Direct imaging of low-energy gamma rays from the Milky Way’s centre claimed as a first

Analyzing SMILE-2+ observations with an electron-tracking camera

The group, involving JAXA and Japanese universities and research institutions, says its analysis of SMILE-2+ observations produced a direct image of gamma-ray emission from the Galactic centre in the 150–600 keV range. It used an electron-tracking Compton camera (ETCC), which records the direction of recoil electrons to determine a gamma ray’s incoming direction. The group describes the result as the first direct imaging in this energy range using linear imaging, and says it also obtained a high-precision positron-annihilation gamma-ray spectrum.

The research group says low-energy gamma-ray observations of the Milky Way’s centre were difficult to image directly because of background noise, so results relied largely on indirect, model-based estimates.

Using data from the 2018 SMILE-2+ balloon experiment, the group reports direct imaging and a high-precision spectrum of 150–600 keV gamma-ray emission from the Galactic centre through linear imaging based on geometrical optics.

The claimed result addresses a longstanding observational limitation in the low-energy, MeV-scale gamma-ray range: strong background noise has made direct images difficult. Direct imaging, if borne out, would provide a way to assess spatial distribution and intensity without relying as heavily on source-model estimates.