electrodynamics (FFE).

The dynamics of electric and magnetic field is described by four sets of equations called Maxwell’s equations whereas gravity is described by general relativity. Because these compact objects exhibit very strong gravity, general relativistic effects cannot be neglected. Therefore, we have to solve Maxwell’s equation along with the equations of force-free electrodynamics, in a background spacetime. The background space-time here, is the Kerr spacetime which is known to describe the exterior of rotating black holes. Because of the complexity of the set of nonlinear partial differential equations involved, most of the work done in FFE, has been numerical. There are few analytical solutions known but so far no physically reasonable solution has been found that allows for the extraction of energy.

My current work involves finding exact solution to the Equations of FFE in Kerr spacetime using the formalism developed by Menon. I am also studying the full magnetohydrodynamic equations in the Kerr spacetime as well FFE in Robertson-Walker Spacetime.