A very narrow gyrosynchrotron spectrum during a solar flare

Free access article

Issue		A&A Volume 457, Number 2, October II 2006


Page(s)		693 - 697
Section		The Sun
DOI		http://dx.doi.org/10.1051/0004-6361:20054438

A&A 457, 693-697 (2006)
DOI: 10.1051/0004-6361:20054438

A very narrow gyrosynchrotron spectrum during a solar flare

C. G. Giménez de Castro¹, J. E. R. Costa^{1, 2}, A. V. R. Silva¹, P. J. A. Simões^{1, 2}, E. Correia^{1, 2} and A. Magun³

¹ Centro de Rádio Astronomia e Astrofísica Mackenzie, R. da Consolação 896, 01302-907 São Paulo, SP, Brazil
e-mail: guigue@craam.mackenzie.br
² Instituto Nacional de Pesquisas Espaciais, São José dos Campos, Brazil
³ Institute of Applied Physics, University of Bern, Bern, Switzerland

(Received 28 October 2005 / Accepted 13 June 2006)

Abstract
During the rising phase of the radio burst of August 30, 2002, at $\sim$ 1328 UT a short pulse with a duration of approximately 4 s was observed. Here we present a multiwavelength analysis, including microwave and X-ray. Its background-subtracted radio spectrum ranges only from 2.5 to 12 GHz with a maximum flux density of approximately 900 s.f.u. at 7 GHz and a steep optically thin spectral index $\alpha\sim 8$ . The hard X-ray pulse emission above the background in the range of 10-150 keV observed by RHESSI is coincident in time with the microwave observation. Hard X-ray images reveal very compact (~ $10^{\prime\prime}$ ) footpoint sources. A distribution of accelerated electrons represented by a double power law, with $\delta_{E \ < \ \mathrm{250 ~ keV}} = 5.3$ and $\delta_{E\ \ge \ 250 \ \mathrm{keV}} = 13$ , was used to compute the expected gyrosynchrotron and thick target bremsstrahlung fluxes of a homogeneous source. We interpret the very steep electron index above the energy break to represent a high energy cutoff. With these parameters, our results reproduce the observations well. Nevertheless, they pose the still unanswered question about the mechanism that has slectively accelerated these electrons.

Key words: Sun: activity -- Sun: flares -- Sun: radio radiation

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