X-ray emission from Saturn

Abstract

We report the first unambiguous detection of X-ray emission originating from Saturn with a Chandra observation, duration 65.5 ks with ACIS-S3. Beyond the pure detection we analyze the spatial distribution of X-rays on the planetary surface, the light curve, and some spectral properties. The detection is based on 162cts extracted from the ACIS-S3 chip within the optical disk of Saturn. We found no evidence for smaller or larger angular extent. The expected background level is 56 cts, i.e., the count rate is (1.6 ± 0.2) × 10-3 cts/s. The extracted photons are rather concentrated towards the equator of the apparent disk, while both polar caps have a relative photon deficit. The inclination angle of Saturn during the observation was ∼-27°, so that the northern hemisphere was not visible during the complete observation. In addition, it was occulted by the ring system. We found a small but significant photon excess at one edge of the ring system. The light curve shows a small dip twice at identical phases, but rotational modulation cannot be claimed at a significant level. Spectral modeling results in a number of statistically, but not necessarily physically, acceptable models. The X-ray flux level we calculate from the best-fit spectral models is ∼6.8 × 10-15 erg cm-2 s-1(in the energy interval 0.1-2keV), which corresponds to an X-ray luminosity of ∼8.7 × 10 14erg s-1. A combination of scatter processes of solar X-rays require a relatively high albedo favoring internal processes, but a definitive explanation remains an open issue.