Characterising the X-ray Emissions from Uranus

After the discovery that Jupiter’s and Saturn’s atmospheres scatter solar X-rays, it was postulated that Uranus would follow a similar behaviour (Ness & Schmitt 2000). However, Uranian X-rays remained elusive until a few years ago. A 30-ks-long (~0.5 Uranus rotation) observation in 2002 by the Chandra X-ray Observatory (CXO) revealed a statistically significant, but low signal detection of 5 ± 2.2 X-ray photons in the energy range 0.5-1.2 keV (Dunn et al., 2021). The flux measured during this observation was also higher than what models had predicted if these emissions were only due to solar scattering. Chandra’s unrivalled spatial resolution also showed that some of the Uranian X-ray photons may coincide with the planet’s rings. Two further CXO campaigns from 2017 each lasting ~25 ks (~0.3 Uranus rotation) resulted in non-detections of Uranus, however, there were hints of temporal variability in the data. The excess in X-ray flux, timing variability, and location of the X-ray photons suggest that Uranus may have a higher X-ray albedo than its Giant Planet cousins, and/or there are other X-ray production mechanisms at play, such as auroral emissions and ring fluorescence. Both have been witnessed at Jupiter (aurora) and Saturn (ring fluorescence).

A set of three observations were taken by XMM-Newton in August 2022, January 2023 and February 2023. These were much longer in duration than CXO’s with each being 114-126 ks (~1.8-2.0 Uranus rotations) long. After reprocessing the data to Uranus-centric coordinates, the same method as previously used on the CXO dataset (Dunn et al., 2021) and on Saturn X-ray studies (Ness & Schmitt 2000; Weigt et al., 2021) were used to compare the number of Uranian X-ray photons with energies between 0.4-1.0 keV with the background counts. In chronological order, each observation gave counts of 35 ± 5.9, 62 ± 7.9, and 21 ± 4.6 and were 5.0, 6.0, and 2.0 median absolute deviations away from the median of the respective background counts.

Despite soft proton events significantly contaminating 50% of the total exposure time, spectra and ligthtcurves from XMM-Newton’s European Photon Imaging Camera (EPIC) instrument were extracted from each observation.  We present our initial results of the XMM-Newton dataset and highlight whether the European Space Agency’s flagship X-ray observatory’s superior sensitivity and spectral resolution can constrain the atomic composition of the Uranian rings and upper atmosphere and through detecting Solar Wind Charge Exchange X-rays, explore whether the Uranian aurorae are from the planet’s cusps.