We investigated particle acceleration and shock structure associated with an unmagnetized relativistic jet propagating into an unmagnetized plasma. Strong magnetic fields generated in the trailing shock contribute to the electrons transverse deflection and acceleration. We have calculated, self-consistently, the radiation from electrons accelerated in these turbulent magnetic fields. We found that the synthetic spectra depend on the bulk Lorentz factor of the jet, its temperature and strength of the generated magnetic fields. We have also investigated accelerated electrons in strong magnetic fields generated by kinetic shear (Kelvin-Helmholtz) instabilities. The calculated properties of the emerging radiation will guide our understanding of the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets in general, and supernova remnants. 1 Center for Space Plasma and Aeronomic Research, University of Alabama in Huntsville, 320 Sparkman Drive, ZP12, Huntsville, AL 35805, USA 2 Department of Physics, University of Nevada, Las Vegas, NV 89154, USA 3 Institute of Space Science, Atomistilor 409, Bucharest-Magurele 077125, Romania 4 Department of Physics and Astronomy, University of Kansas, KS 66045, USA 5 Department of Physics and Astronomy, The University of Alabama, Tuscaloosa, AL 35487, USA 6 Korea Advanced Institute of Science and Technology, Daejeon 305-701, South Korea 7 Institute of Nuclear Physics PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland 8 Institute of Astronomy National Tsing-Hua University, Hsinchu, Taiwan 30013, R.O.C 9 Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark 10 LUTH, Observatoire de Paris-Meudon, 5 place Jules Jansen, 92195 Meudon Cedex, France 11 Institue of Physics and Astronomy, University of Potsdam, Karl-Liebknecht-Strasse 24/25, 14476 Potsdam-Golm, Germany 12 Department of Physics and Astronomy, Clemson University, Clemson, SC 29634, USA c © EAS, EDP Sciences 2013 DOI: 10.1051/eas/1361026 178 Gamma-ray Bursts: 15 Years of GRB Afterglows 1 Simulation of kinetic Kelvin-Helmholz instability Recent kinetic simulations have focused on magnetic field generation via electromagnetic plasma instabilities in unmagnetized flows without velocity shears. Three-dimensional (3D) particle-in-cell (PIC) simulations of Weibel turbulence (Nishikawa et al. 2009) have demonstrated subequipartition magnetic field generation. We have calculated, self-consistently, the radiation from electrons accelerated in the turbulent magnetic fields. We found that the synthetic spectra depend on the Lorentz factor of the jet, its thermal temperature and strength of the generated magnetic fields (Nishikawa et al. 2011, 2012). These works have neglected the role of velocity shear in the flow, which are an alternative mechanism to generate subequipartition magnetic fields in relativistic outflows (Alves et al. 2012). Furthermore, a shear flow upstream of a shock can lead to density inhomogeneities via the Kelvin-Helmholtz instability (KHI) which may constitute important scattering sites for particle acceleration. We have performed simulations with a relativistic core jet surrounded by a sheared velocity layer with the stationary sheath plasmas as simulated in our RMHD simulations (Mizuno et al. 2007). In our initial simulations the initial conditions for the shear flow will have a core jet with vcore = 0.9978c (γ = 15) pointing in the positive x direction in the middle of the simulation box as in Alves et al. (2012); the upper and lower quarter of the simulation box will contain a sheath also moving in the positive x direction with vsheath = 0 (for this simulation). Overall, this structure is similar in spirit, although not in scale, to that proposed for active galactic nuclei (AGN) relativistic jet cores surrounded by a slower moving sheath, and is also relevant to gamma-ray burst (GRB) jets. In particular, we note that this structure is also relevant to the “jet-in-a-jet” or “needles” in a jet scenarios (Giannios et al. 2009, papers therein) which have been invoked to provide smaller scale high speed structures within a much larger more slowly moving AGN jet. Similar smaller scale structures within GRB jets are also conceivable. This more realistic setup is different from the initial conditions used by the previous simulations with counter-steaming flow of Alves et al. (2012), and hence allows us to compute synthetic spectra in the observer frame far along the x-direction. As discussed by Alves et al. (2012), in our setup the growing kinetic KHI will propagate with the flow. For GRB jets, the relativistic jet core will have much higher density relative to the external medium. On the other hand, for an AGN the relativistic core is less dense than the surrounding sheath. We have performed simulations using a system with (Lx, Ly, Lz) = (1005Δ, 205Δ, 205Δ) with the mass ratio of ion and electron, mi/me = 20. Figure 1 shows the magnetic field structures generated by shearing relativistic electronion flows with γ = 15 with stationary sheath plasmas taken at time t = 70 ω−1 pe . Figure 1a shows the magnetic field intensity of By plotted in the y − z plane at the center of the box x = 500Δ (jet out of the plane) with the magnetic finds By (red), Bx (black), and Bz (blue) at x = 500Δ and y = 100Δ. Figure 1c shows the x component of current. The relativistic jet is directed out of the plane and the positive current is generated at the jet side, whereas the negative current is K.-I. Nishikawa et al.: Radiation from Accelerated Particles 179 0 50 100 150 200 Y/Δ Z/ Δ

The central radio source in M87 provides the best opportunity to study jet formation because it has a large angular size for the gravitational radius of the black hole and has a bright jet that is well resolved by very long baseline interferometry observations. We present intensive monitoring observations from 2007 and 2008, plus roughly annual observations that span 17 years, all made with the the Very Long Baseline Array at 43 GHz with a resolution of about 30 by 60RS. Our high dynamic range images clearly show the wide opening angle structure and the counterjet. The jet and counterjet are nearly symmetric in the inner 1.5 mas (0.12 pc in projection), with both being edge brightened. Both show deviations from parabolic shape in the form of an initial rapid expansion and subsequent contraction followed by further rapid expansion and, beyond the visible counterjet, subsequent collimation. Proper motions and counterjet/jet intensity ratios both indicate acceleration from apparent speeds of ≲0.5c to ≳2c in the inner ∼2 mas and suggest a helical flow. The jet displays a sideways shift with an approximately 8–10 yr quasi-periodicity. The shift propagates outward nonballistically and significantly more slowly than the flow speed revealed by the fastest-moving components. Polarization data show a systematic structure with magnetic field vectors that suggest a toroidal field close to the core.

AbstractWe study the interaction of relativistic jets with their environment, using 3-dimen- sional relativistic particle-in-cell simulations for two cases of jet composition: (i) electron-proton (e− − p+) and (ii) electron-positron (e±) plasmas containing helical magnetic fields. We have performed simulations of “global” jets containing helical magnetic fields in order to examine how helical magnetic fields affect kinetic instabilities such as the Weibel instability, the kinetic Kelvin-Helmholtz instability and the Mushroom instability. We have found that these kinetic instabilities are suppressed and new types of instabilities can grow. For the e− − p+ jet, a recollimation-like instability occurs and jet electrons are strongly perturbed, whereas for the e± jet, a recollimation-like instability occurs at early times followed by kinetic instability and the general structure is similar to a simulation without a helical magnetic field. We plan to perform further simulations using much larger systems to confirm these new findings.

The most promising mechanisms for producing and accelerating relativistic jets, and maintaining collimated structure of relativistic jets involve magnetohydrodynamical (MHD) processes. We have investigated the magnetic dissipation mechanism in relativistic jets via relativistic MHD simulations. We found that the relativistic jets involving a helical magnetic field are unstable for the current-driven kink instability, which leads to helically distorted structure in relativistic jets. We identified the regions of high current density in filamentary current sheets, indicative of magnetic reconnection, which are associated to the kink unstable regions and correlated to the converted regions of magnetic to kinetic energies of the jets. We also found that an over-pressured relativistic jet leads to the generation of a series of stationary recollimation shocks and rarefaction structures by the nonlinear interaction of shocks and rarefaction waves. The differences in the recollimation shock structure due to the difference of the magnetic field topologies and strengths may be observable through mm-VLBI observations and space-VLBI mission.

M87 is the best source in which to study a jet at high resolution in gravitational units because it has a very high mass black hole and is nearby. The angular size of the black hole is second only to Sgr A*, which does not have a strong jet. The jet structure is edge brightened with a wide opening angle base and a weak counterjet. We have roughly annual observations for 17 years plus intensive monitoring at three week intervals for a year and five day intervals for 2.5 months made with the Very Long Baseline Array (VLBA) at 43 GHz. The inner jet shows very complex dynamics, with apparent motions both along and across the jet. Speeds from zero to over 2c are seen, with acceleration observed over the first 3 milli-arcseconds. The counterjet decreases in brightness much more rapidly than the main jet, as is expected from relativistic beaming in an accelerating jet oriented near the line-of-sight. Details of the structure and dynamics are discussed. The roughly annual observations show side-to-side motion of the whole jet with a characteristic time scale of about 9 years.

This work is supported by NSF AST-0908010, AST-0908040, NASA-NNG05GK73G, NNX07AJ88G, NNX08AG83G, NNX08AL39G, NNX09AD16G, NNX12AH06G, NNX13AP21G, and NNX13AP14G grants. The work of J.N. has been supported by Narodowe Centrum Nauki through research project DEC-2013/10/E/ST9/00662. J.L.G. acknowledges support through grant AYA2013-40825-P of the Spanish Ministry of Economy and Competitiveness. Y.M. is supported by the ERC Synergy Grant u003eBlackHoleCam-Imaging the Event Horizon of Black Holesu003e (Grant No. 610058). M.P. acknowledges support through grant PO 1508/1-2 of the Deutsche Forschungsgemeinschaft.

In the study of relativistic jets one of the key open questions is their interaction with the environment on the microscopic level. Here, we study the initial evolution of both electron–proton ( e − – p + ) and electron–positron ( e ± ) relativistic jets containing helical magnetic fields, focusing on their interaction with an ambient plasma. We have performed simulations of “global” jets containing helical magnetic fields in order to examine how helical magnetic fields affect kinetic instabilities such as the Weibel instability, the kinetic Kelvin-Helmholtz instability (kKHI) and the Mushroom instability (MI). In our initial simulation study these kinetic instabilities are suppressed and new types of instabilities can grow. In the e − – p + jet simulation a recollimation-like instability occurs and jet electrons are strongly perturbed. In the e ± jet simulation a recollimation-like instability occurs at early times followed by a kinetic instability and the general structure is similar to a simulation without helical magnetic field. Simulations using much larger systems are required in order to thoroughly follow the evolution of global jets containing helical magnetic fields.

We have performed two-dimensional special-relativistic magnetohydrodynamic simulations of non-equilibrium over-pressured relativistic jets in cylindrical geometry. Multiple stationary recollimation shock and rarefaction structures are produced along the jet by the nonlinear interaction of shocks and rarefaction waves excited at the interface between the jet and the surrounding ambient medium. Although initially the jet is kinematically dominated, we have considered axial, toroidal, and helical magnetic fields to investigate the effects of different magnetic-field topologies and strengths on the recollimation structures. We find that an axial field introduces a larger effective gas pressure and leads to stronger recollimation shocks and rarefactions, resulting in larger flow variations. The jet boost grows quadratically with the initial magnetic field. On the other hand, a toroidal field leads to weaker recollimation shocks and rarefactions, significantly modifying the jet structure after the first recollimation rarefaction and shock. The jet boost decreases systematically. For a helical field, instead, the behavior depends on the magnetic pitch, with a phenomenology that ranges between the one seen for axial and toroidal magnetic fields, respectively. In general, however, a helical magnetic field yields a more complex shock and rarefaction substructure close to the inlet that significantly modifies the jet structure. The differences in shock structure resulting from different field configurations and strengths may have observable consequences for disturbances propagating through a stationary recollimation shock.

We have investigated the generation of magnetic fields associated with velocity shear between an unmagnetized relativistic (core) jet and an unmagnetized sheath plasma by the kinetic Kelvin-Helmholtz instability for different mass ratios (m i /m e = 1, 20, and 1836) and different jet Lorentz factors. We found that electron-positron cases have alternating magnetic fields instead of the DC magnetic fields found in electron-ion cases. We have also investigated particle acceleration and shock structure associated with an unmagnetized relativistic jet propagating into an unmagnetized plasma for electron-positron and electron-ion plasmas. Strong magnetic fields generated in the trailing shock lead to transverse deflection and acceleration of the electrons. We have self-consistently calculated the radiation from the electrons accelerated in the turbulent magnetic fields for different jet Lorentz factors. We find that the synthetic spectra depend on the bulk Lorentz factor of the jet, the jet temperature, and the strength of the magnetic fields generated in the shock.

We investigated the influence of velocity shear and a radial density profile on the spatial development of the current-driven (CD) kink instability along helically magnetized relativistic jets via three-dimensional relativistic magnetohydrodynamic simulations. In this study, we use a nonperiodic computational box, the jet flow is initially established across the computational grid, and a precessional perturbation at the inlet triggers growth of the kink instability. If the velocity shear radius is located inside the characteristic radius of the helical magnetic field, a static nonpropagating CD kink is excited as the perturbation propagates down the jet. Temporal growth disrupts the initial flow across the computational grid not too far from the inlet. On the other hand, if the velocity shear radius is outside the characteristic radius of the helical magnetic field, the kink is advected with the flow and grows spatially down the jet. In this case, flow is maintained to much larger distances from the inlet. The effect of different radial density profiles is more subtle. When the density increases with radius, the kink appears to saturate by the end of the simulation without apparent disruption of the helical twist. This behavior suggests that relativistic jets consisting of a tenuous spine surrounded by a denser medium with a velocity shear radius outside the radius of the maximum toroidal magnetic field have a relatively stable configuration.

We have investigated the influence of jet rotation and differential motion on the linear and nonlinear development of the current-driven (CD) kink instability of force-free helical magnetic equilibria via threedimensional relativistic magnetohydrodynamic simulations. In this study, we follow the temporal development within a periodic computational box. Displacement of the initial helical magnetic field leads to the growth of the CD kink instability. In rotating relativistic jet case, developed helical kink structure propagates along jet axis with continuous growth of kink amplitude. The growth rate of CD kink instability does not depend on the jet rotation. The coupling of multiple unstable wavelengths is crucial to determining whether the jet is eventually disrupted in nonlinear stage. The strongly deformed magnetic field via CD kink instability may trigger of magnetic reconnection in the jet.

Relativistic jets, be they Poynting flux or kinetic flux dominated, are current driven ( CD) and/or Kelvin-Helmholtz (KH) velocity shear driven unstable. These macroscopic MHD instabilities may be respon- sible for some of the observed larger scale twisted jet structures and typically do not disrupt jets on less than kiloparsec scales. Here I review our understanding of the jet properties that will lead to the observed relative stability of astrophysical jets. In addition, I review the progress made on the microscopic scale plasma instabil- ities in shocks and velocity shears that may lead to magnetic field generation and that does lead to the particle acceleration required to produce the observed emission from radio wavelengths to TeV energies. Finally, I discuss these instabilities in the context of the jet in M 87.

We have investigated particle acceleration and shock structure associated with an unmagnetized relativistic jet propagating into an unmagnetized plasma for electron-positron and electron-ion plasmas. Strong magnetic fields generated in the trailing jet shock lead to transverse deflection and acceleration of the electrons. We have self-consistently calculated the radiation from the electrons accelerated in the turbulent magnetic fields for different jet Lorentz factors. We find that the synthetic spectra depend on the bulk Lorentz factor of the jet, the jet temperature, and the strength of the magnetic fields generated in the shock. We have investigated the generation of magnetic fields associated with velocity shear between an unmagnetized relativistic (core) jet and an unmagnetized sheath plasma. We discuss particle acceleration in collimation shocks for AGN jets formed by relativistic MHD simulations. Our calculated spectra should lead to a better understanding of the complex time evolution and/or spectral structure from gamma-ray bursts, relativistic jets, and supernova remnants.

We have investigated the influence of velocity shear on the linear and non-linear development of the CD kink instability. We follow temporal development of the instability within a periodic computational box. We find that helically distorted density structure propagates along the jet with speed and flow structure dependent on the location of the velocity shear relative to the characteristic radius of the helically twisted force-free magnetic field. At small radius the plasma flows through the kink. The kink propagation speed increases as the radius increases and the kink becomes more embedded in the plasma flow. Larger velocity shear radius leads to slower linear growth, makes a later transition to the nonlinear stage, and with larger maximum amplitude than occurs for a static plasma column. However, when the velocity shear radius is much greater than the characteristic radius of the helical magnetic field, linear and non-linear development become more similar to the development of a static plasma column.

Recent PIC simulations of relativistic electron-positron (electron-ion) jets injected into a stationary medium show that particle acceleration occurs in the shocked regions. Simulations show that the Weibel instability is responsible for generating and amplifying highly nonuniform, small-scale magnetic fields and for particle acceleration. These magnetic fields contribute to the electron's transverse deflection behind the shock. The "jitter" radiation from deflected electrons in turbulent magnetic fields has properties different from synchrotron radiation calculated in a uniform magnetic field. This jitter radiation may be important for understanding the complex time evolution and/or spectral structure of gamma-ray bursts, relativistic jets in general, and supernova remnants. In order to calculate radiation from first principles and go beyond the standard synchrotron model, we have used PIC simulations. We present synthetic spectra to compare with the spectra obtained from Fermi observations.

Abridged: The giant radio galaxy M 87 with its proximity, famous jet, and very massive black hole provides a unique opportunity to investigate the origin of very high energy (VHE; E>100 GeV) gamma-ray emission generated in relativistic outflows and the surroundings of super-massive black holes. M 87 has been established as a VHE gamma-ray emitter since 2006. The VHE gamma-ray emission displays strong variability on timescales as short as a day. In this paper, results from a joint VHE monitoring campaign on M 87 by the MAGIC and VERITAS instruments in 2010 are reported. During the campaign, a flare at VHE was detected triggering further observations at VHE (H.E.S.S.), X-rays (Chandra), and radio (43 GHz VLBA). The excellent sampling of the VHE gamma-ray light curve enables one to derive a precise temporal characterization of the flare: the single, isolated flare is well described by a two-sided exponential function with significantly different flux rise and decay times. While the overall variability pattern of the 2010 flare appears somewhat different from that of previous VHE flares in 2005 and 2008, they share very similar timescales (~day), peak fluxes (Phi(>0.35 TeV) ~= (1-3) x 10^-11 ph cm^-2 s^-1), and VHE spectra. 43 GHz VLBA radio observations of the inner jet regions indicate no enhanced flux in 2010 in contrast to observations in 2008, where an increase of the radio flux of the innermost core regions coincided with a VHE flare. On the other hand, Chandra X-ray observations taken ~3 days after the peak of the VHE gamma-ray emission reveal an enhanced flux from the core. The long-term (2001-2010) multi-wavelength light curve of M 87, spanning from radio to VHE and including data from HST, LT, VLA and EVN, is used to further investigate the origin of the VHE gamma-ray emission. No unique, common MWL signature of the three VHE flares has been identified.

Radio and optical images of the M 87 jet show bright filaments, twisted into an apparent double helix, extending from HST-1 to knot A. Proper motions within the jet suggest a decelerating jet flow passing through a slower, accelerating wave pattern. A mass and energy flux conserving model requires the jet to be an internally hot, but subrelativistic plasma, from HST-1 to knot A. Kelvin-Helmholtz (KH) elliptical mode generated twisted filaments require the cocoon to be cooler than the jet at HST-1 but nearly as hot as the jet at knot A. A pseudo-synchrotron image of our model jet shows both similarities and differences when compared to an image of the real jet.