SIMBAD references

Synchrotron emission from astrophysical nonthermal sources usually assumes that the emitting particles are isotropic. By means of large-scale two- and three-dimensional particle-in-cell simulations, we demonstrate that the dissipation of magnetically dominated (σ₀≫1) turbulence in pair plasmas leads to strongly anisotropic particle distributions. At Lorentz factors ∼σ₀γ_th0 (here, γ_th0 is the initial Lorentz factor), the particle velocity is preferentially aligned with the local magnetic field; instead, the highest energy particles are preferentially oriented in the plane perpendicular to the field. This energy-dependent anisotropy leads to a synchrotron spectral flux νF_ν∝ν^s that is much harder than for isotropic particles. Remarkably, for σ₀≫1 we find that the angle-integrated spectral slope in the slow cooling regime is s∼0.5-0.7 for a wide range of turbulence fluctuations, 0.25≲δB_rms0²/B₀²≲4, despite significant variations in the power-law energy spectrum of nonthermal particles. This is because weaker turbulence levels imprint a stronger degree of anisotropy, thereby counteracting the effect of the steeper particle spectrum. The synchrotron spectral slope may be even harder, s≳0.7, if the observer is in the plane perpendicular to the mean magnetic field. Our results are independent of domain size and dimensionality. Our findings may help explain the origin of hard synchrotron spectra of astrophysical nonthermal sources, most notably the radio spectrum of pulsar wind nebulae.