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Particle Acceleration by Magnetic Reconnection in Striped Pulsar Winds and Relativistic Jets

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The relativistic wind of pulsars consists of toroidal
stripes of opposite magnetic field polarity, separated by current sheets of hot
plasma. By means of 2D and 3D particle-in-cell simulations, we investigate
particle acceleration and magnetic field dissipation at the termination shock
of a striped pulsar wind. At the 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 the alternating fields to the
particles. As the value of lambda/(r_L*sigma) increases (here, r_L is the
relativistic Larmor radius in the wind), the post-shock spectrum passes from a
thermal Maxwellian to a flat power-law tail with slope around -1.5, populated
by particles accelerated by the reconnection electric field.

The limit lambda/(r_L*sigma)>>1 is realized in
relativistic jets, where kink instabilities may seed the conditions for
magnetic reconnection. Here, we find that the particle spectrum in the current
sheet approaches a flat power-law tail with slope between -1.5 and -2,
regardless of the conditions in the jet. The spectrum extends to higher
energies for larger magnetizations or colder plasma temperatures, everything
else being fixed. Our results place important constraints on the emission
models of Pulsar Wind Nebulae and magnetically-dominated astrophysical jets.