RELATIVISTIC JETS: The Common Physics of AGN, Microquasars, and Gamma-Ray Bursts
856(2006); http://dx.doi.org/10.1063/1.2356381View Description Hide Description
This review discusses the basic characteristics of the jets in active galactic nuclei (AGN). It concentrates on the highly relativistic variety found in blazars, since these are the closest cousins to the jets seen in some X‐ray binaries (microquasars) and the collimated, ultra‐fast flows inferred to exist in gamma ray bursts.
The fastest AGN jets are gamma‐ray bright, with flow Lorentz factors that range up to at least 45, although such high values must be rare unless the radiative efficiency is usually very low. If jets are magnetically launched, observations indicate that the acceleration and collimation occur over an extended region and the Poynting flux is converted into both flow energy and turbulence. Particle domination of the pressure occurs during outbursts, as might be possible in shocks. The cores of AGN jets may have different origins depending on frequency: points where the Doppler factor becomes high, conical recollimation shocks, or the optically thick/thin transition region. Superluminal knots and outbursts are well explained by shocks, but the polarization properties require than in many cases the shock fronts are oblique to the jet axis.
In two radio galaxies, appearances of superluminal knots in the jet follow low‐hard X‐ray states, reminiscent of the behavior of microquasars. Models that relate the accretion state to the geometry of the magnetic field must explain how the transition back to a radiative inner disk releases an ultra‐fast moving energetic disturbance into the jet.
856(2006); http://dx.doi.org/10.1063/1.2356382View Description Hide Description
In this paper I give a brief and subjective review of the current state of knowledge regarding jets from X‐ray binary systems, also known as microquasars. In particular I discuss how the presence of different ‘types’ of jet, as observed in the radio band, appears to relate to the accretion ‘state’ of the source, as revealed by X‐ray spectral and timing properties. Finally, I make some attempt to connect the observed patterns of behaviour to Active Galactic Nuclei.
856(2006); http://dx.doi.org/10.1063/1.2356383View Description Hide Description
Gamma‐Ray Bursts are extreme astrophysical events, which emit the bulk of their energy as photons in the 0.1 – 1.0 MeV range, and whose durations span milliseconds to tens of minutes. They are formed in extreme relativistic outflows with Lorentz factors of hundreds, and reside at cosmological distances. They are followed by X‐ray, optical and radio afterglows which can be observed for over a year after the event. Observations of afterglows showed that the emission is from jets, and when corrected for this geometry the energies of GRBs appear to cluster around 5 × 1050 erg — very comparable to that of supernovae. Evidence in the last several years shows that a significant fraction of long GRBs are related to a peculiar type of supernova explosions. These supernovae most likely mark the birth events of stellar mass black holes as the final products of the evolution of very massive stars. Short bursts are still somewhat mysterious, but it is known that some of them are produced by an old population of stars. Neutron star merger is a leading candidate as the progenitor of short GRBs.
856(2006); http://dx.doi.org/10.1063/1.2356384View Description Hide Description
The basic theoretical stability properties and normal mode structures of astrophysical jets are reviewed. Simulations designed to examine the non‐linear development of instability and some results from comparison between simulations and theory are presented. The potential use of observed normal mode structures in jets is discussed.
856(2006); http://dx.doi.org/10.1063/1.2356385View Description Hide Description
Connections between accretion disks and jets in accreting black holes are anticipated theoretically. In recent years, potential evidence for such connections has been emerging, most vividly in the convenient regime of stellar‐mass black holes. In this contribution, various lines of evidence for disk‐jet connections are briefly examined, from the standpoint of an observer focused on the role of the disk. While many lines of investigation may be promising in the future, obtaining multi‐wavelength lightcurves and correlating jet flux in the radio band with physical parameters and phenomena tied to the accretion disk in X‐rays may be the most direct.
856(2006); http://dx.doi.org/10.1063/1.2356386View Description Hide Description
Since their discovery in the late 1960s, gamma‐ray bursts (GRBs) have established a reputation of being mysterious astrophysical phenomena. This well‐deserved reputation is a result of complex physical processes hidden from observation, and extreme energies previously unobserved. GRB observations have provided measurements of several physical parameters, including information about the energy and the dynamics of these events. From the several decades of observational data, it is widely believed that GRBs have beamed emission, leading to their association with relativistic jets. The progenitor of the observed emission is still unknown, although recent clues have advanced our understanding. Due to observational limitations, we must rely on theoretical studies and computational simulations to further understand the complex physical processes. In this paper we discuss the link between GRB observable parameters and those required for computational simulations. Following the spirit of this conference on AGN, Microquasar and GRB jets, this information is intended for an audience with a wide appreciation of jets, but without detailed knowledge of GRBs.
856(2006); http://dx.doi.org/10.1063/1.2356387View Description Hide Description
Many problems in astrophysics involve relativistic outflows. The plasma dynamics in these scenarios is critical to determine the conditions for the self‐consistent evolution of the fields and particle acceleration. Advances in computer power now allow for kinetic plasma simulations, based on the particle‐in‐cell (PIC) paradigm, capable of providing information about the role of plasma instabilities in relativistic outflows. A discussion of the key issues associated with PIC simulations is presented, along with some the most important results and open questions, with a particular emphasis on the long time evolution of the filamentation, or Weibel, instability, and on the possible collisionless mechanisms for particle acceleration arising in the collision of relativistic plasma shells.
856(2006); http://dx.doi.org/10.1063/1.2356388View Description Hide Description
In recent years we have witnessed the rapid development of new numerical methods for Relativistic Magnetohydrodynamics. It is not going to be long before they become standard computational tools available to any keen researcher interested in relativistic astrophysics. In this paper I provide a very broad and yet brief review that is intended to help those who are not yet expert in the field, but who wish to become one in the future.