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Title:
Evidence for a Weak Global Galactic Center Magnetic Field from the Detection of Low Frequency Diffuse Nonthermal Radio Emission
Authors:
LaRosa, T. N.; Brogan, C. L.; Shore, S. N.; Lazio, T. J. W.; Kassim, N. E.; Nord, M. E.
Affiliation:
AA(Kennesaw State Univ.), AB(Inst. for Astronomy), AC(Univ. of Pisa), AD(Naval Research Lab.), AE(Naval Research Lab.), AF(Naval Research Lab.)
Publication:
American Astronomical Society Meeting 205, #32.07; Bulletin of the American Astronomical Society, Vol. 36, p.1399
Publication Date:
12/2004
Origin:
AAS
Bibliographic Code:
2004AAS...205.3207L

Abstract

High resolution 74 MHz continuum observations made with the VLA, together with single dish 330 MHz observations acquired with the GBT, reveal a diffuse but distinct source of nonthermal emission coincident with the Galactic center (GC) region. This emission extends roughly 3 degrees along the plane on either side of the Galactic center. A minimum energy analysis yields a total energy of ˜ 1052 ergs with a corresponding magnetic field strength of 6 f-2/7 microgauss and an equipartition particle energy density of 1.3 f-2/7eV cm-3, where f is the filling factor of the ionized gas. We model this source as residual electrons from an enhanced rate of Galactic center supernova (SN) explosions. The source particle energy density is consistent with the EGRET gamma-ray data but the derived magnetic field is several orders of magnitude below the milligauss field commonly invoked for the GC. We show that depending on the Galactic center cosmic ray energy density, a 1 mG magnetic field can generate orders of magnitude more emission than is observed. The shorter electron radiation lifetimes in a strong magnetic field also require that the energy be resupplied to this source at a rate faster than any plausible SN rate. We conclude that any mean global space filling magnetic field in the Galactic center region must be weak, of order 10 microgauss.

TNL thanks the INFN/Pisa for travel support. Basic research in radio astronomy at the NRL is supported by the Office of Naval Research.

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