Physical conditions of the acceleration region of a solar flare with an unusually narrow gyrosynchrotron spectrum

During the rising phase of the August 30, 2002 X1.5 flare a short pulse with a total duration of 8 seconds was observed. Its background-subtracted radio spectrum ranges only from 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 α 8. Maximum degree of polarization at 7 GHz is around 5%. The hard X-ray pulse emission above the background in the range of 30 150 keV observed by RHESSI is coincident in time with the microwave observation. Hard X-ray images reveal very compact (∼ 10″) footpoint sources. Below 30 keV, a thermal source is observed.