Imaging the 511 keV positron annihilation sky with COSI
Thomas Siegert, Steven E. Boggs, John A. Tomsick, Andreas Zoglauer, Carolyn Kierans, Clio Sleator, Jacqueline Beechert, Theresa Brandt, Pierre Jean, Hadar Lazar… 展开作者
Thomas Siegert, Steven E. Boggs, John A. Tomsick, Andreas Zoglauer, Carolyn Kierans, Clio Sleator, Jacqueline Beechert, Theresa Brandt, Pierre Jean, Hadar Lazar, Alex Lowell, Jarred M. Roberts, Peter von Ballmoos
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 $γ$-ray line emission. The strongest persistent and diffuse $γ$-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.
Hydrodynamic and kinetic models for spin-1/2 electron-positron quantum plasmas: Annihilation interaction, helicity conservation, and wave dispersion in magnetized plasmas
Electron-positron Annihilation Lines and Decaying Sterile Neutrinos
如果存在大量的惰性中微子，它们衰变为光子和/或电子-正电子对可能会引起可观察到的后果。我们考虑了MeV无菌中微子衰变导致银河系中心弥散正电子湮没线的可能性，并由此从相关的天体物理/宇宙学数据中得到了无菌中微子衰变率$Γ_e \ge 10^{-28}$ s $^{-1}$的界.此外，我们预计软伽马流量$1.2 \times 10^{-4}-9.7 \times 10^{-4}$ ph cm $^{-2}$ s $^{-1}$从银河系中心，显示为一个小MeV的颠簸在背景光子谱。此外，我们还估计了无菌中微子衰变产生的活动中微子穿过地球的通量为$0.02-0.1$ cm $^{-2}$ s $^{-1}$。
