The production of transient megagauss kilotesla [kT] fields in laser-induced explosions is probably due to inertial forces but may still turn out to be relevant to accretion flows. Large magnetic Reynolds numbers are more difficult to achieve in HED experiments because of the large magnetic diffusivity of these plasmas, though values of Re M of about 10 2 to 10 3 seem feasible. The instability has been found in two-dimensional hydrodynamic simulations and has been shown to give rise to a small number 3 to 5 of long-lived vortices that give outward transport of angular momentum.
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These vortices may have a key role in planet formation in disks around young stars. These present us with some of the most visually captivating images encountered in astronomy and astrophysics. In some of the most active quasar sources, the electromagnetic emissions extend from the radio about 10 8 hertz Hz to extreme gamma-. Recent observations, such as the Hubble Space Telescope images of HH 30, and theoretical and simulation studies support models where the twisting of an ordered magnetic field threading an accretion disk acts to magnetically accelerate the jets.
The power in the jets is thought to come from matter accretion in the disk, but it may include power extracted electromagnetically from a spinning black hole. There are two main regimes:. The hydromagnetic regime, w here energy and angular momentum are carried by both the electromagnetic field and the kinetic flux of matter, is relevant to the jets from young stellar objects; and.
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The Poynting flux regime, where energy and angular momentum from the disk are carried dominantly by the electromagnetic field, is relevant to extragalactic and microquasar jets and possibly to gamma-ray burst sources. The theory of the origin of hydromagnetic outflows from accretion disks has been developed by many authors, starting from seminal work carried out in the early s. MHD simulations axisymmetric —first done using a Lax-Wendroff method— have greatly increased the physical understanding of these outflows.
Stationary MHD outflows have been discovered using a Godunov-type method.
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These stationary solutions have the five constants of the motion predicted by ideal magnetohydrodynamics. Within the simulation region, the outflows are observed to accelerate from thermal speed in the disk to a much larger asymptotic poloidal flow velocity of the order of 0.
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This asymptotic velocity is much larger than the local escape speed and is larger than the fast magnetosonic speed by a factor of about 1. The acceleration distance for the outflow, over which the flow accelerates from about 0 to, say, 90 percent of the asymptotic speed, occurs at a flow distance of about 80 r i. The flows are approximately spherical outflows, with only small collimation within the simulation region.
This lack of collimation conflicts with the common notion that the jets collimate due to the pinching force associated with the axial current flow. Laboratory experiments on supersonic hydromagnetic jets relevant to understanding astrophysical jets can be done using conical, linear, and radial wire arrays in a Z-pinch facility, and the interaction of these flows—both collisional and collisionless—with target plasmas and with counter-propagating flows can be studied.
In collisional systems, one can study the region of kinetic energy deposition,. Reducing the target density allows the study of the collisionless penetration of jets into target plasmas. Indeed, experiments on high-Mach-number, hydrodynamic jets and radiatively collapsing jets have been demonstrated both on lasers and on pulsed-power facilities, with internal Mach numbers of 5 to 10 for purely hydrodynamic jets, and Mach numbers as high as 50 to 60 for strongly radiatively cooled jets.
Finally, a new class of very energetic, electrically neutral proton jets has been observed on ultrahigh-intensity, short-pulse lasers. The mechanism behind their formation is still being debated, but their existence at energies of up to MeV is well established. Poynting jets have been discovered in axisymmetric MHD simulations of the opening of magnetic-field loops threading a Keplerian disk. A typical initial disk magnetic-field configuration is a dipolelike field threading the disk where the field lines are in meridian planes B r B z in cylindrical coordinates.
Appropriate diagnostics would allow measurements of the collimation and energy outflow of such a diode. Exciting challenges in future magnetohydrodynamic simulation studies are understanding highly relativistic jets or Poynting flows with Lorentz factors of about 10 that are observed to emanate from some active galaxies, and the jets in gamma-ray burst sources that may have Lorentz factors of about Recent progress has been made in the development of robust relativistic magnetohydrodynamic codes, including the Kerr metric of a rotating black hole.
Ultimately, we would like to know the following:. Is there a universal jet formation mechanism operative for stellar, microquasar, active galaxy, and gamma-ray burst jets? Can the central engine dynamics, energetics, and history be inferred from the jet morphology? A further puzzle is the nature of the acceleration of the radiating particles electrons, and possibly positrons that give rise to the observed synchrotron radiation from jets.
It has been long recognized that this acceleration must be in situ in at least some jets such as those of the quasar 3C and the giant elliptical galaxy M87 : the very high energy up to x-ray of the synchrotron radiation implies radiative lifetimes much shorter than the travel time from the central source. Lepton acceleration may occur in collisionless shocks distributed along the length of the jet. Alternative ideas such as particle acceleration in sporadic reconnection events have also been discussed.
Thus, a remaining puzzle for jets is—. Physics seeks to determine how matter interacts with the nonbaryonic components of the universe—ranging from photons to neutrinos to yet more exotic forms of matter. Here again, recent results from astrophysics have led us into new and unusual domains in which these interactions can be studied. One of the most exciting areas of modern astrophysics is understanding the nature of accretion-powered compact objects.
Their enormous luminosities are thought to result from the energy conversion of matter falling into supermassive black holes at the center of galaxies. The intense x-ray emissions from these compact objects produce photoionized plasma conditions in the infalling accretion disk. One of the goals of high-energy astrophysics is to understand the dynamics of these black-hole accretion disk systems.
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The most promising observational tool is high-resolution spectroscopy of the emerging x-ray emissions. Two new space-based x-ray observatories currently in orbit, Chandra and XMM, are acquiring high-quality data of just such x-ray emissions. Turning these data into a better understanding of the dynamics of accreting black holes, however, will require a better understanding of photoionized plasmas, both in equilibrium and in nonequilibrium conditions.
Computer models for photoionized plasmas exist, but these complex codes differ in their predictions and have not been directly validated owing to a lack of.
Modern Z-pinches and large lasers are now capable of generating intense bursts of photoionizing x rays. Experiments on the Z-machine have shown that astrophysically relevant equilibrium photoionized plasmas can be created and diagnosed.
It may also be possible to access photoionized plasma conditions on large laser facilities. These new HED laboratory capabilities will allow complex x-ray photoionization theories and models to be tested under relevant conditions, thereby serving as a critical component in the effort to understand the dynamics of accreting black holes. Gamma-ray bursts are among the greatest enigmas in contemporary astrophysics. Detected at a rate of more than one per day from random directions in the sky, GRBs have typical burst durations of a few seconds, but signal variability as short as about 1 ms, at photon energies of 0.
At least some of the GRBs are at cosmological distances of several billion light-years, and their total source energies of 10 51 to 10 53 ergs per burst appear to be emitted from very compact sources. Their power law spectral shape is often interpreted as suggesting that the source plasma is optically thin to the radiation observed. The fireball scenario is the most widely discussed model of GRBs. Here, an initial release of about 10 52 ergs of energy into a volume of spatial extent about 10 7 cm creates a relativistically hot fireball of photons and leptons, with a small admixture of baryons.
Spectroscopic diagnostics of astrophysical plasmas
This initial fireball of leptons and photons at an initial temperature of 1 to 10 MeV expands relativistically; the gamma rays are produced by synchrotron or inverse Compton radiation from Fermi-accelerated electrons in optically thin shocks in the fireball. A small admixture of baryons is also accelerated to relativistic velocities, thereby transferring the fireball thermal energy to the kinetic energy of the radially expanding baryons.
The baryons sweep into the interstellar medium ISM , creating a system of a forward shock and several reverse shocks, with the observed GRB emission coming from the reverse shocks. The much-longer-lived x-ray afterglow then comes from the forward shock in the ISM, and the prompt optical emission from the corresponding reverse shock. In this scenario, the GRB can be thought of as a relativistic supernova remnant. The shocks producing the gamma rays must occur after the fireball has emerged from the stellar envelope and later, also an x-ray, optical, radio afterglow, as the fireball decelerates by sweeping up an increasing amount of matter encountered.
The preferred escape. These binary mergers would also lead to a central black hole plus a shorter-lived accreting torus, and a probably less collimated MHD or pair-dominated jet along the rotation axis. The difference between the two classes of bursts will be further probed with spacecraft such as Swift and GLAST, now under construction. As the fireball expands, inelastic nuclear collisions are expected when the n and p fluids decouple and their relative drift velocity becomes comparable to the speed of light. These, interacting with turbulently generated magnetic fields, lead to nonthermal gamma rays and, subsequently, in the jet deceleration phase, to an x-ray, optical, and radio afterglow, which has served to pinpoint the location of dozens of bursts and their distances.
The interaction of waste heat bubbles or a decaying jet with the stellar envelope or with external debris can also lead to characteristic x-ray line spectra, in addition to a power law continuum. Interesting issues of shock physics arise in connection with the GRB fireball radiation mechanism. While these are, in a sense, time-averaged shock properties, specifically time-dependent effects would be expected to affect the electron energy distribution and photon spectral slopes, leading to time-integrated observed spectra that could differ from those in the simple time-averaged picture.
The back-reaction of protons accelerated in the same shocks and magnetic fields may also be important, as in supernova remnants. Turbulence may be important for the electron-proton energy exchange, while reactions leading to neutrons and vice versa can influence the escaping proton spectrum. New experiments under construction, such as the Pierre Auger Observatory array, will provide much stronger constraints on whether GRBs are associated with such events. Protons accelerated in internal shocks in the buried jet while it advances through the star interact with thermal x rays to produce teraelectronvolt neutrinos, for which the detection probability is maximized in cubic kilometer Cherenkov detectors such as ICECUBE or ANTARES.
In the laboratory, the most promising means for accessing these relativistic plasma dynamics and flows are with experiments done on ultraintense, short-pulse lasers see Figure 2. Another intriguing observation in these ultraintense laser experiments was the generation of ultrastrong magnetic fields. Wilks, Lawrence Livermore National Laboratory.
Such extreme conditions, albeit over small volumes and exceedingly short times, may overlap with aspects of the relativistic fireball dynamics thought to occur in gamma-ray bursts. Pairs can then be created when the relativistic electrons interact with high-Z target ions via the trident and Bethe-Heitler processes. This demonstrates that petawatt lasers are indeed capable of producing copious pairs and has led to a proposal to use double-sided illumination to partially confine the pairs and to create multiple generations of pairs via reacceleration. PIC simulations show that, after the lasers are turned off, the pairs will expand much faster than the heavier gold ions.
Scientists Simulate Astrophysical Jets
Hence a miniature fireball of relativistically expanding pure pairs can be created. This pair fireball can be made to collide with another pair fireball to mimic the internal shock model of gamma-ray bursts, allowing us to study how the expansion energy of pair fireballs can be converted into internal energy and gamma rays. Another exotic future application is to study the static pair-balanced plasmas theorized to be the source of the episodic annihilation line flares from black-hole candidates.
Two megajoule-class 0. When a blast wave sweeps up a high-density medium, the optically thin radiative cooling time at the shock front may become shorter than the dynamical time.