Revised equipartition and minimum energy formula for magnetic field strength estimates from radio synchrotron observations

By Rainer Beck, M. Krause. 2005

308 citations recorded by the source corpus.

The commonly used classical equipartition or minimum-energy estimate of total magnetic fields strengths from radio synchrotron intensities is of limited practical use because it is based on the hardly known ratio K of the total energies of cosmic ray protons and electrons and also has inherent problems. We present a revised formula, using the number density ratio K for which we give estimates. For particle acceleration in strong shocks K is about 40 and increases with decreasing shock strength. Our revised estimate for the field strength gives larger values than the classical estimate for flat radio spectra with spectral indices of about 0.5–0.6, but smaller values for steep spectra and total fields stronger than about 10 µG. In very young supernova remnants, for example, the classical estimate may be too large by up to 10×. On the other hand, if energy losses of cosmic ray electrons are important, K increases with particle energy and the equipartition field may be underestimated significantly. Our revised larger equipartition estimates in galaxy clusters and radio lobes are consistent with independent estimates from Faraday rotation measures, while estimates from the ratio between radio synchrotron and X-ray inverse Compton intensities generally give much weaker fields. This may be explained e.g. by a concentration of the field in filaments. Our revised field strengths may also lead to major revisions of electron lifetimes in jets and radio lobes estimated from the synchrotron break frequency in the radio spectrum. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/ba139926f678c51107234d42d827741e5ad80110

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Citations in the corpus

The Starburst Contribution to the Extragalactic γ-Ray Background (2006, background): “Typical estimates for the ratio of the total energy injected into protons and primary electrons are 20Y40 (e.g., Beck & Krause 2005), in which case secondary electrons/positrons likely dominate the radio emission in starbursts (see T06; Rengarajan 2005).”
The Astrophysics of Ultrahigh-Energy Cosmic Rays (2011, background): “…the field strength using the synchrotron emission (assuming equipartition between the total energy density of non thermal particles and of the magnetic field for example), depends notably on the hardly known ratio between the leptonic and hadronic accelerated particles (e.g., Beck & Krause 2005).”
Diffuse Radio Emission from Galaxy Clusters (2019, background): “Some estimates have relied on the idea that the energies in cosmic rays and magnetic fields in the radio emitting regions are the same (“equipartition”; Beck & Krause 2005).”
CALIBRATING EXTINCTION-FREE STAR FORMATION RATE DIAGNOSTICS WITH 33 GHz FREE–FREE EMISSION IN NGC 6946 (2011, methodology): “‡ Calculated using the total 1.4GHz flux densities and the revised minimum energy calculation of Beck & Krause (2005).”
Magnetic Fields in Starburst Galaxies and the Origin of the FIR-Radio Correlation (2006, background): “Because of the potential importance of this conclusion, it is worth critically examining the applicability of the minimum energy estimate in the context of luminous starbursts (see Beck & Krause 2005 for a more general discussion).”
Magnetic fields and spiral arms in the galaxy M51 (2010, methodology): “8, applying the revised formulae by Beck & Krause (2005).”