A Low Signal Detection of X-Rays From Uranus

Within the solar system, X-ray emissions have been detected from every planet except the Ice Giants: Uranus and Neptune. We analyze the three archival Chandra X-ray observations of Uranus (each 24-30 ks duration) to date: a stand-alone Advanced CCD Imaging Spectrometer (ACIS) observation on August 7, 2002 and two High Resolution Camera (HRC) observations on November 11 and 12, 2017 coordinated with optical observations. For the earlier ACIS observation, the Uranus-coincident photons were clustered in the 0.6-1.1 keV spectral range, consistent with emission from Jupiter and Saturn. To test the significance of the detected signal, we distributed a grid of similar to 10,000 Uranus-sized regions across the field of view (FoV). The number of Uranus-coincident X-ray photons in the 0.5-1.2 keV range exceeded 99.9% of Uranus-sized regions across the FoV (10.2 standard deviations > FoV mean; probability of chance occurrence similar to 10(-6)-10(-7)). However, the planetary signal was low with only 5 +/- 2.2 X-ray photons against a FoV mean background of 0.16 photons. Without the possibility of energy filtering, the recent HRC observations had a much brighter background (FoV mean similar to 10 photons). Consequently, neither of the new observations provided a second unambiguous Uranus detection, although a 40-min interval of brightening on November 12, 2017 did produce a signal above 99.9% of the FoV. The observed Uranus X-ray fluxes of 10(-15)-10(-16) erg/cm(2)/s are consistent with previous observational limits and modeling predictions. These fluxes exceed expectations from scattered solar emission alone, suggesting either a larger X-ray albedo than Jupiter/Saturn or the possibility of additional X-ray production processes at Uranus. Further observations are needed to test this.