Relativistic Plasmoids Research Articles

Plasmas, particles and photons—spotlights on multimessenger astronomy

During the past decennia, progress in the area of high-energy astroparticle physics was exceptional, mainly due to the great success of the bridging of particle- and astrophysics both in theory and in the instrumentation of astroparticle physics observatories. Multimessenger data coming from charged cosmic-ray-, gamma-ray- and neutrino-observatories start to shed more and more light on the nature and origin of cosmic rays. At the same time, the development of methods for the investigation of cosmic-ray transport, acceleration and interaction has advanced to the true potential of tying these different pieces of multimessenger data together, this way closing in on the origin of cosmic rays. In recent years, this rapid interplay between modeling and observations has made it clear that it is essential to add the ingredient of plasma physics to the problem. It has been shown that even the interpretation of data of highly relativistic cosmic rays at TeV energies and above is in need of a proper modeling of the plasma physics involved. One of the most important examples is the understanding of wave-particle interactions. In simulations of cosmic-ray transport in the Galaxy, the cosmic-ray diffusion coefficient is typically approximated with a Kolmogorov-type cascade model, resulting in an energy-dependent parallel diffusion coefficient with . Here, we show how the energy dependence of the diffusion coefficient can be investigated systematically as a function of . The complex energy behavior that goes well beyond a simple powerlaw interpretation will be presented together with a formal definition of an energy range that indeed can be approximated as a powerlaw. These results are applied to cosmic-ray transport in the Milky Way. Finally, the transition between the ballistic and diffusive regime will be investigated for astrophysical sources with special focus on relativistic plasmoids of active galaxies.

Properties of GRB light curves from magnetic reconnection

The energy dissipation mechanism within Gamma-Ray Burst (GRB) outflows, driving their extremely luminous prompt $\gamma$-ray emission is still uncertain. The leading candidates are internal shocks and magnetic reconnection. While the emission from internal shocks has been extensively studied, that from reconnection still has few quantitative predictions. We study the expected prompt-GRB emission from magnetic reconnection and compare its temporal and spectral properties to observations. The main difference from internal shocks is that for reconnection one expects relativistic bulk motions with Lorentz factors $\Gamma'\gtrsim\;$a few in the jet's bulk frame. We consider such motions of the emitting material in two anti-parallel directions (e.g. of the reconnecting magnetic-field lines) within an ultra-relativistic (with $\Gamma\gg1$) thin spherical reconnection layer. The emission's relativistic beaming in the jet's frame greatly affects the light-curves. For emission at radii $R_0<R<R_0+\Delta{}R$ (with $\Gamma=\rm{const}$) the observed pulse width is $\Delta{}T\sim(R_0/2c\Gamma^2)\max(1/\Gamma',\,\Delta{}R/R_0)$, i.e. up to $\sim\Gamma'$ times shorter than for isotropic emission in the jet's frame. We consider two possible magnetic reconnection modes: a quasi steady-state with continuous plasma flow into and out of the reconnection layer, and sporadic reconnection in relativistic turbulence that produces relativistic plasmoids. Both of these modes can account for many observed prompt-GRB properties: variability, pulse asymmetry, the very rapid declines at their end, and pulse evolutions that are either hard to soft (for $\Gamma'\lesssim2$) or intensity tracking (for $\Gamma'>2$). However, the relativistic turbulence mode is more likely to be relevant for the prompt sub-MeV emission and can naturally account also for the peak luminosity$\,$--$\,$peak frequency correlation.

ON THE RECENTLY DISCOVERED CORRELATIONS BETWEEN GAMMA-RAY AND X-RAY PROPERTIES OF GAMMA-RAY BURSTS

Recently, several new correlations between the observed $\gamma$-ray and the X-ray properties of gamma-ray bursts (GRBs) were inferred from a comprehensive analysis of the X-ray light curves of more than 650 GRBs measured with the Swift X-ray telescope (Swift/XRT) during the years 2004-2010. Like previously well established correlations, they challenge GRB models. Here we show that in the cannonball (CB) model of GRBs, the newly discovered correlations and those that follow from them, have the same simple kinematic origin as those discovered earlier. They result from the strong dependence of the observed radiations on the bulk motion Lorentz and Doppler factors of the jet of highly relativistic plasmoids (CBs) that produces these radiations by interaction with the medium through which it propagates.

KINEMATIC ORIGIN OF CORRELATIONS BETWEEN GAMMA-RAY BURST OBSERVABLES

Recently, several new correlations between gamma ray burst (GRB) observables have been discovered. Like previously well established correlations, they challenge GRB models. Here we show that in the cannonball (CB) model of GRBs, these newly discovered correlations have the same simple kinematic origin as those discovered earlier. They all result from the strong dependence of the observed radiations on the Lorentz and Doppler factors of the jet of highly relativistic plasmoids (CBs) that produces the observed radiations by interaction with the medium through which it propagates.

FLARES IN LONG AND SHORT GAMMA-RAY BURSTS

The many similarities between the prompt emission pulses in gamma ray bursts (GRBs) and X-ray flares during the fast decay and afterglow phases of GRBs suggest a common origin. In the cannonball (CB) model of GRBs, this common origin is mass accretion episodes of fall-back matter on a newly born compact object. The prompt emission pulses are produced by a bipolar jet of highly relativistic plasmoids (CBs) ejected in the early, major episodes of mass accretion. As the accretion material is consumed, one may expect the engine's activity to weaken. X-ray flares ending the prompt emission and during the afterglow phase are produced in such delayed episodes of mass accretion. The common engine, environment and radiation mechanisms (inverse Compton scattering and synchrotron radiation) produce their observed similarities. Flares in both long GRBs and short hard gamma ray bursts (SHBs) can also be produced by bipolar ejections of CBs following a phase transition in compact objects due to loss of angular momentum and/or cooling. Optical flares, however, are mostly produced in collisions of CBs with massive stellar winds/ejecta or with density bumps along their path. In this paper we show that the master formulae of the CB model of GRBs and SHBs, which reproduce very well their prompt emission pulses and their smooth afterglows, seem to reproduce also very well the lightcurves and spectral evolution of the prominent X-ray and optical flares that are well sampled.

The origin of cosmic rays

It is generally regarded that the bulk of cosmic rays originate in the galaxy and that those below the ‘knee’ (the rapid steepening in the energy spectrum) at a few PeV, come from galactic supernovae, the particles being accelerated by the shocks in the supernova remnants. At higher energies, there are problems in that conventional SNR—which surely constitute the bulk of the sources—have a natural limit at a few tens of PeV (for iron nuclei). The question of the origin of particles above this limit is thus an open one. Here we examine a number of possibilities: a variety of supernovae and hypernovae, pulsars, a giant galactic halo and an extragalactic origin. A relevant property of any model is the extent to which it can provide the lack of significant irregularity of the energy spectrum, its intensity and shape together with structures such as the ‘second knee’ at the sub-EeV energy, in addition to the well-known ‘knee’ and ‘ankle’. Although it is appreciated that spectral measurements are subject to systematic as well as random errors, we consider that contemporary data are good enough to allow at least some progress in this new field. These aspects are examined for particles of all energies and it is shown that they can constrain some parameters of the proposed models. In the search for origin above PeV energies, we conclude that shocks in the galactic halo, whatever their source (galactic wind, relativistic plasmoids—‘cannonballs’, multiple shocks from supernovae etc) are most likely, pulsars such as B0656+14 and hypernovae come a close second although such a suggestion is not without its difficulties. What is most important is that trapping of particles in the halo is needed to reduce irregularities of the energy spectra both below and above the ‘knee’ caused by the stochastic nature of supernova explosions and other potential (discrete) galactic sources. We argue that precise experimental studies of spectral ‘irregularities’ will provide considerable help in the search for cosmic ray origin.

The origin of cosmic rays

It is generally regarded that the bulk of cosmic rays originate in the galaxy and that those below the 'knee' (the rapid steepening in the energy spectrum) at a few PeV, come from galactic supernovae, the particles being accelerated by the shocks in the supernova remnants. At higher energies, there are problems in that conventional SNR—which surely constitute the bulk of the sources—have a natural limit at a few tens of PeV (for iron nuclei). The question of the origin of particles above this limit is thus an open one. Here we examine a number of possibilities: a variety of supernovae and hypernovae, pulsars, a giant galactic halo and an extragalactic origin. A relevant property of any model is the extent to which it can provide the lack of significant irregularity of the energy spectrum, its intensity and shape together with structures such as the 'second knee' at the sub-EeV energy, in addition to the well-known 'knee' and 'ankle'. Although it is appreciated that spectral measurements are subject to systematic as well as random errors, we consider that contemporary data are good enough to allow at least some progress in this new field. These aspects are examined for particles of all energies and it is shown that they can constrain some parameters of the proposed models. In the search for origin above PeV energies, we conclude that shocks in the galactic halo, whatever their source (galactic wind, relativistic plasmoids—'cannonballs', multiple shocks from supernovae etc) are most likely, pulsars such as B0656+14 and hypernovae come a close second although such a suggestion is not without its difficulties. What is most important is that trapping of particles in the halo is needed to reduce irregularities of the energy spectra both below and above the 'knee' caused by the stochastic nature of supernova explosions and other potential (discrete) galactic sources. We argue that precise experimental studies of spectral 'irregularities' will provide considerable help in the search for cosmic ray origin.

The Dual Nature of Hard X-Ray Outbursts from the Superluminal X-Ray Transient Source GRO J1655−40

We report the results of multiwavelength observations of the superluminal X-ray transient GRO J1655-40 during and following the prominent hard X-ray outburst of 1995 March-April. GRO J1655-40 was monitored continuously by BATSE on board the Compton Gamma Ray Observatory and observed repeatedly in the radio and optical bands from the ground. About a month after the onset of the hard X-ray outburst, GRO J1655-40 was observed twice by the Hubble Space Telescope in 1995 April 25 and 27, with the aim of detecting faint optical emission from ejected plasmoids. Despite the similarity of the hard X-ray emission in 1995 April with previous events in 1994, no radio or optical emission from GRO J1655-40 related to plasmoids was detected. We conclude that GRO J1655-40 is subject to complex behavior showing radio-loud hard X-ray outbursts with strong radio emission (of flux fr 100 mJy), both from a core source and from propagating plasmoids (as those in 1994), and radio-quiet hard X-ray outbursts with no detectable radio emission and plasmoid activity (fr 0.5 mJy) (as those in 1995). Our results can constrain models of particle acceleration and radiation of relativistic plasmoids.

The dynamics of relativistic plasmoids and rapidly varying radio sources

Variability and superluminal expansion of compact radio sources are investigated in terms of a theoretical model for the dynamics of radio sources involving relativistic motion of ram-pressure confined plasmoids (defined to be aggregates of magnetized plasma and relativistic particles). It is shown that relativistic spherical blast waves are subject to fragmentation leading to the formation of a number of discrete, relativistic plasmoids. It is shown that the initial temperature of the thermal component of the plasma fragments is likely to be on the order of 10/sup 12/ K. At this temperture the electrons in the plasma are relativistic, although the proton thermal velocity is nonrelativistic. As a result, the Faraday depolarization of such plasmoids is negligible, in agreement with observations of compact sources. Because of relativistic beaming, the plasmoids whose trajectories lie closest to the observer's line of sight are preferentially seen, causing the sources to exhibit variability and superluminal expansion effects which are also consistent with observations of rapidly varying radio sources.