An Observational Signature of Sub-Equipartition Magnetic Fields in the Spectra of Black Hole Binaries

TL;DR

This study models spectra of quiescent black hole binaries and finds that their magnetic fields are significantly weaker than the traditionally assumed equipartition levels, impacting jet launching theories.

Contribution

The paper provides observational evidence that magnetic fields in quiescent black hole binaries are much weaker than previously assumed, challenging the equipartition assumption.

Findings

Magnetic fields are between 0.1% and 1% of equipartition value.

Spectral shapes are reproduced with weaker magnetic fields.

Implications for jet launching and magnetic dissipation in accretion flows.

Abstract

A common assumption used in the study of accretion disks is that the magnetic energy density and the kinetic energy density should be in equipartition. This assumption relies on the faster growth rate of the magnetic field strength against the kinetic energy of the particles in the flow, for decreasing radius, combined with a dissipation mechanism that tends towards equipartition. In this paper, we examine this assumption by modeling the radio, mm and optical spectra of several black hole binaries in their quiescent state. We use a standard two-component disk model, consisting of an inner geometrically thick and optically thin disk, emitting thermal synchrotron radiation, along with an outer, thin disk, which radiates as a multicolor blackbody. We find that at the low accretion rates typical of the quiescent state, the spectral shape is qualitatively reproduced using magnetic fields that are between 0.1% and 1% of the equipartition value, considerably smaller than previously thought. We discuss our findings in view of (1) the launching of jets in these objects, which is commonly believed to rely on the presence of a strong magnetic field in the central region of the disk; and (2) the role of magnetic dissipation in the structure of the inflow.