The All-sky Medium Energy Gamma-ray Observatory eXplorer (AMEGO-X) is designed to identify and characterize gamma rays from extreme explosions and accelerators. The main science themes include supermassive black holes and their connections to neutrinos and cosmic rays; binary neutron star mergers and the relativistic jets they produce; cosmic ray particle acceleration sources including galactic supernovae; continuous monitoring of other astrophysical events and sources over the full sky in this important energy range. AMEGO-X will probe the medium energy gamma-ray band using a single instrument with sensitivity up to an order of magnitude greater than previous telescopes in the energy range 100 keV to 1 GeV that can be only realized in space. During its 3-year baseline mission, AMEGO-X will observe nearly the entire sky every two orbits, building up a sensitive all-sky map of gamma-ray sources and emissions. AMEGO-X was submitted in the recent 2021 NASA MIDEX announcement of opportunity.

In 2016, the Compton Spectrometer and Imager (COSI) had a successful 46 day flight on board NASA’s Super Pressure Balloon platform. In this work, we report measurements of the Galactic diffuse continuum emission (GDCE) observed toward the inner Galaxy during the flight, which in the COSI energy band (0.2–5 MeV) is primarily generated from inverse Compton radiation. Within uncertainties, we find overall good agreement with previous measurements from INTEGRAL/SPI and COMPTEL. Based on these initial findings, we discuss the potential for further probing the GDCE with the 2016 COSI balloon data, as well as prospects for the upcoming satellite mission.

Gamma-ray bursts are the most luminous electromagnetic events in the Universe. Their prompt gamma-ray emission has typical durations between a fraction of a second and several minutes. A rare subset of these events have durations in excess of a thousand seconds, referred to as ultra-long gamma-ray bursts. Here, we report the discovery of the longest gamma-ray burst ever seen with a $\sim$25 000 s gamma-ray duration, GRB 250702B, and characterize this event using data from four instruments in the InterPlanetary Network and the Monitor of All-sky X-ray Image. We find a hard spectrum, subsecond variability, and high total energy, which are only known to arise from ultrarelativistic jets powered by a rapidly spinning stellar-mass central engine. These properties and the extreme duration are together incompatible with all confirmed gamma-ray burst progenitors and nearly all models in the literature. This burst is naturally explained with the helium merger model, where a field binary ends when a black hole falls into a stripped star and proceeds to consume and explode it from within. Under this paradigm, GRB 250702B adds to the growing evidence that helium stars expand and that some ultra-long GRBs have similar evolutionary pathways as collapsars, stellar-mass gravitational wave sources, and potentially rare types of supernovae.

AMEGO-X, the All-sky Medium Energy Gamma-ray Observatory eXplorer, is a proposed instrument designed to bridge the so-called “MeV gap” by surveying the sky with unprecedented sensitivity from ∼100 keV to about 1 GeV. This energy band is of key importance for multimessenger and multiwavelength studies but it is nevertheless currently underexplored. AMEGO-X addresses this situation by proposing a design capable of detecting and imaging gamma rays via both Compton interactions and pair production processes. However, some of the objects that AMEGO-X will study, such as gamma-ray bursts and magnetars, extend to energies below ∼100 keV where the dominant interaction becomes photoelectric absorption. These events deposit their energy in a single pixel of the detector. In this work we show how the ∼3500 cm² effective area of the AMEGO-X tracker to events between ∼25 and ∼100 keV will be utilized to significantly improve its sensitivity and expand the energy range for transient phenomena. Although imaging is not possible for single-site events, we show how we will localize a transient source in the sky using their aggregate signal to within a few degrees. This technique will more than double the number of cosmological gamma-ray bursts seen by AMEGO-X, allow us to detect and resolve the pulsating tails of extragalactic magnetar giant flares, and increase the number of detected less-energetic magnetar bursts—some possibly associated with fast radio bursts. Overall, single-site events will increase the sensitive energy range, expand the science program, and promptly alert the community of fainter transient events.