Radiative relativistic reconnection powering the hard-state emission of X-ray binaries: synchrotron-driven pair loading, MeV tail, and seed photons
Abstract
In the hard state, X-ray emission from black hole X-ray binaries is attributed to Comptonization in the corona, a highly magnetized region near the black hole. Relativistic magnetic reconnection drives bulk motions that can upscatter soft photons and reproduce the spectral peak at ∼ 100 keV for parameters relevant to Cygnus X-1. However, the origin of the seed photons and of the MeV tail above the peak remains unclear. In 3D reconnection, so-called "free" particles escape the flux ropes and accelerate at the maximum rate set by the reconnection electric field. Their synchrotron emission extends to tens of MeV, which enables γγ pair production with the Compton-upscattered photons. We present 3D particle-in-cell simulations of relativistic reconnection in pair plasma, in which particles cool by inverse Compton (IC) scattering below the synchrotron self-absorption threshold and by synchrotron radiation above it. Of the incoming magnetic power, ∼ 40 % is converted to IC radiation and ≳ 10-20% to synchrotron. Using analytical estimates and radiative calculations for Cygnus X-1, we model how the radiation from reconnection-accelerated particles is reprocessed in the corona. Pair production attenuates the synchrotron emission above ∼ 1 MeV, and the escaping γ-rays form an MeV tail consistent with the observations, extending up to at least ∼ 10 MeV. Most of the absorbed power goes into secondary pairs, whose Thomson optical depth can reach order unity. Their synchrotron emission, peaking at ∼ 0.1 keV, can supply the seed photons for Comptonization. Our results support reconnection-based synchrotron self-Compton models of the hard-state emission in X-ray binaries.
Related
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- Coherent Radio Emission from Flux Rope Mergers in 3D Relativistic Reconnection
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- Two-Phase Structure of Synchrotron-Cooling-Unstable Relativistic Plasma
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BibTeX
@misc{rohoza2026radiative,
title = {{Radiative relativistic reconnection powering the hard-state emission of X-ray binaries: synchrotron-driven pair loading, MeV tail, and seed photons}},
author = {Valeriia Rohoza and Maria Petropoulou and Hayk Hakobyan and Despina Karavola and Lorenzo Sironi},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2611.05307},
eprint = {2611.05307},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2611.05307},
}
RIS
TY - UNPB
TI - Radiative relativistic reconnection powering the hard-state emission of X-ray binaries: synchrotron-driven pair loading, MeV tail, and seed photons
AU - Valeriia Rohoza
AU - Maria Petropoulou
AU - Hayk Hakobyan
AU - Despina Karavola
AU - Lorenzo Sironi
PY - 2026
DA - 2026/11/03
PB - arXiv
DO - 10.48550/arxiv.2611.05307
UR - https://arxiv.org/abs/2611.05307
AB - In the hard state, X-ray emission from black hole X-ray binaries is attributed to Comptonization in the corona, a highly magnetized region near the black hole. Relativistic magnetic reconnection drives bulk motions that can upscatter soft photons and reproduce the spectral peak at ∼ 100 keV for parameters relevant to Cygnus X-1. However, the origin of the seed photons and of the MeV tail above the peak remains unclear. In 3D reconnection, so-called "free" particles escape the flux ropes and accelerate at the maximum rate set by the reconnection electric field. Their synchrotron emission extends to tens of MeV, which enables γγ pair production with the Compton-upscattered photons. We present 3D particle-in-cell simulations of relativistic reconnection in pair plasma, in which particles cool by inverse Compton (IC) scattering below the synchrotron self-absorption threshold and by synchrotron radiation above it. Of the incoming magnetic power, ∼ 40 % is converted to IC radiation and ≳ 10-20% to synchrotron. Using analytical estimates and radiative calculations for Cygnus X-1, we model how the radiation from reconnection-accelerated particles is reprocessed in the corona. Pair production attenuates the synchrotron emission above ∼ 1 MeV, and the escaping γ-rays form an MeV tail consistent with the observations, extending up to at least ∼ 10 MeV. Most of the absorbed power goes into secondary pairs, whose Thomson optical depth can reach order unity. Their synchrotron emission, peaking at ∼ 0.1 keV, can supply the seed photons for Comptonization. Our results support reconnection-based synchrotron self-Compton models of the hard-state emission in X-ray binaries.
ER -