On Lorentz Variability of Magnetically Dominated Relativistic Outflows

Abstract

We show that magnetized relativistic outflows can exhibit a relativistic effect in which Lorentz transformation maps spatially extended magnetic field structure into apparent temporal variability in the observer's frame. Using a force-free Beltrami configurations as representative equilibria of magnetically dominated outflows, we demonstrate that Lorentz mapping of stationary helical magnetic field produces quasi-periodic modulation of observable electromagnetic signatures, without invoking intrinsic plasma variability. The same mechanism naturally generates temporal evolution of the observed polarization properties, producing smooth, generally aperiodic polarization-angle swings in the observer's frame. This effect may be described as an aberration of force-free magnetic fields under Lorentz transformation. The characteristic frequency of the time variability is determined by the helical wave-number of the magnetic field, the viewing angle, and the bulk Lorentz factor of the jet outflow, and scales linearly with $\hat{I}“$. This establishes a purely kinematic relativistic origin of variability and introduces the concept of magnetic Lorentz seismology: the inference of magnetic field structure in relativistic outflows directly from observed temporal variability.

16 pages, 2 figures. Accepted for publication in Physica Scripta