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A common assumption used in the study of accretion disks is that the magneticenergy density and the kinetic energy density should be in equipartition. Thisassumption relies on the faster growth rate of the magnetic field strengthagainst the kinetic energy of the particles in the flow, for decreasing radius,combined with a dissipation mechanism that tends towards equipartition. In thispaper, we examine this assumption by modeling the radio, mm and optical spectraof several black hole binaries in their quiescent state. We use a standardtwo-component disk model, consisting of an inner geometrically thick andoptically thin disk, emitting thermal synchrotron radiation, along with anouter, thin disk, which radiates as a multicolor blackbody. We find that at thelow accretion rates typical of the quiescent state, the spectral shape isqualitatively reproduced using magnetic fields that are between 0.1% and 1% ofthe equipartition value, considerably smaller than previously thought. Wediscuss 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 fieldin the central region of the disk; and (2) the role of magnetic dissipation inthe structure of the inflow.

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