MHD Outflows from Hot Coronae

We discuss the application of meridionally self-similar models to winds and jets from hot coronae, in particular in the central region of accretion disks. A summary of how they may help understanding the evolution of jets from young stars is outlined. Then we discuss their application to the classification of AGN jets and extension to the relativistic regime of these exact axisymmetric solutions. Finally we discuss how it is possible to extend the polytropic equation of state and Parker winds to the relativistic regime to have a simple toy model for understanding thermal acceleration.     

Relativistic Jets from Accretion Disks

The jets observed to emanate from many compact accreting objects may arise from the twisting of a magnetic field threading a differentially rotating accretion disk which acts to magnetically extract angular momentum and energy from the disk. Two main regimes have been discussed, hydromagnetic jets, which have a significant mass flux and have energy and angular momentum carried by both matter and electromagnetic field and, Poynting jets, where the mass flux is small and energy and angular momentum are carried predominantly by the electromagnetic field. Here, we describe recent theoretical work on the formation of relativistic Poynting jets from magnetized accretion disks. Further, we describe new relativistic, fully electromagnetic, particle-in-cell (PIC) simulations of the formation of jets from accretion disks. Analog Z-pinch experiments may help to understand the origin of astrophysical jets.     

Distributed intelligence in an astronomical Distributed Sensor Network

The Telescope Alert Operations Network System (TALONS) was designed and developed in the year 2000, around the architectural principles of a distributed sensor network. This network supported the original Rapid Telescopes for Optical Response (RAPTOR) project goals; however, only with further development could TALONS meet the goals of the larger Thinking Telescope Project. The complex objectives of the Thinking Telescope project required a paradigm shift in the software architecture – the centralised intelligence merged into the TALONS network operations could no longer meet all of the new requirements. The intelligence needed to be divorced from the network operations and developed as a series of peripheral intelligent agents, distributing the decision making and analytical processes based on the temporal volatility of the data. This paper is presented as only one part of the poster from the workshop and in it we will explore the details of this architecture and how that merges with the current Thinking Telescope system to meet our project goals. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Reviving the stalled shock by jittering jets in core collapse supernovae: jets from the standing accretion shock instability

I present a scenario by which an accretion flow with alternating angular momentum on to a newly born neutron star in a core collapse supernova(CCSN) efficiently amplifies magnetic fields and by that launches jets. The accretion flow of a collapsing core on to the newly born neutron star suffers spiral standing accretion shock instability(SASI). This instability leads to a stochastically variable angular momentum of the accreted gas, which in turn forms an accretion flow with alternating directions of the angular momentum, and hence alternating shear, at any given time. I study the shear in this alternating-shear sub-Keplerian inflow in published simulations, and present a new comparison with Keplerian accretion disks. From that comparison I argue that it might be as efficient as Keplerian accretion disks in amplifying magnetic fields by a dynamo. I suggest that although the average specific angular momentum of the accretion flow is small,namely, sub-Keplerian, this alternating-shear accretion flow can launch jets with varying directions, namely,jittering jets. Neutrino heating is an important ingredient in further energizing the jets. The jittering jets locally revive the stalled accretion shock in the momentarily polar directions, and by that they explode the star. I repeat again my call for a paradigm shift from a neutrino-driven explosion of CCSNe to a jet-driven explosion mechanism that is aided by neutrino heating.     

What Drives Molecular Outflows from Young Stars?

After briefly reviewing observations of molecular outflows from young stars, we discuss current ideas as to how they might be accelerated. Broadly speaking it is thought that such outflows represented either deflected accreted gas, or ambient material that has been pushed by a poorly collimated wind or accelerated by a highly collimated jet. Observations tend to favour the latter model, with jets being the clear favourite at least for the youngest flows. Jets from young stars may accelerate ambient gas either through the development of a boundary layer, where ambient and jet material are turbulently mixed, or at the working surface of the jet, i.e. the bow shock, via the prompt entrainment mechanism. Recently, we (Downes and Ray, 1999) have investigated, through simulations, the efficiency of prompt entrainment in jets from young stars as a means of accelerating ambient molecular gas without causing dissociation. Prompt entrainment was found to be very poor at transferring momentum from the jet to its surroundings in both the case of ``heavy' (not surprizingly) but also ``equi-density' (with respect to the ambient environment) jets. Moreover the transfer efficiency decreases with increasing density as the bow shock takes on a more aerodynamic shape. Models, however, in which jets are the ultimate prime movers, do have the advantage that they can reproduce several observational features of molecular outflows. In particular a power law relationship for mass versus velocity, similar to what is observed, is predicted by the simulations and the so-called ``Hubble Law' for molecular outflows is naturally explained. Pulsing of the jet, i.e. varying its velocity, is found to have little effect on the momentum transfer efficiency at least for the dynamically young jets we have studied. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Environment of YSO Jets

It is commonly accepted that stars form in molecular clouds by the gravitational collapse of dense gas. However, it is precisely not the infalling but the outflowing material that is primarily observed. Outflow motions prevail around both low and high mass young stellar objects. We present here results from a family of self-similar models that could possibly help to understand this paradox. The models take into account the heating of the central protostar for the deflection and acceleration of the gas. The models make room for all the ingredients observed around the central objects, i.e. molecular outflows, fast jets, accretion disks and infalling envelopes. We suggest that radiative heating and magnetic field may ultimately be the main energy sources driving outflows for both low and high mass stars. The models show that the ambient medium surrounding the jet is unhomogeneous in density, velocity, magnetic field. Consequently, we suggest that jets and outflows have a prehistory that is inprinted in their environment, and that this should have direct consequences on the setting of jet numerical simulations.     

Study of Coronal Jets During Solar Minimum Based on STEREO/SECCHI Observations

During the 2007 – 2008 minimum of solar activity, the internally occulted coronagraphs SECCHI-COR1 onboard the STEREO space mission recorded numerous jet-like ejections over a great range of latitudes. We have found more than 10000 white-light jets in the above-mentioned period. Sometimes they can be identified on the disk with bright points observed in ultraviolet images by EUVI. In this study we present a catalog consisting of jets observed by the SECCHI-COR1 instrument and their association with lower coronal activity (bright points, UV jets). Furthermore, their association with bright points in the context of previously proposed models is discussed. From the complete catalog we have selected 106 jets observed in both STEREO-A and STEREO-B images for which it is possible to derive their kinematics and point of origin.     

Laboratory Experiments of Stellar Jets from the Perspective of an Observer

It has been two decades since astronomers first discovered that accretion disks around young stars drive highly collimated supersonic jets. Thanks to concerted efforts to understand emission line ratios from jets, we know that velocity variations dominate the heating within these flows, and motions in stellar jets, now observed in real time, are primarily radial. The fluid dynamics of the cooling zones can be complex, with interacting shocks, clumps, and instabilities that could benefit from insights into the physics that only experiments can provide. Recent laboratory experiments have reproduced jets with velocities and Mach numbers similar to those within stellar jets, and the field seems poised to make significant advances by connecting observations and theories with experiments. This article points out several aspects of stellar jets that might be clarified by such experiments.     

Magnetohydrodynamic properties of extragalactic jets

The theory that magnetic fields are instrumental in the formation and propagation of jets in active galactic nuclei dates back four decades. Despite a recent growing consensus on this notion stemming from the results of numerical simulations of magnetohydrodynamic (MHD) flows near black holes, the precise dynamical role of magnetic fields in observed parsec and kiloparsec jets remains uncertain. Some of the unanswered fundamental questions about extragalactic jets include the location where the flow becomes relativistic and where acceleration and collimation terminate, as well as the specifics of how the flow interacts with the ISM. Such observed properties as superluminal motions and wiggled structures based on numerical simulations to constitute the foundation of an MHD paradigm for extragalactic jets. We particularly focus our attention to the M87 jet, which is one of the best candidates to investigate relativistic outflows in extragalactic system.     

On the content of cold electrons in blazar and microquasar jets

We explore the possibility of determining the corpuscular composition of the plasma in the relativistic jets of blazars and microquasars from data on the polarization and intensity of their radio synchrotron emission. We have constructed a universal diagram that allows the relative content of nonrelativistic electrons to be established in specific objects using information about their frequency spectra and polarization at individual frequencies. As a result, we have found that the electron plasma component in the jets of the blazars 3C 279 and BL Lac is relativistic. In the jets of the microquasar GRS 1915+105, the cold plasma density may be comparable to or considerably higher than the relativistic particle density.     

Physical parameters of low-ionization knots and jets in PNe: NGC 7009, K 4-47, and NGC 6543

In addition to the large-scale outflows, which form their round, elliptical, and bipolar shells, planetary nebulae (PNe) also have, usually on smaller scales, pairs of highly collimated outflows, or jets. These jets, as well as the pairs of knots that appear at their tips (very prominent in the low-ionization emission lines), are the subject of the present study. We show our results on the temperatures and densities of jets and knots, compare these physical parameters with those of the main shells of PNe, and compare them with theoretical model predictions. We note particularly that the knots at the tips of the jets are not denser than the jets, and that neither is their emission collisionally excited, as one would expect if they were by-products of the associated supersonic jets.     

SDO Observations of Solar Jets

We present an analysis of high cadence observations of solar jets observed in the Extreme Ultraviolet (EUV), at 304 Å, with the Atmospheric Imaging Assembly instrument aboard the Solar Dynamics Observatory (SDO). The jets in our sample lie very close to the solar limb to minimize projection effects. Two of the events show clear helical patterns during ejection. We also find that some of the jets are recurrent and that most of them cannot overcome solar gravity. We investigate the temporal evolution of the jets by measuring the height of their leading edge as a function of time. By fitting the resulting height–time diagrams, we derive the magnitude of their initial ejection speed and plasma acceleration by assuming ballistic motion. Moreover, we calculate the upward acceleration of the jets based on the dynamical velocity of the plasma, without assuming a ballistic motion. In both models, the acceleration profiles suggest the influence of forces other than gravity. In particular, we find indications of an upwards driving force which weakens the decelerating effect of the solar gravitational field along the motion of the jet. This force is larger in the dynamical model, which indicates that the ballistic approximation does not properly determine the rising motion of the plasma jets.     

Testing the beamed inverse-Compton model for jet X-ray emission: velocity structure and deceleration

By considering a small sample of core-dominated radio-loud quasars with X-ray jets, I show, as has been argued previously by others, that the observations require bulk jet deceleration if all of the X-ray emission is to be explained using the widely adopted beamed inverse-Compton model, and argue that jets even in these powerful objects must have velocity structure in order to reconcile their radio and X-ray properties. I then argue that the deceleration model has several serious weaknesses, and discuss the viability of alternative models for the decline in X-ray/radio ratio as a function of position. Although inverse-Compton scattering from the jets is a required process and must come to dominate at high redshifts, adopting an alternative model for the X-ray emission of some nearby, well-studied objects can greatly alleviate some of the problems posed by these observations for the beamed inverse-Compton model.     

Entrainment and Deceleration of Relativistic Jets

Extragalactic radio sources are separated in two classes according to their specific luminosity: Fanaroff-Riley I and II. The origin of this dichotomy can be due either to intrinsec different properties of the AGN or to interaction of the jet with different enviroments. We assume that jets are always relativistic and supersonic close to their source, as recent observations suggest, and we explore the conditions in which the jets decelerate assuming FR I morphology. We have carried out high resolution 3D simulations for a set of parameters and in this paper we concentrate our discussion on two extreme cases.     

Does the plasma composition affect the long-term evolution of relativistic jets?

We study the influence of the matter content of extragalactic jets on their morphology, dynamics and emission properties. For this purpose we consider jets of extremely different compositions, including pure leptonic and baryonic plasmas. Our work is based on two-dimensional relativistic hydrodynamic simulations of the long-term evolution of powerful extragalactic jets propagating into a homogeneous environment. The equation of state used in the simulations accounts for an arbitrary mixture of electrons, protons and electron–positron pairs. Using the hydrodynamic models, we have also computed synthetic radio maps and the thermal bremsstrahlung X-ray emission from their cavities.
Although there is a difference of about three orders of magnitude in the temperatures of the cavities inflated by the simulated jets, we find that both the morphology and the dynamic behaviour are almost independent of the assumed composition of the jets. Their evolution proceeds in two distinct epochs. During the first one, multidimensional effects are unimportant and the jets propagate ballistically. The second epoch starts when the first larger vortices are produced near the jet head, causing the beam cross-section to increase and the jet to decelerate. The evolution of the cocoon and cavity is in agreement with a simple theoretical model. The beam velocities are relativistic  ( Γ ≃4)  at kiloparsec scales, supporting the idea that the X-ray emission of several extragalactic jets may be due to relativistically boosted CMB photons. The radio emission of all models is dominated by the contribution of the hotspots. All models exhibit a depression in the X-rays surface brightness of the cavity interior, in agreement with recent observations.     

Magnetic-Tower Jet Solution for Launching Astrophysical Jets

In spite of the large number of global three-dimensional (3-D) magnetohydrodynamic (MHD) simulations of accretion disks and astrophysical jets, which have been developed since 2000, the launching mechanisms of jets is somewhat controversial. Previous studies of jets have concentrated on the effect of the large-scale magnetic fields permeating accretion disks. However, the existence of such global magnetic fields is not evident in various astrophysical objects, and their origin is not well understood. Thus, we study the effect of small-scale magnetic fields confined within the accretion disk. We review our recent findings on the formation of jets in dynamo-active accretion disks by using 3-D MHD simulations. In our simulations, we found the emergence of accumulated azimuthal magnetic fields from the inner region of the disk (the so-called magnetic tower) and also the formation of a jet accelerated by the magnetic pressure of the tower. Our results indicate that the magnetic tower jet is one of the most promising mechanisms for launching jets from the magnetized accretion disk in various astrophysical objects. We will discuss the formation of cosmic jets in the context of the magnetic tower model.     

High-Speed x-ray Jets Associated with the 18 June 1999 Limb Flares

Peculiar high-speed X-ray jets associated with the 18 June 1999 limb flares have directly been observed with the soft X-ray Telescope (SXT) aboard Yohkoh. The jets have a much shorter lifetime (within 200 s) and a much larger velocity (∼ 1700 km s⁻¹) than the previously reported jets. Judged from their large velocity, the kinetic energy of the jets is approximately one order of magnitude larger than the thermal energy content, which is far different from the jets previously reported by other researchers. Here we present the preliminary results and discuss the particular features of the jets. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1013324126229     

Dynamic confinement of jets by magnetotorsional oscillations

Many quasars and active galactic nuclei (AGN) appear in radio, optical and X-ray maps as bright nuclear sources from which emerge single or double long, thin jets. When observed with high angular resolution, these jets show evidence of structure, with bright knots separated by relatively dark regions. High percentages of polarization, sometimes more then 50 per cent, indicate the non-thermal nature of the radiation, which is well explained as the synchrotron radiation of the relativistic electrons in an ordered magnetic field.
A strong collimation of jets is probably connected with ordered magnetic fields. The mechanism of magnetic collimation first suggested by Bisnovatyi-Kogan et al. was based on the initial charge separation, which led to the creation of an oscillating electrical current, which in turn produced an azimuthal magnetic field, preventing jet expansion and disappearance. Here we consider magnetic collimation associated with the torsional oscillations of a cylinder with an elongated magnetic field. Instead of initial blobs with charge separation, we consider a cylinder with a periodically distributed initial rotation around the cylinder axis. The stabilizing azimuthal magnetic field is created by torsional oscillations, meaning that charge separation is unnecessary. An approximate simplified model is developed, and an ordinary differential equation is derived and solved numerically, making it possible to estimate quantitatively the range of parameters for which jets may be stabilized by torsional oscillations.     

Observations of Rotating Jets of Carbon Monoxide in Comet Hale–Bopp with the Iram Interferometer

CO was observed on March 11, 1997 in comet Hale–Bopp with theIRAM Plateau de Bure interferometer. The maps show evidence for asymmetrical patterns, due to the Existence of CO jets. Analysis of the spectra and their velocity shifts shows that there is a spiral CO jet rotating in a plane almost perpendicular to the sky plane.This is the first time that rotating jets are observed for parent molecules.We have developed a 3-D model simulating rotating spiral jets of CO gas.We present here the comparison between the observations and our model.     

Energization of interstellar media and cosmic ray production by jets from X-ray binaries

Drawing on recent estimates of the power of jets from X-ray binary systems as a function of X-ray luminosity, combined with improved estimates of the relevant  log( N )–log( L _X)  luminosity functions, we calculate the total energy input to the interstellar medium (ISM) from these objects. The input of kinetic energy to the ISM via jets is dominated by those of the black hole systems, in contrast to the radiative input, which is dominated by accreting neutron stars. Summing the energy input from black hole jets L _J in the Milky Way, we find that it is likely to correspond to ≥1 per cent of L _SNe, the time-averaged kinetic luminosity of supernovae, and ≥5 per cent of L _CR, the cosmic ray luminosity. Given uncertainties in jet power estimates, significantly larger contributions are possible. Furthermore, in elliptical galaxies with comparable distributions of low mass X-ray binaries, but far fewer supernovae, the ratio   L _J/ L _SNe  is likely to be larger by a factor of ∼5. We conclude that jets from X-ray binaries may be an important, distributed, source of kinetic energy for the ISM in the form of relativistic shocks, and as a result are likely to be a major source of cosmic rays.