  We describe microwave and hard X-ray observations of strong quasiperiodic
pulsations from the GOES X1.3 solar flare on 15 June 2003. The radio
observations were made jointly by the Owens Valley Solar Array (OVSA), the
Nobeyama Polarimeter (NoRP), and the Nobeyama Radioheliograph (NoRH). Hard
X-ray observations were made by the Ramaty High Energy Solar Spectroscopic
Imager (RHESSI). Using Fourier analysis, we study the frequency- and
energy-dependent oscillation periods, differential phase, and modulation
amplitudes of the radio and X-ray pulsations. Focusing on the more complete
radio observations, we also examine the modulation of the degree of circular
polarization and of the radio spectral index. The observed properties of the
oscillations are compared with those derived from two simple models for the
radio emission. In particular, we explicitly fit the observed modulation
amplitude data to the two competing models. The first model considers the
effects of MHD oscillations on the radio emission. The second model considers
the quasi-periodic injection of fast electrons. We demonstrate that
quasiperiodic acceleration and injection of fast electrons is the more likely
cause of the quasiperiodic oscillations observed in the radio and hard X-ray
emission, which has important implications for particle acceleration and
transport in the flaring sources.
