The Galactic Annihilation Line Explorer (GALE) Balloon-borne Mission to Study 511-keV Annihilation Emission Morphology in the Inner Galaxy with Makoto Sasaki
The 511 keV signature of electron–positron annihilation was first observed from the Galactic center region in the 1970s, but the source of these positrons is still not well understood. Measurements by OSSE/CGRO in 1990s, later by SPI/INTEGRAL, and recently by COSI in the balloon flight, confirmed that result, concluding to the ~1x10-3 cm-2 s-1 flux from a few degree diameter spot at the GC, but the morphology, and consequently an origin of positrons is still enigmatic: they can either be produced in unresolved faint point sources, or can be genuinely diffuse from of “exotic” nature. Being a subject of a large number of arguing papers, this problem cannot be resolved by the available observation results or by planned (COSI) gamma-ray missions, since none of them has a needed angular resolution of a fraction of degree, combined with a point source sensitivity of <1x10-4 cm-2 s-1to resolve an emission structure of that spot.
The GALE (Galactic Annihilation Line Explorer) coded-aperture telescope can achieve both the required point-source sensitivity and angular resolution in an Ultra-Long Duration Balloon (ULDB) flight, allowing resolution of this mystery. At the heart of the instrument is a cadmium zinc telluride (CZT) 3D calorimeter based on well-developed commercially available pixel detectors. GALE will combine the CZT calorimeter with a coded-aperture mask (CAM), and the resulting telescope will be heavily shielded with both active bismuth germanium oxide(BGO) and passive (tungsten) materials. It will use StarSpec’s Precision Attitude Control System (PACS) to achieve the requisite pointing accuracy (< 1’). In this talk, the science goals, GALE instrument design, anticipated performance and mission will be discussed.
Sponsored by the McDonnell Center for the Space Sciences