Upper limits to Jovian hard X radiation from the Ulysses gamma ray burst experiment

Abstract

We describe the first in situ search for Jovian X radiation with a dedicated X ray instrument. Upper limits to Jupiter's X ray power in the 27-48 keV range are presented using data taken by the gamma ray burst experiment on the inbound Ulysses trajectory, at distances between 163 and 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 3σ upper limits lie between 1.1×10⁸ and 5.6×10⁸ W. The results are consistent with the idea that the bulk of the Jovian UV auroral emissions are due to energetic electrons, while the previously observed low-energy X radiation is due to ion precipitation.