Discovery of Diffuse Hard X-ray Emission around Jupiter with Suzaku

We report the discovery of diffuse hard (1-5 keV) X-ray emission around Jupiter in a deep 160 ks Suzaku XIS data. The emission is distributed over ~16x8 Jovian radius and spatially associated with the radiation belts and the Io Plasma Torus. It shows a flat power-law spectrum with a photon index of 1.4+/-0.2 with the 1-5 keV X-ray luminosity of (3.3+/-0.5)x10^15 erg/s. We discussed its origin and concluded that it seems to be truly diffuse, although a possibility of multiple background point sources can not be completely rejected with a limited angular resolution. If it is diffuse, the flat continuum indicates that X-rays arise by the non-thermal electrons in the radiation belts and/or the Io Plasma Torus. The synchrotron and bremsstrahlung models can be rejected from the necessary electron energy and X-ray spectral shape, respectively. The inverse-Compton scattering off solar photons by ultra-relativistic (several tens MeV) electrons can explain the energy and the spectrum but the necessary electron density is >~10 times larger than the value estimated from the empirical model of Jovian charge particles.

💡 Research Summary

The authors present the first detection of diffuse hard X‑ray emission surrounding Jupiter, based on a deep 160 ks observation with the Suzaku X‑ray Imaging Spectrometer (XIS). By carefully removing the bright point‑like emission from Jupiter itself and the variable solar background, they isolate a faint, extended component that spans roughly 16 × 8 Jupiter radii (R_J) in the 1–5 keV band. The morphology of this component aligns closely with the known locations of the Jovian radiation belts and the Io Plasma Torus (IPT), suggesting a physical connection to the planet’s magnetospheric plasma environment.

Spectral analysis shows that the emission is well described by a simple power‑law model with photon index Γ = 1.4 ± 0.2 and an unabsorbed 1–5 keV luminosity of (3.3 ± 0.5) × 10¹⁵ erg s⁻¹. The flat spectrum rules out a purely thermal plasma origin and is inconsistent with bremsstrahlung from low‑energy electrons, which would produce characteristic line features not observed.

Three emission mechanisms are examined in detail. Synchrotron radiation would require ultra‑relativistic electrons (≫ GeV) in magnetic fields far stronger than those measured in the Jovian belts, making this scenario implausible. Bremsstrahlung from keV‑MeV electrons also fails to reproduce the observed continuum shape. Inverse‑Compton (IC) scattering of solar photons by electrons with energies of several tens of MeV can generate a power‑law spectrum with the measured index, and the required photon energy density (solar flux at Jupiter) is well known. However, quantitative IC modeling indicates that the electron density needed to produce the observed X‑ray flux is at least an order of magnitude higher than the density predicted by empirical models of the Jovian radiation belts. This discrepancy points to either an underestimation of the high‑energy tail of the electron population or to a localized enhancement of electrons in the region coincident with the IPT.

The limited angular resolution of Suzaku (≈ 2 arcmin) leaves open the possibility that a population of unresolved background point sources contributes partially to the apparent diffuse emission. The authors therefore advocate follow‑up observations with higher‑resolution X‑ray telescopes such as Chandra or future missions (e.g., Athena) to separate genuine diffuse emission from point‑source contamination.

In summary, the paper establishes that Jupiter is surrounded by a previously unknown, spatially extended hard X‑ray glow. The most plausible physical explanation is inverse‑Compton scattering by ultra‑relativistic electrons, but the required electron density exceeds current magnetospheric models, implying that our understanding of particle acceleration and distribution in the Jovian system is incomplete. This discovery opens a new observational window on planetary magnetospheres, linking X‑ray astronomy with in‑situ plasma measurements and prompting further theoretical and observational work to reconcile the X‑ray data with existing models of Jovian charge‑particle populations.