The balloon-borne Compton Spectrometer and Imager (COSI) had a successful 46-day flight in 2016. The instrument is sensitive to photons in the energy range $0.2$-$5$ MeV. Compton telescopes have the advantage of a unique imaging response and provide the possibility of strong background suppression. With its high-purity germanium detectors, COSI can precisely map $\gamma$-ray line emission. The strongest persistent and diffuse $\gamma$-ray line signal is the 511 keV emission line from the annihilation of electrons with positrons from the direction of the Galactic centre. While many sources have been proposed to explain the amount of positrons, $\dot{N}_{\mathrm{e^+}} \sim 10^{50}\,\mathrm{e^+\,yr^{-1}}$, the true contributions remain unsolved. In this study, we aim at imaging the 511 keV sky with COSI and pursue a full-forward modelling approach, using a simulated and binned imaging response. For the strong instrumental background, we describe an empirical approach to take the balloon environment into account. We perform two alternative methods to describe the signal: Richardson-Lucy deconvolution, an iterative method towards the maximum likelihood solution, and model fitting with pre-defined emission templates. Consistently with both methods, we find a 511 keV bulge signal with a flux between $0.9$ and $3.1 \times 10^{-3}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$, confirming earlier measurements, and also indications of more extended emission. The upper limit we find for the 511 keV disk, $< 4.3 \times 10^{-3}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$, is consistent with previous detections. For large-scale emission with weak gradients, coded aperture mask instruments suffer from their inability to distinguish isotropic emission from instrumental background, while Compton-telescopes provide a clear imaging response, independent of the true emission.

中文翻译：

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使用 COSI 对 511 keV 正电子湮灭天空成像

气球携带的康普顿光谱仪和成像仪 (COSI) 在 2016 年成功飞行了 46 天。该仪器对能量范围为 $0.2$-$5$ MeV 的光子敏感。康普顿望远镜具有独特的成像响应优势，并提供强大的背景抑制的可能性。凭借其高纯度锗探测器，COSI 可以精确绘制 $\gamma$ 射线线发射。最强的持久和弥散的 $\gamma$ 射线线信号是来自银河中心方向的正电子与电子湮灭的 511 keV 发射线。虽然已经提出了许多来源来解释正电子的数量，但 $\dot{N}_{\mathrm{e^+}} \sim 10^{50}\,\mathrm{e^+\,yr^{- 1}}$，真正的贡献仍未解决。在这项研究中，我们的目标是使用 COSI 对 511 keV 天空进行成像，并采用全前向建模方法，使用模拟和分级的成像响应。对于强大的仪器背景，我们描述了一种将气球环境考虑在内的经验方法。我们执行两种替代方法来描述信号：Richardson-Lucy 反卷积，一种针对最大似然解的迭代方法，以及使用预定义发射模板的模型拟合。与这两种方法一致，我们发现了一个 511 keV 的凸起信号，其通量在 $0.9$ 和 $3.1 之间 \times 10^{-3}\,\mathrm{ph\,cm^{-2}\,s^{-1} }$，确认了早期的测量结果，以及更多扩展排放的迹象。我们发现 511 keV 磁盘的上限，$< 4.3 \times 10^{-3}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$，与以前的检测。对于梯度较弱的大规模发射，