Origin of power-law X-ray emission in the Steep power-law state of X-ray Binaries

TL;DR

This paper proposes a new model where the steep power-law X-ray emission in black hole binaries originates from synchrotron radiation of relativistic electrons in magnetized spots near the ISCO, explaining high-energy spectra and QPOs.

Contribution

It introduces a novel scenario linking synchrotron emission in magnetized spots to the steep power-law state, connecting spectral features with accretion dynamics.

Findings

Synchrotron radiation from relativistic electrons explains the steep X-ray spectrum.

Model accounts for high-frequency QPOs and high luminosity in SPL state.

Down-scattering by plasma shapes the observed spectrum.

Abstract

We present a new scenario of the emissive origin in the Steep Power Law (SPL) state of X-ray Binaries. The power-law component of X-ray emission is the synchrotron radiation of relativistic electrons in highly magnetized compact spots orbiting near the inner stable circular orbit (ISCO) of black hole and has a hard spectrum that extends to above MeV bands determined by electron acceleration rate. These photons are then down-scattered by the surrounding plasma and form an observed steep spectrum. The relevance of this model with high-frequency quasi-periodic oscillations (HFQPOs) and extremely high luminosity of the SPL state is discussed.