UPPER LIMITS TO JOVIAN HARD X-RADIATION FROM THE ULYSSES GAMMA-RAY BURST EXPERIMENT

We describe the first in situ search for Jovian X radiation with a ded icated X ray instrument. Upper limits to Jupiter's X my power in the 2 7-48 keV range are presented using data taken by the gamma my burst ex periment on the inbound Ulysses trajectory, at distances between 163 a nd 110 R(J). The data were analyzed using three methods for detection of (1) a steady flux, (2) a flux linked to System III rotation, and (3 ) a sporadic flux. The 3sigma upper limits lie between 1.1 x 10(8) and 5.6 x 10(8) W. The results are consistent with the idea that the bulk of the Jovian UV auroral emissions are due to energetic electrons, wh ile the previously observed low-energy X radiation is due to ion preci pitation.