0220078

We propose a 100ks TOO observation of the transient Black-Hole candidate GX 339-4 in its short-lived Very-High State with the aim to investigate the nature of its hard component. For this component in the Very High State of other Black-Hole Candidates there has been no high-energy cutoff measured pointing towards the presence of non-thermal plasma but the complete picture from spectra and timing is inconsistent. We will be able to establish the presence/absence of such a cutoff with high confidence to determine the shape of the broad-band spectrum and to detect a possible 511 keV line. The trigger will be provided either by RXTE or by Swift. A simultaneous RXTE and radio campaign will be arranged. [truncated! Please see actual data for full text]