[论文解读] The Compton Spectrometer and Imager

John A. Tomsick, Andreas Zoglauer+22 more|arXiv (Cornell University)|Aug 12, 2019

Gamma-ray bursts and supernovae参考文献 24被引用 38

一句话总结

论文介绍 COSI，一种 SMEX 伽玛射线望远镜（0.2–5 MeV），使用锗十字条探测器阵列实现高分辨谱学、成像和偏振，任务设计旨在绘制 Galactic 正电子湮灭与核合成线的图谱。

ABSTRACT

In this Astro2020 APC White Paper, we describe a Small Explorer (SMEX) mission concept called the Compton Spectrometer and Imager. COSI is a Compton telescope that covers the bandpass often referred to as the "MeV Gap" because it is the least explored region of the whole electromagnetic spectrum. COSI provides a significant improvement in sensitivity along with high-resolution spectroscopy, enabling studies of 511 keV electron-positron annihilation emission and measurements of several radioactive elements that trace the Galactic history of supernovae. COSI also measures polarization of gamma-ray bursts (GRBs), accreting black holes, and pulsars as well as detecting and localizing multimessenger sources. In the following, we describe the COSI science, the instrument, and its capabilities. We highlight many Astro2020 science WPs that describe the COSI science in depth.

研究动机与目标

• Motivate a Small Explorer mission (COSI-SMEX) to address the MeV gap in gamma-ray astronomy.
• Enable high-sensitivity mapping of Galactic 511 keV positron annihilation and nucleosynthesis lines (26Al, 60Fe, 44Ti).
• Provide polarization measurements of GRBs, AGN, pulsars, and multimessenger counterparts.
• Characterize the MeV gamma-ray sky to study cosmic-ray and background processes.
• Demonstrate a mature instrument concept with proven detectors and software for Compton imaging.

提出的方法

• Use a stacked array of germanium cross-strip detectors for high-resolution spectroscopy and 3D interaction localization.
• Employ Compton imaging with MEGAlib software for event reconstruction of multiple interactions.
• House detectors in a cryostat cooled to <80 K with active shielding to reduce background and define a 25% sky field of view.
• Implement an all-sky survey mode via 12-hour North-South re-pointing to achieve 100% sky coverage per day.
• Utilize an active BGO shield to extend the effective field of view to ~50% for GRB detection and localization.

实验结果

研究问题

• RQ1 How can MeV gamma-ray sensitivity be significantly improved to map positron annihilation and radionuclide lines across the Galaxy?
• RQ2 What are the capabilities of a MeV Compton telescope in terms of angular resolution, spectral resolution, and polarization measurements for diverse sources (GRBs, pulsars, AGN)?
• RQ3 Can COSI provide accurate localization and polarization measurements to enable multimessenger counterpart searches (GW/neutrino associations)?
• RQ4 What is the expected impact on understanding Galactic nucleosynthesis and positron sources with wide-field MeV observations?

主要发现

参数	COSI 值
Energy Range	0.2–5 MeV
Spectral Resolution	0.2–1%
Field of View	25% sky
Sky Coverage	100% per day
Angular Resolution (511 keV)	3.2° FWHM
Angular Resolution (1.809 MeV)	1.5° FWHM
Line Sensitivity (511 keV)	7.9×10^-6 photons cm^-2 s^-1
Line Sensitivity (1.809 MeV)	1.7×10^-6 photons cm^-2 s^-1
Flux polarization limit	15 mCrab
GRB polarization fluence limit	4×10^-6 erg cm^-2

• COSI offers 0.2–5 MeV coverage with 0.2–1% spectral resolution and 25% sky FOV, enabling wide-field MeV gamma-ray astronomy.
• Angular resolutions: 3.2° FWHM at 511 keV and 1.5° FWHM at 1.809 MeV, improving imaging of positron annihilation and radionuclide maps.
• Projected line sensitivities in 2 years: 511 keV = 7.9×10^-6 photons cm^-2 s^-1; 1.809 MeV = 1.7×10^-6 photons cm^-2 s^-1.
• Polarization sensitivity: flux polarization limit of 15 mCrab; GRB polarization fluence limit of 4×10^-6 erg cm^-2 for 50% minimum detectable polarization in two years.
• COSI enables multimessenger science by detecting and localizing short GRBs associated with gravitational-wave events and potential high-energy neutrino counterparts.
• The instrument has demonstrated progress via balloon flights, including Crab Nebula detection and GRB localization, validating the concept for a SMEX mission.