Nonthermal emission from the plunging region: a model for the high-energy tail of black hole X-ray binary soft states

Abstract

X-ray binaries exhibit a soft spectral state comprising thermal blackbody emission at 1 keV and a power-law tail above 10 keV. Empirical models fit the high-energy power-law tail to radiation from a nonthermal electron distribution, but the physical location of the nonthermal electrons and the reason for their power-law index and high-energy cut-off are still largely unknown. Here, we propose that the nonthermal electrons originate from within the black hole's innermost stable circular orbit (the ''plunging region''). Using an analytic model for the plunging region dynamics and electron distribution function properties from particle-in-cell simulations, we outline a steady-state model that can reproduce the observed spectral features. In particular, our model reproduces photon indices of $\Gamma \gtrsim 2$ and power-law luminosities on the order of a few percent of the disk luminosity for strong magnetic fields, consistent with observations of the soft state. Because the emission originates so close to the black hole, we predict that the power-law luminosity should strongly depend on the system inclination angle and black hole spin. This model could be extended to the power-law tails observed above 400 keV in the hard state of X-ray binaries.

10 pages, 8 figures. Accepted in MNRAS

References in corpus (8)

• X-ray Properties of Black-Hole Binaries
• Black hole flares: ejection of accreted magnetic flux through 3D plasmoid-mediated reconnection
• Three-Dimensional Simulations of Magnetized Thin Accretion Disks around Black Holes: Stress in the Plunging Region
• A Grid of Relativistic, non-LTE Accretion Disk Models for Spectral Fitting of Black Hole Binaries
• Sgr A* X-ray flares from non-thermal particle acceleration in a magnetically arrested disc
• Reconnection and particle acceleration in 3D current sheet evolution in moderately-magnetized astrophysical pair plasma
• Hard Synchrotron Spectra from Magnetically Dominated Plasma Turbulence
• Pair-Regulated Klein-Nishina Relativistic Magnetic Reconnection with Applications to Blazars and Accreting Black Holes

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• Thermal solutions of strongly magnetized disks and the hysteresis in X-ray binaries
• The inner structure and thermodynamics of a thin accretion disc
• Accretion Geometry of Black Hole X-ray Binaries: Insights from X-ray Observations
• A New Approach for Testing Einstein's Theory of Gravity Close to Rapidly Spinning Black Holes
• Fully kinetic shearing-box simulations of magnetorotational turbulence in 2D and 3D. I. Pair plasmas