Evolution of Global Relativistic Jets: Collimations and Expansion with kKHI and the Weibel Instability

One of the key open questions in the study of relativistic jets is their interaction with the environment. Here, we study the initial evolution of both electron-proton and electron-positron relativistic jets, focusing on their lateral interaction with the ambient plasma. We trace the generation and evolution of the toroidal magnetic field generated by both kinetic Kelvin-Helmholtz (kKH) and Mushroom instabilities (MI). This magnetic field collimates the jet. We show that in electron-proton jet, electrons are perpendicularly accelerated with jet collimation. The magnetic polarity switches from the clockwise to anti-clockwise in the middle of jet, as the instabilities weaken. For the electron-positron jet, we find strong mixture of electron-positron with the ambient plasma, that results in the creation of a bow shock. Merger of magnetic field current filaments generate density bumps which initiate a forward shock. The strong mixing between jet and ambient particles prevents full development of the jet on the studied scale. Our results therefore provide a direct evidence for both jet collimation and particle acceleration in the created bow shock. Differences in the magnetic field structures generated by electron-proton and electron-positron jets may contribute to observable differences in the polarized properties of emission by electrons.Comment: 25 pages, 12 figures, ApJ, accepte

Magnetic Field Generation in Core-Sheath Jets via the Kinetic Kelvin-Helmholtz Instability

We have investigated magnetic field generation in velocity shears via the kinetic Kelvin-Helmholtz instability (kKHI) using a relativistic plasma jet core and stationary plasma sheath. Our three-dimensional particle-in-cell simulations consider plasma jet cores with Lorentz factors of 1.5, 5, and 15 for both electron-proton and electron-positron plasmas. For electron-proton plasmas we find generation of strong large-scale DC currents and magnetic fields which extend over the entire shear-surface and reach thicknesses of a few tens of electron skin depths. For electron-positron plasmas we find generation of alternating currents and magnetic fields. Jet and sheath plasmas are accelerated across the shear surface in the strong magnetic fields generated by the kKHI. The mixing of jet and sheath plasmas generates transverse structure similar to that produced by the Weibel instability.Comment: 28 pages, 12 figures, in press, ApJ, September 10, 201

Particle Acceleration in Relativistic Electron-positron Jets with Helical Magnetic Fields

The properties of relativistic jets, their interaction with the ambient environment and particle acceleration due to kinetic instabilities are studied self-consistently with Particle-in-Cell (PIC) simulations. In this work we study how a relativistic electron-positron jet containing a helical magnetic field evolves by focusing on its interaction with the external ambient plasma. Particularly, 3D PIC simulations are performed using a longer simulation system than previous studies with an embedded helical magnetic field. An important key issue in this work is how such a magnetic field affects an electron-positron jet and how this excites kinetic instabilities such as the Weibel instability (WI), the kinetic Kelvin-Helmholtz instability (kKHI) and others by further focusing on how particles accelerate. We do find that kinetic instabilities along with generated magnetic turbulence are present and consequently accelerate particles. At the linear stage we observe recollimation-like features at the center of the simulated jet and later-on as the electron-positron jet evolves, the magnetic fields generated by the instabilities become untangled and reorganized into a new topology near the non-linear phase. We additionally report indications of reconnection near the end of the non-linear stage, before the magnetic-field becomes untangled, as electrons get accelerated by multiple magnetic islands in the jet. In the present study the untangled magnetic field becomes turbulent without any reformation as it happened in our previous study of an electron-proton jet, which we will use to additionally compare the present results, obtaining important insights about the nature of these phenomena applicable to high-energy astrophysical environments such as Active Galactic Nuclei jets and Gamma-ray bursts.Comment: 12 pages, 24 figures, submitted to MNRAS Journa

Magnetic field generation in a jet-sheath plasma via the kinetic Kelvin-Helmholtz instability

We have investigated generation of magnetic fields associated with velocity shear between an unmagnetized relativistic jet and an unmagnetized sheath plasma. We have examined the strong magnetic fields generated by kinetic shear (Kelvin-Helmholtz) instabilities. Compared to the previous studies using counter-streaming performed by Alves et al. (2012), the structure of KKHI of our jet-sheath configuration is slightly different even for the global evolution of the strong transverse magnetic field. In our simulations the major components of growing modes are the electric field $E_{\rm z}$ and the magnetic field $B_{\rm y}$. After the $B_{\rm y}$ component is excited, an induced electric field $E_{\rm x}$ becomes significant. However, other field components remain small. We find that the structure and growth rate of KKHI with mass ratios $m_{\rm i}/m_{\rm e} = 1836$ and $m_{\rm i}/m_{\rm e} = 20$ are similar. In our simulations saturation in the nonlinear stage is not as clear as in counter-streaming cases. The growth rate for a mildly-relativistic jet case ($\gamma_{\rm j} = 1.5$) is larger than for a relativistic jet case ($\gamma_{\rm j} = 15$).Comment: 6 pages, 6 figures, presented at Dynamical processes in space plasmas II, Isradinamic 2012, in press, ANGEO. arXiv admin note: text overlap with arXiv:1303.256

Radiation from Accelerated Particles in Relativistic Jets with Shocks, Shear-flow, and Reconnection

We have investigated particle acceleration and shock structure associated with an unmagnetized rel-ativistic 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

Measurement of the Depth of Maximum of Extensive Air Showers above 10^18 eV

We describe the measurement of the depth of maximum, Xmax, of the longitudinal development of air showers induced by cosmic rays. Almost four thousand events above 10^18 eV observed by the fluorescence detector of the Pierre Auger Observatory in coincidence with at least one surface detector station are selected for the analysis. The average shower maximum was found to evolve with energy at a rate of (106 +35/-21) g/cm^2/decade below 10^(18.24 +/- 0.05) eV and (24 +/- 3) g/cm^2/decade above this energy. The measured shower-to-shower fluctuations decrease from about 55 to 26 g/cm^2. The interpretation of these results in terms of the cosmic ray mass composition is briefly discussed.Comment: Accepted for publication by PR

Operations of and Future Plans for the Pierre Auger Observatory

Technical reports on operations and features of the Pierre Auger Observatory, including ongoing and planned enhancements and the status of the future northern hemisphere portion of the Observatory. Contributions to the 31st International Cosmic Ray Conference, Lodz, Poland, July 2009.Comment: Contributions to the 31st ICRC, Lodz, Poland, July 200

The Pierre Auger Observatory III: Other Astrophysical Observations

Astrophysical observations of ultra-high-energy cosmic rays with the Pierre Auger ObservatoryComment: Contributions to the 32nd International Cosmic Ray Conference, Beijing, China, August 201