Retardation magnification and the appearance of relativistic jets

TL;DR

This paper clarifies how relativistic effects like retardation magnification influence the interpretation of observational data from relativistic jets, resolving discrepancies in de-beaming estimates and emphasizing the importance of spatial considerations.

Contribution

It distinguishes between fixed-source and fixed-projection scenarios, providing a clearer framework for interpreting jet observations and addressing existing literature inconsistencies.

Findings

Retardation magnification affects apparent jet structures.

Discrepancies in magnetic field estimates are due to misinterpretation of beaming effects.

Most jet regions are hidden in the observer's image due to relativistic effects.

Abstract

Thanks to the availability of high-resolution high-sensitivity telescopes such as the Very Large Array, the Hubble Space Telescope, and the Chandra X-ray Observatory, there is now a wealth of observational data on relativistic jets from active galactic nuclei (AGN) as well as galactic sources such as Black-Hole X-ray Binaries. Since the jet speeds cannot be constrained well from observations, but are generally believed to be relativistic, physical quantities inferred from observables are commonly expressed in terms of the unknown beaming parameters: the bulk Lorentz factor and the line-of-sight angle, usually in their combination as relativistic Doppler factor. This paper aims to resolve the discrepancies existing in the literature about such "de-beaming" of derived quantities, in particular regarding the minimum-energy magnetic field estimate. The discrepancies arise because the distinction is not normally made between the case of a fixed source observed with different beaming parameters and the case where the source projection on the sky is held fixed. The former is usually considered, but it is the latter that corresponds to interpreting actual jet observations. Furthermore, attention is drawn to the fact that apparent superluminal motion has a spatial corollary, here called "retardation magnification", which implies that most parts of a relativistic jet that are actually present in the observer's frame (a "world map" in relativity terminology) are in fact hidden on the observer's image (the "world picture" in general, or "supersnapshot" in the special case of astronomy).