Kinetic Beaming in Radiative Relativistic Magnetic Reconnection: A Mechanism for Rapid Gamma-Ray Flares in Jets. (arXiv:2002.07243v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Mehlhaff_J/0/1/0/all/0/1">J. M. Mehlhaff</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Werner_G/0/1/0/all/0/1">G. R. Werner</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Uzdensky_D/0/1/0/all/0/1">D. A. Uzdensky</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Begelman_M/0/1/0/all/0/1">M. C. Begelman</a>
Rapid gamma-ray flares pose an astrophysical puzzle, requiring mechanisms
both to accelerate energetic particles and to produce fast observed
variability. These dual requirements may be satisfied by collisionless
relativistic magnetic reconnection. On the one hand, relativistic reconnection
can energize gamma-ray emitting electrons. On the other, as previous kinetic
simulations have shown, the reconnection acceleration mechanism preferentially
focuses high-energy particles — and their emitted photons — into beams, which
may create rapid blips in flux as they cross a telescope’s line of sight. Using
a series of 2D pair-plasma particle-in-cell simulations, we explicitly
demonstrate the critical role played by radiative cooling in mediating the
observable signatures of this `kinetic beaming’ effect. Only in our efficiently
cooled simulations do we measure kinetic beaming beyond one light crossing time
of the reconnection layer. We find a correlation between the cooling strength
and the photon energy range across which persistent kinetic beaming occurs:
stronger cooling coincides with a wider range of beamed photon energies. We
also apply our results to rapid gamma-ray flares in flat-spectrum radio
quasars, suggesting that a paradigm of radiatively efficient kinetic beaming
constrains relevant emission models. In particular, beaming-produced
variability may be more easily realized in two-zone (e.g. spine-sheath) setups,
with Compton seed photons originating in the jet itself, rather than in
one-zone external Compton scenarios.
Rapid gamma-ray flares pose an astrophysical puzzle, requiring mechanisms
both to accelerate energetic particles and to produce fast observed
variability. These dual requirements may be satisfied by collisionless
relativistic magnetic reconnection. On the one hand, relativistic reconnection
can energize gamma-ray emitting electrons. On the other, as previous kinetic
simulations have shown, the reconnection acceleration mechanism preferentially
focuses high-energy particles — and their emitted photons — into beams, which
may create rapid blips in flux as they cross a telescope’s line of sight. Using
a series of 2D pair-plasma particle-in-cell simulations, we explicitly
demonstrate the critical role played by radiative cooling in mediating the
observable signatures of this `kinetic beaming’ effect. Only in our efficiently
cooled simulations do we measure kinetic beaming beyond one light crossing time
of the reconnection layer. We find a correlation between the cooling strength
and the photon energy range across which persistent kinetic beaming occurs:
stronger cooling coincides with a wider range of beamed photon energies. We
also apply our results to rapid gamma-ray flares in flat-spectrum radio
quasars, suggesting that a paradigm of radiatively efficient kinetic beaming
constrains relevant emission models. In particular, beaming-produced
variability may be more easily realized in two-zone (e.g. spine-sheath) setups,
with Compton seed photons originating in the jet itself, rather than in
one-zone external Compton scenarios.
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