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

TL;DR

This study demonstrates a direct MeV gamma-ray observation of the Galactic center using an electron-tracking Compton camera on a balloon platform, achieving a $7.9\sigma$ detection in the 150–600 keV band. By modeling backgrounds from atmospheric gamma-rays, cosmic rays, and accidents, and subtracting known point sources with external catalogs, the authors perform imaging-spectroscopy rather than template fitting. Among three emission models, a multi-component configuration provides a notable positronium-related flux of $(3.2 \pm 1.4)\times 10^{-2}$ photons cm$^{-2}$ s$^{-1}$, about a factor of two higher than INTEGRAL, hinting at unresolved sources or diffuse emission and potential exotic origins such as primordial black holes. The results showcase ETCC’s potential for high-precision MeV gamma-ray surveys and outline improvements—such as angular-resolution upgrades and CdZnTe detectors—that could significantly boost sensitivity and enable extensive future Galactic MeV mapping.

Abstract

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.

Figures (3)

• Figure 1: Top panel: Zenith angles of the Crab Nebula (blue) and the Galactic center (red) as a function of time. Second panel: Observed gamma-ray count rate (black points with error bars). The green line shows the estimated background model. The blue line adds contribution from known point sources. Third panel: Count rate after subtracting the background and point-source contributions , overlaid with the predicted diffuse emission of multi-component model (red). Bottom panel: Residuals between the observed data and the total model (including diffuse emission), expressed as a percentage.
• Figure 2: Top-left, top-right, and bottom-left panels show the observed skymap, the estimated background skymap, and the background-subtracted skymap, respectively, all displayed in Galactic coordinates. The orange line traces the trajectory of the zenith direction of the ETCC during the observation. The red and green contours represent the 75% and 50% exposure levels at 511 keV, respectively. The bottom-right panel presents the significance map.
• Figure 3: Measured gamma-ray intensity in the Galactic center region obtained with the ETCC, shown as a function of energy (red points with error bars), compared with previous measurements from INTEGRAL/SPI Berteaud_2022 and COSI Karwin_2023. Upper limits are shown at the 95% confidence level. The ETCC data correspond to the region $|l|<33^{\circ}$ and $|b|<33^{\circ}$, whereas the COSI and INTEGRAL measurements cover larger regions of $|l|<65^{\circ}, |b|<45^{\circ}$ and $|l|<47.5^{\circ}, |b|<47.5^{\circ}$, respectively. The black box and purple point with error bars indicate higher-energy measurements from COMPTEL Strong_1999 and INTEGRAL/SPI Siegert_2022. Solid black, red, and cyan curves represent inverse Compton emission models based on the PDDE, DRE, and DRELowV cosmic-ray propagation scenarios, respectively, for the region $|l|<33^{\circ}$ and $|b|<33^{\circ}$Orland_2017.