Probing dissipation mechanisms in BL Lac jets through X-ray polarimetry

The dissipation of energy flux in blazar jets plays a key role in the acceleration of relativistic particles. Two possibilities are commonly considered for the dissipation processes, magnetic reconnection -- possibly triggered by instabilities in magnetically-dominated jets -- , or shocks -- for weakly magnetized flows. We consider the polarimetric features expected for the two scenarios analyzing the results of state-of-the-art simulations. For the magnetic reconnection scenario we conclude, using results from global relativistic MHD simulations, that the emission likely occurs in turbulent regions with unstructured magnetic fields, although the simulations do not allow us to draw firm conclusions. On the other hand, with local particle-in-cell simulations we show that, for shocks with a magnetic field geometry suitable for particle acceleration, the self-generated magnetic field at the shock front is predominantly orthogonal to the shock normal and becomes quasi-parallel downstream. Based on this result we develop a simplified model to calculate the frequency-dependent degree of polarization, assuming that high-energy particles are injected at the shock and cool downstream. We apply our results to HBLs, blazars with the maximum of their synchrotron output at UV-soft X-ray energies. While in the optical band the predicted degree of polarization is low, in the X-ray emission it can ideally reach 50\%, especially during active/flaring states. The comparison between measurements in the optical and in the X-ray band made during active states (feasible with the planned {\it IXPE} satellite) are expected to provide valuable constraints on the dissipation and acceleration processes.Comment: 9 pages, 6 figures, accepted for publication by MNRA

Particle-in-cell simulations of shock-driven reconnection in relativistic striped winds

By means of two- and three-dimensional particle-in-cell simulations, we investigate the process of driven magnetic reconnection at the termination shock of relativistic striped flows. In pulsar winds and in magnetar-powered relativistic jets, the flow consists of stripes of alternating magnetic field polarity, separated by current sheets of hot plasma. At the wind termination shock, the flow compresses and the alternating fields annihilate by driven magnetic reconnection. Irrespective of the stripe wavelength "lambda" or the wind magnetization "sigma" (in the regime sigma>>1 of magnetically-dominated flows), shock-driven reconnection transfers all the magnetic energy of alternating fields to the particles, whose average Lorentz factor increases by a factor of sigma with respect to the pre-shock value. In the limit lambda/(r_L*sigma)>>1, where r_L is the relativistic Larmor radius in the wind, the post-shock particle spectrum approaches a flat power-law tail with slope around -1.5, populated by particles accelerated by the reconnection electric field. The presence of a current-aligned "guide" magnetic field suppresses the acceleration of particles only when the guide field is stronger than the alternating component. Our findings place important constraints on the models of non-thermal radiation from Pulsar Wind Nebulae and relativistic jets.Comment: 25 pages, 14 figures, movies available at https://www.cfa.harvard.edu/~lsironi/sironi_movies.tar ; in press, special issue of Computational Science and Discovery on selected research from the 22nd International Conference on Numerical Simulation of Plasma

Happiness, housework and gender inequality in Europe

Although the last few decades have seen a progressive increase of gender equality in almost all dimensions of society, roles concerning childcare and domestic work remain highly gender-specific. Gender division of labour within the family varies considerably within and across countries. Gender systems are likely to have an important impact on individuals' well-being. Improved gender equality has enhanced the general well-being of women, although its extent may depend on the context in which women live and operate. This work considers the effect of the unequal division of labour within the household, between women and their partners, on women's own subjective assessment of happiness. We conducted the analysis using the European Social Survey data. We included 26 European countries and explored, exploiting a multi-level model to investigate the determinants of, women's differing levels of happiness across countries. In particular, we examined the extent to which gender equality at the country level can explain variation in happiness at the individual level. © The Author 2010. Published by Oxford University Press. All rights reserved

Particle Acceleration in Pulsar Wind Nebulae: PIC modelling

We discuss the role of particle-in-cell (PIC) simulations in unveiling the origin of the emitting particles in PWNe. After describing the basics of the PIC technique, we summarize its implications for the quiescent and the flaring emission of the Crab Nebula, as a prototype of PWNe. A consensus seems to be emerging that, in addition to the standard scenario of particle acceleration via the Fermi process at the termination shock of the pulsar wind, magnetic reconnection in the wind, at the termination shock and in the Nebula plays a major role in powering the multi-wavelength signatures of PWNe.Comment: 32 pages, 16 figures, to appear in the book "Modelling Nebulae" edited by D. Torres for Springer, based on the invited contributions to the workshop held in Sant Cugat (Barcelona), June 14-17, 201

Radio light curves during the passage of cloud G2 near Sgr A*

We calculate radio light curves produced by the bow shock that is likely to form in front of the G2 cloud when it penetrates the accretion disk of Sgr A*. The shock acceleration of the radio-emitting electrons is captured self-consistently by means of first-principles particle-in-cell simulations. We show that the radio luminosity is expected to reach maximum in early 2013, roughly a month after the bow shock crosses the orbit pericenter. We estimate the peak radio flux at 1.4 GHz to be 1.4 - 22 Jy depending on the assumed orbit orientation and parameters. We show that the most promising frequencies for radio observations are in the 0.1<nu<1 GHz range, for which the bow shock emission will be much stronger than the intrinsic radio flux for all the models considered.Comment: 15 pages, 10 figures, accepted for publication in MNRA

Generation of near-equipartition magnetic fields in turbulent collisionless plasmas

The mechanisms that generate "seed" magnetic fields in our Universe and that amplify them throughout cosmic time remain poorly understood. By means of fully-kinetic particle-in-cell simulations of turbulent, initially unmagnetized plasmas, we study the genesis of magnetic fields via the Weibel instability and follow their dynamo growth up to near-equipartition levels. In the kinematic stage of the dynamo, we find that the rms magnetic field strength grows exponentially with rate $\gamma_B \simeq 0.4\,u_{\rm rms}/L$, where $L/2 \pi$ is the driving scale and $u_{\rm rms}$ is the rms turbulent velocity. In the saturated stage, the magnetic field energy reaches about half of the turbulent kinetic energy. Here, magnetic field growth is balanced by dissipation via reconnection, as revealed by the appearance of plasmoid chains. At saturation, the integral-scale wavenumber of the magnetic spectrum approaches $k_{\rm int}\simeq 12\pi/L$. Our results show that turbulence -- induced by, e.g., the gravitational build-up of galaxies and galaxy clusters -- can magnetize collisionless plasmas with large-scale near-equipartition fields.Comment: 10 pages, 10 figures, PRL in pres

Acceleration in perpendicular relativistic shocks for plasmas consisting of leptons and hadrons

We investigate the acceleration of light particles in perpendicular shocks for plasmas consisting of a mixture of leptonic and hadronic particles. Starting from the full set of conservation equations for the mixed plasma constituents, we generalize the magneto-hydrodynamical jump conditions for a multi-component plasma, including information about the specific adiabatic constants for the different species. The impact of deviations from the standard model of an ideal gas is compared in theory and particle-in-cell simulations, showing that the standard-MHD model is a good approximation. The simulations of shocks in electron-positron-ion plasmas are for the first time multi-dimensional, transverse effects are small in this configuration and 1D simulations are a good representation if the initial magnetization is chosen high. 1D runs with a mass ratio of 1836 are performed, which identify the Larmor frequency \omega_{ci} as the dominant frequency that determines the shock physics in mixed component plasmas. The maximum energy in the non-thermal tail of the particle spectra evolves in time according to a power-law proportional to t^\alpha with \alpha in the range 1/3 < \alpha < 1, depending on the initial parameters. A connection is made with transport theoretical models by Drury (1983) and Gargate & Spitkovsky (2011), which predict an acceleration time proportional to \gamma and the theory for small wavelength scattering by Kirk & Reville (2010), which predicts a behavior rather as proportional to \gamma^2. Furthermore, we compare different magnetic field orientations with B_0 inside and out of the plane, observing qualitatively different particle spectra than in pure electron-ion shocks