Relativistic Jets in Active Galactic Nuclei (1812.06025v1)

Abstract: The nuclei of most normal galaxies contain supermassive black holes, which can accrete gas through a disk and become active. These Active Galactic Nuclei, AGN, can form jets which are observed on scales from AU to Mpc and from meter wavelengths to TeV gamma ray energies. High resolution radio imaging and multi-wavelength/messenger campaigns are elucidating the conditions under which this happens. Evidence is presented that: AGN jets are formed when the black hole spins and the accretion disk is strongly magnetized, perhaps on account of gas accretingat high latitude beyond the black hole sphere of influence; AGN jets are collimated close to the black hole by magnetic stress associated with a disk wind; higher power jets can emerge from their galactic nuclei in a relativistic, supersonic and proton-dominated state and they terminate in strong, hot spot shocks; lower power jets are degraded to buoyant plumes and bubbles; jets may accelerate protons to EeV energies which contribute to the cosmic ray spectrum and which may initiate pair cascades that can efficiently radiate synchrotron gamma rays; jets were far more common when the universe was a few billion years old and black holes and massive galaxies were growing rapidly; jets can have a major influence on their environments, stimulating and limiting the growth of galaxies. The observational prospects for securing our understanding of AGN jets are bright.

• The paper presents a comprehensive review of AGN jets, demonstrating that spinning black holes with magnetized accretion disks collimates and accelerates the jets.
• It finds that high-power jets, moving at relativistic speeds, generate shocks and accelerate protons to EeV energies, impacting cosmic ray production.
• The study employs advanced multi-wavelength observations and MHD modeling to uncover the structure and environmental effects of AGN jets.

Overview of "Relativistic Jets in Active Galactic Nuclei" by Blandford, Meier, and Readhead

The paper "Relativistic Jets in Active Galactic Nuclei," authored by Roger Blandford, David Meier, and Anthony Readhead, presents an extensive review of the observational and theoretical understanding of relativistic jets emanating from Active Galactic Nuclei (AGN). The paper explores the formation, dynamics, and influence of these jets, integrating observational data across the electromagnetic spectrum, from radio wavelengths to TeV gamma-ray energies.

Key Findings

The authors compile several critical hypotheses and findings regarding AGN jets:

• Jet Formation and Dynamics: The paper discusses the conditions necessary for jet formation, emphasizing the role of a spinning supermassive black hole surrounded by a strongly magnetized accretion disk. AGN jets are thought to be collimated by magnetic stresses that originate close to the black hole and extend to larger distances.
• Jet Composition: High-power jets are seen emerging in a relativistic, supersonic, and proton-dominated state, terminating in significant hotspot shocks. In contrast, lower-power jets are characterized by their evolution into buoyant plumes and bubbles.
• Cosmic Ray Production: The authors propose that AGN jets can accelerate protons to energies on the order of EeV, thus contributing to the cosmic ray spectrum. Such highly energetic protons may trigger pair cascades that efficiently emit synchrotron gamma rays.
• Cosmological Implications: Historically, jets were more prevalent when the universe was younger, aligning with the rapid growth phase of black holes and massive galaxies. This aligns with the broader evolutionary narrative of galaxies and their central black holes.
• Environmental Impact: AGN jets significantly impact their environment by both stimulating and limiting star formation and galaxy growth. Their influence extends to modifying large-scale structures within galaxies and potentially affecting intergalactic medium dynamics.

Observational Backdrop

Observational insights into AGN jets have expanded significantly with technological advancements in radio imaging and multi-wavelength surveys. Key telescopic advancements, like the Event Horizon Telescope (EHT), have enhanced our understanding of jet origins and behaviors.

• Resolution of Jet Structures: High-resolution radio observations reveal intricate jet structures on various scales, providing insights into the mechanics of jet acceleration and collimation.
• Polarization and Magnetic Fields: Polarization studies have been crucial in inferring the magnetic field configurations within jets, which are pivotal for understanding jet dynamics and stability.
• Gamma-ray Observations: The Fermi Gamma-ray Space Telescope and ground-based Cherenkov telescopes have been central to unearthing the high-energy behaviors and variability of AGN jets.

Discourses on Jet Modeling

The paper discusses various theoretical models for jet formation and propagation, emphasizing scenarios driven by magnetohydrodynamics (MHD). These models attempt to reconcile the observations with the physics of rotating black holes and accretion processes.

• Jet Collimation and Confinement: Magnetic fields are proposed as primary agents for jet collimation near black holes, transitioning to gas pressure dominance at larger scales.
• Energy Dissipation: The paper reflects on how jets dissipate energy from relativistic to subrelativistic flows across different environments, highlighting efficient particle acceleration mechanisms like shocks and magnetic reconnection.

Future Directions and Challenges

The authors outline several future challenges and directions, notably emphasizing the need for:

1. Better understanding of disk magnetic field configurations and their role in jet dynamics.
2. Exploration of the particle acceleration mechanisms that can account for observed high-energy phenomena.
3. Comprehensive modeling efforts that combine kinetic particle simulations with conventional MHD approaches.

The paper serves as both a compendium of our current understanding and a call to action for addressing the myriad open questions surrounding AGN jets. It underscores the importance of upcoming observational capabilities and multi-messenger astronomy in deepening our understanding of these extraordinary astrophysical phenomena.