AMEGO-X: Medium Energy Gamma-ray Observatory

• AMEGO-X is a NASA MIDEX-class mission concept that fills the MeV gap by integrating Compton, pair-production, and photoelectric detection methods.
• It achieves enhanced sensitivity and sub-degree angular resolution, enabling rapid transient alerts and multimessenger astrophysics studies.
• The mission supports investigations of supermassive black holes, neutron star mergers, and cosmic ray accelerators by covering 100 keV to 1 GeV.

The All-sky Medium Energy Gamma-ray Observatory eXplorer (AMEGO-X) is a NASA MIDEX-class mission concept engineered to achieve a transformative increase in sensitivity for medium-energy gamma-ray astronomy, covering the energy range from approximately 100 keV up to 1 GeV. Designed to fill the “MeV gap” between hard X-ray and high-energy gamma-ray instrumentation, AMEGO-X leverages a hybrid detection architecture that combines Compton, pair-production, and low-energy photoelectric absorption event modes. Its primary scientific objectives focus on multimessenger astrophysics, probing the mechanisms underlying supermassive black holes, neutron star mergers, cosmic ray acceleration, and continuous monitoring of astrophysical transients through full-sky coverage.

1. Mission Scientific Objectives

AMEGO-X targets several classes of high-energy astrophysical phenomena, aiming to provide comprehensive data for both time-domain and multimessenger science. Core objectives include:

• Characterization of supermassive black holes and AGN jets, especially their potential to accelerate protons and produce astrophysical neutrinos and cosmic rays via hadronic emission mechanisms. Observations of blazar flares directly test for hadronic signatures such as pion-decay “bumps” and polarization distinguishing between leptonic and hadronic models.
• Direct detection of prompt gamma-ray bursts (GRBs) and afterglows associated with compact binary mergers (e.g., neutron star–neutron star), crucial for resolving the physics of relativistic jet formation and facilitating joint gravitational–wave/gamma-ray investigations.
• Survey of cosmic ray accelerators in the Galaxy, including supernova remnants, novae, and pulsar wind nebulae, emphasizing signatures of hadronic processes and positron excess phenomena.
• Continuous all-sky transient monitoring, enabling detection and rapid onboard alerting of energetic events—from short GRBs to magnetar flares—across the full sensitive band, with the capability for multimessenger coordination.

Contextually, AMEGO-X advances multimessenger astronomy by complementing gravitational wave and neutrino observatories, rapidly identifying gamma-ray counterparts to extreme events and enabling precise electromagnetic follow-up (Caputo et al., 2022).

2. Instrument Design and Detection Principles

AMEGO-X utilizes a single-instrument architecture built to exploit three distinct photon detection regimes:

• Compton Scattering (<10 MeV): Gamma-ray interactions below ~10 MeV are dominated by Compton scattering. AMEGO-X registers both “untracked” (photon-only, confining the source location to a sky ring) and “tracked” (in which the direction of the Compton-scattered electron is measured, reducing the uncertainty to an arc) Compton events. The key reconstructive formula for Compton events is:

$$\cos\theta_C = 1 - \frac{m_e c^2}{E'_\gamma} - \frac{m_e c^2}{E'_\gamma + E_e}$$

where $E'_\gamma$ and $E_e$ are the energies of the scattered photon and recoil electron, respectively.

• Pair Production (>10 MeV): Gamma rays above ~10 MeV are primarily detected via electron–positron pair production. The silicon pixel tracker provides spatially resolved tracking of both leptons, allowing reconstruction of the incident gamma-ray direction via energy-weighted averaging.
• Photoelectric Single-site Events (~25–100 keV): At energies below ~100 keV, photoelectric absorption dominates. AMEGO-X records “single-site events” in which the photon deposits its full energy in one detector element, extending its transient sensitivity and effective area at low energies (Martinez-Castellanos et al., 2021).

This detection framework is realized via modular towers each consisting of stacked silicon pixel tracker segments (AstroPix), a hodoscopic CsI calorimeter for energy deposition measurement, and a plastic scintillator anti-coincidence detector (ACD) for cosmic-ray background veto (Caputo et al., 2022, Fleischhack, 2021).

3. Detector Technologies and Subsystem Integration

Key subsystems and their functions include:

Subsystem	Function	Technology Highlights
Silicon Pixel Tracker (AstroPix)	Event localization, electron/positron track measurement	Monolithic CMOS active pixel sensors; ∼500 μm pitch; ≤25 keV threshold
CsI:Tl Calorimeter	Shower energy measurement, DOI reconstruction	Orthogonal layers of CsI:Tl bars; SiPM arrays on both ends; energy via $\sqrt{L \times R}$ (Woolf et al., 2019)
ACD	Charged particle background rejection	Plastic scintillator panels; SiPM readouts

• AstroPix Pixels: These implement high-voltage CMOS technology, enabling low-noise, compact integration and time-over-threshold (TOT) measurements for both energy and timing. The dynamic range covers 25–700 keV; measured energy resolution is σ_E ≈ 5 keV at 122 keV, and position resolution is 500 μm (Caputo et al., 3 Dec 2024).
• CsI Calorimeter: The calorimeter subsystem employs SiPM readout arrays for both ends of orthogonally packed CsI:Tl bars, with a geometric mean for deposited energy ($E = \sqrt{L \times R}$), and position reconstruction via light asymmetry. Beam tests have validated ∼3% energy resolution at 662 keV and ~1 cm position resolution (Woolf et al., 2019).
• ACD System: Multi-panel plastic scintillators surround the instrument to efficiently suppress charged particle backgrounds while minimizing self-veto in high-energy events. SiPM readouts ensure fast response (Kierans et al., 2021).

Subsystem optimization addresses both Compton and pair event reconstruction, and single-site events for enhanced transient capability.

4. Observational Strategy and Performance

AMEGO-X will be deployed in a low Earth orbit, utilizing a zenith-rocking survey mode to observe nearly the entire sky every two orbits (~3 hours). This enables:

• Accumulation of a sensitive all-sky map for persistent and variable sources.
• Real-time transient detection, with rapid onboard event reconstruction and alert capabilities (≤30 seconds for GRBs).
• Sensitivity up to an order of magnitude greater than prior instruments in the crucial 100 keV to 1 GeV band.

Instrumental performance metrics are:

• Energy Range: 100 keV–1 GeV, facilitating studies from photoelectric to pair production regimes.
• Angular Resolution: Improved over prior missions—e.g., sub-degree localization for GRBs, Compton events constrained to arcs/rings, and positional accuracy of ∼1 cm for calorimeter events.
• Sensitivity: Minimum detectable flux as a function of observation time, background, and effective area is given by

$$F_{\mathrm{min}}(E) = \frac{n^2 + n\sqrt{n^2 + 4\,N_b}}{2\,A_\mathrm{eff}\,T_\mathrm{Obs}}$$

where n is the desired significance, $N_b$ the background count, $A_\mathrm{eff}$ the effective area, and $T_\mathrm{Obs}$ the observation time (Caputo et al., 2022).

AMEGO-X’s design prioritizes maximizing field-of-view, minimizing passive material, and using neural-network and algorithmic techniques for advanced event reconstruction and background rejection (Angelis et al., 2021).

5. Scientific Impact and Multiprobe Synergies

Probing the MeV regime with AMEGO-X is expected to yield:

• Discriminatory testing of hadronic vs. leptonic jet models in AGN and blazar spectra, including direct polarization and spectral measurements.
• Constraining the physics of relativistic jet formation and post-merger evolution in neutron star mergers via joint GW–gamma-ray observations.
• Identification of pion-decay gamma signatures from galactic cosmic-ray sources.
• Surveying the diffuse MeV background, nuclear line spectroscopy for nucleosynthetic diagnostics (e.g., ^26Al, ^56Ni decay lines), and positron annihilation mapping.
• Large-scale transient detection, with doubled rates for GRB and magnetar identification below 100 keV due to inclusion of single-site events (Martinez-Castellanos et al., 2021).

In dark matter searches, AMEGO-X’s continuum sensitivity enables exploration of parameter regimes inaccessible to line-search missions like COSI, particularly for box-shaped or cascade spectra expected in MeV-scale vector-scalar portal models (Dutra et al., 21 Aug 2025).

The instrument’s modularity and technology choices (pixelated silicon, dual-gain SiPMs for calorimeter, advanced ASIC readouts) are actively being refined with prototype demonstrators such as ComPair and ComPair-2 (Caputo et al., 3 Dec 2024), with measured increases in effective area and energy resolution.

6. Technology Development and Future Prospects

AMEGO-X leverages extensive heritage from Fermi/LAT, COSI, and COSI-X, and prototype validation through ComPair balloon flights. Ongoing development encompasses:

• Advanced AstroPix CMOS pixel sensors meeting high sensitivity and energy resolution requirements under space-radiation conditions (Suda et al., 23 Aug 2024).
• Next-generation calorimeter designs with expanded dynamic range (dual-gain SiPMs) and improved position determination.
• Experimentation in CubeSat platforms like MeVCube, providing rapid pathfinding and low-cost technology demonstration for mission-critical detectors (Lucchetta et al., 2022).

The baseline mission duration is 3 years, with collaborative teams at NASA Goddard, NRL, Argonne, university partners, and Lockheed Martin Space advancing the instrument for prospective flight. AMEGO-X is thus expected to serve as a centerpiece for gamma-ray and multimessenger astrophysics through the late 2020s and beyond.

7. Comparative Context and Controversies

AMEGO-X shares scientific objectives and technical approaches with the European All-Sky-ASTROGAM and ASTROGAM concepts (Tatischeff et al., 2019, Angelis et al., 2021), though with differing implementation. ASA emphasizes nearly 4π sr field of view, a deployable boom for shadow avoidance, and a finely segmented silicon tracker, while AMEGO-X adopts a modular tower design targeting order-of-magnitude sensitivity increases through advanced pixel and calorimeter technologies.

Differences in background rejection strategies, detector geometry, and the focus on low-energy transient events position AMEGO-X as complementary to these missions. Discrepancies in polarimetric capability, survey strategy, and low-energy extension (single-site event mode) remain points for technical comparison and optimization in future mission planning.

In conclusion, AMEGO-X aims to revolutionize medium-energy gamma-ray astrophysics by combining multi-modal photon detection, field-leading sensitivity, and full-sky coverage with advanced detector technologies, playing a central role in the advancement of multiwavelength and multimessenger studies.