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ABSTRACT Relativistic magnetic reconnection is a powerful agent through which magnetic energy can be tapped in astrophysics, energizing particles that then produce observed radiation. In some systems, the highest energy photons come from particles Comptonizing an ambient radiation bath supplied by an external source. If the emitting particle energies are high enough, this inverse Compton (IC) scattering enters the Klein–Nishina regime, which differs from the low-energy Thomson IC limit in two significant ways. First, radiative losses become inherently discrete, with particles delivering an order-unity fraction of their energies to single photons. Secondly, Comptonized photons may pair produce with the ambient radiation, opening up another channel for radiative feedback on magnetic reconnection. We analytically study externally illuminated highly magnetized reconnecting systems for which both of these effects are important. We identify a universal (initial magnetization-independent) quasi-steady state in which gamma-rays emitted from the reconnection layer are absorbed in the upstream region, and the resulting hot pairs dominate the energy density of the inflow plasma. However, a true pair cascade is unlikely, and the number density of created pairs remains subdominant to that of the original plasma for a wide parameter range. Future particle-in-cell simulation studies may test various aspects. Pair-regulated Klein–Nishina reconnection may explain steep spectra (quiescent and flaring) from flat-spectrum radio quasars and black hole accretion disc coronae.
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Kinetic simulations and gamma-ray signatures of Klein–Nishina relativistic magnetic reconnection
ABSTRACT Black hole and neutron star environments often comprise collisionless plasmas immersed in strong magnetic fields and intense baths of low-frequency radiation. In such conditions, relativistic magnetic reconnection can tap the magnetic field energy, accelerating high-energy particles that rapidly cool by inverse Compton (IC) scattering the dense photon background. At the highest particle energies reached in bright gamma-ray sources, IC scattering can stray into the Klein–Nishina regime. Here, the Comptonized photons exceed pair-production threshold with the radiation background and may thus return their energy to the reconnecting plasma as fresh electron–positron pairs. To reliably characterize observable signatures of such Klein–Nishina reconnection, in this work, we present first-principles particle-in-cell simulations of pair-plasma relativistic reconnection coupled to Klein–Nishina and pair-production physics. The simulations show substantial differences between the observable signatures of Klein–Nishina reconnection and reconnection coupled only to low-energy Thomson IC cooling (without pair production). The latter regime exhibits strong harder-when-brighter behaviour; the former involves a stable spectral shape independent of overall brightness. This spectral stability is reminiscent of flat-spectrum radio quasar (FSRQ) GeV high states, furnishing evidence that Klein–Nishina radiative physics operates in FSRQs. The simulated Klein–Nishina reconnection pair yield spans from low to order-unity and follows an exponential scaling law in a single governing parameter. Pushing this parameter beyond its range studied here might give way to a copious pair-creation regime. Besides FSRQs, we discuss potential applications to accreting black hole X-ray binaries, the M87* magnetosphere, and gamma-ray binaries.
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- Award ID(s):
- 1903335
- PAR ID:
- 10485175
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 527
- Issue:
- 4
- ISSN:
- 0035-8711
- Format(s):
- Medium: X Size: p. 11587-11626
- Size(s):
- p. 11587-11626
- Sponsoring Org:
- National Science Foundation
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