Hard X-radiation from a fast coronal ejection

We have observed a high-speed coronal ejection in hard X-rays, detectable t o an altitude of some 2 x 10(5) km in the Yohkoh 23-53 keV energy bands. Si multaneous imaging at 17 and 34 GHz from the Nobeyama radioheliograph shows complex moving features simultaneous with the ejection, including a compac t source that we identify with the rapid X-ray source motion. The hard X-ra y and microwave observations agree on ejection velocities in the vicinity o f 1000 km s(-1). The hard X-ray sources also corresponded in position angle to a bright coronal mass ejection (CME) detected about 15 minutes later an d temporally to both fast-drift and slow-drift radio bursts in the decimete r-meter bands. Other components of coronal hard X-ray emission were also de tected, including an extended long-duration event with a nonthermal spectru m. We suggest that a major eruptive flare occurred in NOAA Active Region 94 15, approximately 26 degrees beyond the west limb at the time of the event. Estimating a source density of 4 x 10(9) cm(-3) from the compact source ob served at 17 GHz, we find a total electron number (>20 keV) of approximatel y N-20 similar to 1.3 x 10(36) for the compact part of the source. We infer that these electrons were trapped in expanding loops forming a part of the CME and may have contributed substantial pressure within these loops.