Abstract
Abstract
The recent discovery of astrophysical neutrinos from the Seyfert galaxy NGC 1068 suggests the presence of nonthermal protons within a compact “coronal” region close to the central black hole. The acceleration mechanism of these nonthermal protons remains elusive. We show that a large-scale magnetic reconnection layer, of the order of a few gravitational radii, may provide such a mechanism. In such a scenario, rough energy equipartition between magnetic fields, X-ray photons, and nonthermal protons is established in the reconnection region. Motivated by recent 3D particle-in-cell simulations of relativistic reconnection, we assume that the spectrum of accelerated protons is a broken power law, with the break energy being constrained by energy conservation (i.e., the energy density of accelerated protons is at most comparable to the magnetic energy density). The proton spectrum is
dn
p
/
dE
p
∝
E
p
−
1
below the break and
dn
p
/
dE
p
∝
E
p
−
s
above the break, with IceCube neutrino observations suggesting s ≃ 3. Protons above the break lose most of their energy within the reconnection layer via photohadronic collisions with the coronal X-rays, producing a neutrino signal in good agreement with the recent observations. Gamma rays injected in photohadronic collisions are cascaded to lower energies, sustaining the population of electron–positron pairs that makes the corona moderately Compton thick.
Funder
Simons Foundation
National Science Foundation
Villum Fonden
EC ∣ Horizon 2020 Framework Programme
NASA ATP
Agence Nationale de la Recherche
MERAC Foundation
Hellenic Foundation for Research and Innovation
U.S. Department of Energy
Publisher
American Astronomical Society
Subject
Space and Planetary Science,Astronomy and Astrophysics