The Galactic Explorer with a Coded Aperture Mask Compton Telescope (GECCO) is a novel Explorer-class concept for a next-generation telescope covering the poorly explored hard X-ray and soft gamma-ray energies. This concept builds upon the heritage of past and current missions, improving sensitivity and, very importantly, angular resolution. GECCO uses the combined Coded Aperture Mask and Compton telescope techniques to employ the benefits of both: superior angular resolution provided by the deployable Coded Aperture Mask, and good background rejection and wide field-of-view (FoV) provided by the Compton telescope. It is being developed at NASA/GSFC in collaboration with other US and foreign institutions. GECCO observations will extend arcminute angular resolution to high-energy images of the Galactic plane, combining the spectral capabilities of INTEGRAL/IBIS and the x-ray imaging of NuSTAR and eROSITA, and will make a bridge to the Fermi-LAT observations, enabling a broad potential for discoveries in the MeV γ-ray sky.

With the unprecedented angular resolution of the coded mask telescope combined with the sensitive, wide FoV Compton telescope, GECCO will focus on two main science objectives:

1. Explore and resolve heavily populated sky regions, in particular the Galactic Center to resolve the nature and environment of the central massive black hole, to decipher the nature of its emission, such as the emission associated with the dark matter, new types of sources, or currently unresolved population of point sources. The GC long-term observation with ~1 arcmin angular resolution could be crucial in disentangling new physics in unexplored energy range. The Carina OB1 and OB2 associations, home to the most massive and luminous stars in the Milky Way, and Cygnus region, are other crowded places where high angular resolution is needed to resolve sources.
2. Large FoV monitoring for transient events, detected with high sensitivity and accurate localization, performing multimessenger investigations to support GW and neutrino discoveries.

Also, GECCO’s observational capability to disentangle discrete sources from truly diffuse emission and high energy resolution will contribute to understanding the gamma-ray Galactic Center excess and the Fermi Bubbles, and to tracing low-energy cosmic rays and their propagation in the Galaxy. Nuclear and annihilation lines will be spatially and spectrally resolved from continuum emission and from sources, addressing the role of low-energy cosmic rays in star formation and galaxy evolution, the origin of the 511 keV positron line, fundamental physics, and Galactic chemical evolution. Of special interest will be the exploration of sites of explosive element synthesis by conducting high-sensitivity measurements of nuclear lines from Type 1a supernovae and from other objects.

The GECCO design is based on the novel CZT Imaging calorimeter, which serves as a standalone Compton telescope and as a focal plane detector for the Coded Mask. It also is a powerful tool to measure the γ-radiation polarization. GECCO’s octagon-shaped active shield also serves as a powerful all-sky detector of gamma-ray bursts, prompting the instrument to slew towards the burst direction and localize it with the Coded Aperture’s arcminute accuracy.

GECCO will operate in the 100 keV - 10 MeV energy range, with energy resolution of ~ 1% from 0.5 - 5 MeV. The Coded Aperture Mask provides the angular resolution of ∼1 arcmin with a 2 × 2 deg² fully coded field-of-view, while the Compton telescope provides the angular resolution of 3 − 6 deg with a 60 × 60 deg² field-of-view. The 3σ, 10⁶ s sensitivity is expected to be about 10^-5 MeV cm^-2 s^-1 over the entire energy range. Primary mode of observation is fixed pointing, with extended exposure of the regions of interest. However, as a standalone Compton telescope with wide FoV, Imaging Calorimeter will simultaneously provide wide-area sky exploration, significantly broadening GECCO’s observational scope.

Contact local : Philippe LAURENT

Organisateur : Frédéric GALLIANO