Observation of the Galactic Center in the Sub-MeV Gamma-Ray Band with an Electron-Tracking Compton Camera

We report the direct detection of gamma-ray emission from the Galactic center in the 150--600~keV band using the electron-tracking Compton camera (ETCC), which has a wide field of view of 3.1~sr and represents the first application of this linear, imaging-spectroscopy method to observations of the Galactic center. A one-day flight over Australia resulted in a significant gamma-ray detection in the light curve and revealed a $7.9σ$ excess \revise{over the background} in the image map from the Galactic center region. These results, obtained through a simple and unambiguous analysis, demonstrate the high reliability and sensitivity of the ETCC and establish its potential for future high-precision MeV gamma-ray observations. The measured intensity and spatial distribution were tested against three emission models: a single point-like source, a multi-component structure, and a symmetric two-dimensional Gaussian. All models were found to be statistically consistent with the data. The positronium-related flux in the multi-component model is $(3.2~\pm~1.4)~\times~10^{-2}$~photons~cm$^{-2}$s$^{-1}$, which is approximately a factor of two higher than the value reported by INTEGRAL, with a discrepancy at the $2σ$ level. While remaining subject to a systematic uncertainty of at most 12\% in the detector response and to uncertainties in the inverse-Compton modeling, this difference may arise from unresolved sources or truly diffuse emission, such as exotic processes involving light dark matter or primordial black holes.