An oversized magnetic sheath wrapping around the parsec-scale jet in 3C 273 (2102.04563v1)

Abstract: In recent studies, several AGN have exhibited gradients of the Faraday Rotation Measure (RM) transverse to their parsec-scale jet direction. Faraday rotation likely occurs as a result of a magnetized sheath wrapped around the jet. In the case of 3C 273, using Very Long Baseline Array multi-epoch observations at 5, 8 and 15 GHz in 2009--2010, we observe that the jet RM has changed significantly towards negative values compared with that previously observed. These changes could be explained by a swing of the parsec-scale jet direction which causes synchrotron emission to pass through different portions of the Faraday screen. We develop a model for the jet-sheath system in 3C 273 where the sheath is wider than the single-epoch narrow relativistic jet. We present our oversized sheath model together with a derived wide jet full intrinsic opening angle $\alpha_\mathrm{int}=2.1^\circ$ and magnetic field strength $B_{||}=3$ $\mu$G and thermal particle density $N_\mathrm{e}=125~\mathrm{cm}^{-3}$ at the wide jet--sheath boundary 230 pc downstream (deprojected) from its beginning. Most of the Faraday rotation occurs within the innermost layers of the sheath. The model brings together the jet direction swing and long-term RM evolution and may be applicable to other AGN jets that exhibit changes of their apparent jet direction.

• The paper identifies an oversized magnetic sheath around 3C 273's parsec-scale jet, with significant Faraday Rotation Measure shifts indicating changes in jet geometry.
• It employs multi-epoch, multi-frequency VLBA data to infer a sheath with an intrinsic opening angle of about 2.1° and estimate magnetic fields and particle densities.
• The findings imply that structured, stable magnetic fields in AGN jets are more common than previously thought, influencing interpretations of polarization observations across similar systems.

An Oversized Magnetic Sheath Observed Around the Parsec-Scale Jet in 3C\,273

The paper by Lisakov et al. provides a comprehensive analysis of the parsec-scale jet of the prominent quasar 3C\,273, particularly focusing on the detection and implications of a magnetic sheath surrounding the relativistic jet. Utilizing high-resolution observations from the Very Long Baseline Array (VLBA) across multiple epochs at several frequencies ranging from 4.6 to 15.4 GHz, the authors document significant changes in the Faraday Rotation Measure (RM) across the jet and propose a detailed model for the jet-sheath system.

Key Observations and Results

The observations reveal that the RM within the jet has experienced significant variation, shifting towards more negative values compared to previous observations. This variation correlates with an inferred change in the parsec-scale jet direction since 2003, suggesting that a change in the visible geometry of the jet could allow the synchrotron emission to pass through different regions of a Faraday screen. The authors propose the existence of an "oversized" magnetic sheath around the jet, wider than the jet itself as observed at a single epoch.

The authors develop a model where the sheath has a larger intrinsic opening angle of $\alpha_{\mathrm{int}} = 2.1^\circ$ compared to the narrow, single-epoch observed jet. Magnetic field strength and particle density within this sheath are estimated, with a magnetic field component $B_{||} = 3\:\mathrm{\mu G}$ and a thermal particle density $N_\mathrm{e} = 125\:\mathrm{cm}^{-3}$ at the boundary 230 pc downstream from the core.

Theoretical and Practical Implications

The model presented by Lisakov et al. illustrates an alternative scenario to the traditional understanding of jet polarization, offering insights into the complex structures and suppression factors that characterize the environments of relativistic jets in Active Galactic Nuclei (AGN). The presence of an expansive, low-variability sheath suggests that magnetic fields in the jet environment might be more structured and stable over long distances and timescales than previously considered.

This discovery has practical implications for interpreting polarization observations in AGN jets. Understanding the sheath's role may lead to an improved interpretation of RM gradients observed in other AGNs. The paper posits that 3C\,273 can be a proxy for similar large-scale magnetized structures in other systems. Additionally, future high-resolution polarimetric observations could further clarify the influence of such magnetic sheaths on the propagation and confinement of jets on different scales.

Future Directions

The authors' findings suggest several avenues for further research. Expanding this model to other AGNs exhibiting significant RM evolution can test whether the oversized sheath is a prevalent feature across different systems. Moreover, detailed simulations incorporating relativistic magnetohydrodynamic (GRMHD) codes might provide deeper insights into the dynamics and stability of such sheaths, including their formation and evolution in response to varying jet conditions. Expanding observational campaigns using VLBI and other high-resolution techniques might also enhance our understanding of the interaction between jets and their surrounding media.

In conclusion, the work of Lisakov et al. represents a significant contribution to the paper of AGN jet dynamics by identifying an oversized magnetic sheath, thereby offering a new lens through which to interpret and understand jet-related phenomena.