Gamma-ray astronomy

Abstract:

Abstract The physics governing the morphology of radio-loud AGN jets is not fully understood. We investigate how magnetization, flickering jet power and their interplay affects the morphology of radio galaxies. We present a grid of relativistic magnetohydrodynamic simulations using the PLUTO code covering constant and variable jets with two levels of magnetisation. We find that the constant high magnetisation jets can lead to highly asymmetrical cocoon morphologies, whilst the variable high magnetisation jet can exhibit a broken morphology, caused by a discontinuous jet beam. Our work highlights the importance of magnetisation and variability on the stability and resulting morphology of radio-loud AGN jets, suggesting both are significant factors in addition to jet power or environment. Furthermore, we show that the interaction between magnetisation and variability can lead to the development of localised kink instabilities along the jet beam. Finally, we discuss the effects of hydrodynamic mixing in low magnetisation jets and the role of viewing angle dependence in comparisons between our simulations and observed sources. To facilitate this comparison we present a library of simulated radio images at different times in the simulations and from various viewing angles, which highlight a diverse set of complex morphologies.

Abstract:

We present the results from an extensive broadband (radio to X-rays) observing campaign of the engine-driven Type Ib SN 2012au in the first 13 yr of evolution. The early time (δt ≤ 190 days) radio and X-ray evolution is well described by conventional models of a forward shock interacting with a wind-like circumstellar medium (CSM; ρCSM ∝ r−2). However, starting at δt ≈ 6.7 yr, we detect a significant radio rebrightening. This late-time emission is dominated by a luminous component characterized by a broad and rapidly evolving spectral peak and a shallow optically thin spectral slope, Fν ∝ ν−0.31 ± 0.02. These properties imply a compact emitting region (R ≲ 1016 cm) expanding at a remarkably slow velocity (vs ≲ 500 km s−1) into a high-density environment (ne ≥ 104 cm−3), accompanied by a hard electron power-law index p ≈ 1.6. No soft or hard X-ray emission is detected at any epoch, indicating that high-energy radiation is either strongly absorbed or intrinsically absent. In the context of aspherical shock–CSM interaction models, these observations imply extreme properties of the CSM (geometry, density, and total mass) that lack clear astrophysical motivation. Instead, we show that the emergence of radiation from a newborn pulsar wind nebula (PWN) naturally explains the radio spectral evolution and high-energy limits, where the emission is governed by the adiabatic expansion of a relic pair plasma. We conclude that SN 2012au represents the most compelling candidate for a young, newborn PWN discovered to date, a scenario that can be directly tested with pending very long baseline interferometry observations.