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HERMES-Pathfinder is a space-borne mission based on a constellation of six nano-satellites flying in a low-Earth orbit. The 3U CubeSats, to be launched in early 2025, host miniaturized instruments with a hybrid Silicon Drift Detector/scintillator photodetector system, sensitive to both X-rays and gamma-rays. A seventh payload unit is installed onboard SpIRIT, an Australian-Italian nano-satellite developed by a consortium led by the University of Melbourne and launched in December 2023. The project aims at demonstrating the feasibility of Gamma-Ray Burst detection and localization using miniaturized instruments onboard nano-satellites. The HERMES flight model payloads were exposed to multiple well-known radioactive sources for spectroscopic calibration under controlled laboratory conditions. The analysis of the calibration data allows both to determine the detector parameters, necessary to map instrumental units to accurate energy measurements, and to assess the performance of the instruments. We report on these efforts and quantify features such as spectroscopic resolution and energy thresholds, at different temperatures and for all payloads of the constellation. Finally we review the performance of the HERMES payload as a photon counter, and discuss the strengths and the limitations of the architecture.

HERMES (High Energy Rapid Modular Ensemble of Satellites) Pathfinder mission aims to observe and localize Gamma Ray Bursts (GRBs) and other transients using a constellation of nanosatellites in low-Earth orbit (LEO). Scheduled for launch in early 2025, the 3U CubeSats will host miniaturized instruments featuring a hybrid Silicon Drift Detector (SDD) and GAGG:Ce scintillator photodetector system, sensitive to X-rays and gamma-rays across a wide energy range. Each HERMES payload contains 120 SDD cells, each with a sensitive area of 45 mm^2, organized into 12 matrices, reading out 60 12.1x6.94x15.0 mm^3 GAGG:Ce scintillators. Photons interacting with an SDD are identified as X-ray events (2-60 keV), while photons in the 20-2000 keV range absorbed by the crystals produce scintillation light, which is read by two SDDs, allowing event discrimination. The detector system, including front-end and back-end electronics, a power supply unit, a chip-scale atomic clock, and a payload data handling unit, fits within a 10x10x10 cm^3 volume, weighs 1.5 kg, and has a maximum power consumption of about 2 W. This paper outlines the development of the HERMES constellation, the design and selection of the payload detectors, and laboratory testing, presenting the results of detector calibrations and environmental tests to provide a comprehensive status update of the mission.

The High Energy Rapid Modular Ensemble of Satellites (HERMES) instrument is a compact X/\\(\\gamma\\)-ray spectrometer operating on board the 6U (11 kg) SpIRIT CubeSat. The payload is particularly well suited for the observation of cosmic transients such as Gamma-Ray Bursts and bright pulsars thanks to its unique broadband sensitivity from a few keV to a few MeV and the temporal resolution down to half a microsecond. We report here the detection of the \\(\\sim\\)33~ms Crab pulsar double-peaked pulse profile obtained by considering the canonical Crab ephemerides as provided by the Jodrell Bank catalog. We collected approximately 5.7\\(\\cdot\\)10\\(^4\\) photons from 730~s of observations, in the 3 keV -- 2 MeV energy band, during a single operation, and achieved a 5\\(\\sigma\\) pulse profile significance in the 3--11.5 keV energy band with binning at the ms scale. The results demonstrate that SpIRIT/HERMES can achieve millisecond timing accuracy at high energies and, thanks to its wide field of view and broad energy band, has the potential to contribute to GRB monitoring in the near future. Such capabilities were previously the domain of flagship observatories, underscoring the performance of the HERMES instrument with its compact form factor.

Gamma Ray Bursts (GRBs) bridge relativistic astrophysics and multi-messenger astronomy. Space-based gamma/X-ray wide field detectors have proven essential to detect and localize the highly variable GRB prompt emission, which is also a counterpart of gravitational wave events. We study the capabilities to detect long and short GRBs by the High Energy Rapid Modular Ensemble of Satellites (HERMES) Pathfinder (HP) and SpIRIT, namely a swarm of six 3U CubeSats to be launched in early 2025, and a 6U CubeSat launched on December 1st 2023. We also study the capabilities of two advanced configurations of swarms of >8 satellites with improved detector performances (HERMES Constellations). The HERMES detectors, sensitive down to ~2-3 keV, will be able to detect faint/soft GRBs which comprise X-ray flashes and high redshift bursts. By combining state-of-the-art long and short GRB population models with a description of the single module performance, we estimate that HP will detect ~195^{+22}_{-21} long GRBs (3.4^{+0.3}_{-0.8} at redshift z>6) and ~19^{+5}_{-3} short GRBs per year. The larger HERMES Constellations under study can detect between ~1300 and ~3000 long GRBs per year and between ~160 and ~400 short GRBs per year, depending on the chosen configuration, with a rate of long GRBs above z>6 between 30 and 75 per year. Finally, we explore the capabilities of HERMES to detect short GRBs as electromagnetic counterparts of binary neutron star (BNS) mergers detected as gravitational signals by current and future ground-based interferometers. Under the assumption that the GRB jets are structured, we estimate that HP can provide up to 1 (14) yr^{-1} joint detections during the fifth LIGO-Virgo-KAGRA observing run (Einstein Telescope single triangle 10 km arm configuration). These numbers become 4 (100) yr^{-1}, respectively, for the HERMES Constellation configuration.

Kaonic atoms, formed when a negatively charged kaon replaces an electron, provide a sensitive probe of the low-energy strong interaction via precision X-ray spectroscopy. The SIDDHARTA-2 experiment at the DA\\(\\Phi\\)NE collider employs high-performance Silicon Drift Detectors (SDDs) optimized for the 4-12 keV range to study light kaonic systems. In preparation for the EXKALIBUR phase, which targets heavier kaonic atoms, new 1 mm-thick SDDs have been developed with Politecnico di Milano and Fondazione Bruno Kessler. Their increased thickness enhances the quantum efficiency by a factor of about two at 30 keV while preserving excellent energy resolution. These detectors are also intended for VIP-3, the next-generation test of the Pauli Exclusion Principle (PEP). Building on VIP-2, which set the most stringent limits on PEP-violating \\(K_{\\alpha}\\) transitions in copper, VIP-3 will extend the search to heavier elements such as Ag, Sn, and Zr. Preliminary measurements demonstrate efficient detection up to 30 keV, supporting future high-precision studies of the kaon-nucleon interaction and PEP in heavier systems.

Kaonic atoms, formed when a negatively charged kaon replaces an electron, provide a sensitive probe of the low-energy strong interaction via precision X-ray spectroscopy. The SIDDHARTA-2 experiment at the DA\\(\\)NE collider employs high-performance Silicon Drift Detectors (SDDs) optimized for the 4-12 keV range to study light kaonic systems. In preparation for the EXKALIBUR phase, which targets heavier kaonic atoms, new 1 mm-thick SDDs have been developed with Politecnico di Milano and Fondazione Bruno Kessler. Their increased thickness enhances the quantum efficiency by a factor of about two at 30 keV while preserving excellent energy resolution. These detectors are also intended for VIP-3, the next-generation test of the Pauli Exclusion Principle (PEP). Building on VIP-2, which set the most stringent limits on PEP-violating \\(K_\\) transitions in copper, VIP-3 will extend the search to heavier elements such as Ag, Sn, and Zr. Preliminary measurements demonstrate efficient detection up to 30 keV, supporting future high-precision studies of the kaon-nucleon interaction and PEP in heavier systems.

The HERMES-TP/SP mission, based on a nanosatellite constellation, has very stringent constraints of sensitivity and compactness, and requires an innovative wide energy range instrument. The instrument technology is based on the \"siswich\" concept, in which custom-designed, low-noise Silicon Drift Detectors are used to simultaneously detect soft X-rays and to readout the optical light produced by the interaction of higher energy photons in GAGG:Ce scintillators. To preserve the inherent excellent spectroscopic performances of SDDs, advanced readout electronics is necessary. In this paper, the HERMES detector architecture concept will be described in detail, as well as the specifically developed front-end ASICs (LYRA-FE and LYRA-BE) and integration solutions. The experimental performance of the integrated system composed by scintillator+SDD+LYRA ASIC will be discussed, demonstrating that the requirements of a wide energy range sensitivity, from 2 keV up to 2 MeV, are met in a compact instrument.

HERMES (High Energy Rapid Modular Ensemble of Satellites) Technological and Scientific pathfinder is a space borne mission based on a LEO constellation of nano-satellites. The 3U CubeSat buses host new miniaturized detectors to probe the temporal emission of bright high-energy transients such as Gamma-Ray Bursts (GRBs). Fast transient localization, in a field of view of several steradians and with arcmin-level accuracy, is gained by comparing time delays among the same event detection epochs occurred on at least 3 nano-satellites. With a launch date in 2022, HERMES transient monitoring represents a keystone capability to complement the next generation of gravitational wave experiments. In this paper we will illustrate the HERMES payload design, highlighting the technical solutions adopted to allow a wide-energy-band and sensitive X-ray and gamma-ray detector to be accommodated in a CubeSat 1U volume together with its complete control electronics and data handling system.

HERMES-Pathfinder is a space-borne mission based on a constellation of six nano-satellites flying in a low-Earth orbit. The 3U CubeSats, to be launched in early 2025, host miniaturized instruments with a hybrid Silicon Drift Detector/scintillator photodetector system, sensitive to both X-rays and gamma-rays. A seventh payload unit is installed onboard SpIRIT, an Australian-Italian nano-satellite developed by a consortium led by the University of Melbourne and launched in December 2023. The project aims at demonstrating the feasibility of Gamma-Ray Burst detection and localization using miniaturized instruments onboard nano-satellites. The HERMES flight model payloads were exposed to multiple well-known radioactive sources for spectroscopic calibration under controlled laboratory conditions. The analysis of the calibration data allows both to determine the detector parameters, necessary to map instrumental units to accurate energy measurements, and to assess the performance of the instruments. We report on these efforts and quantify features such as spectroscopic resolution and energy thresholds, at different temperatures and for all payloads of the constellation. Finally we review the performance of the HERMES payload as a photon counter, and discuss the strengths and the limitations of the architecture.

HERMES Pathfinder is an in-orbit demonstration consisting of a constellation of six 3U cubesats hosting simple but innovative X-ray/gamma-ray detectors for the monitoring of cosmic high-energy transients. HERMES-PF, funded by ASI and by the EC Horizon 2020 grant, is scheduled for launch in Q1 2025. An identical X-ray/gamma-ray detector is hosted by the Australian 6U cubesat SpIRIT, launched on December 1st 2023. The main objective of HERMES-PF/SpIRIT is to demonstrate that high energy cosmic transients can be detected efficiently by miniatured hardware and localized using triangulation techniques. The HERMES-PF X-ray/gamma-ray detector is made by 60 GAGG:Ce scintillator crystals and 12 2x5 silicon drift detector (SDD) mosaics, used to detect both the cosmic X-rays directly and the optical photons produced by gamma-ray interactions with the scintillator crystals. This design provides a unique broad band spectral coverage from a few keV to a few MeV. Furthermore, the use of fast GAGG:Ce crystals and small SDD cells allows us to reach an exquisite time resolution better than a microsecond. We present a progress report on the missions focusing the discussion on the scientific innovation of the project and on the main lessons learned during the project development including: the importance and the challenges of using distributed architectures to achieve ambitious scientific objectives; the importance of developing critical technologies under science agreements for the realization of high-performing but low-cost payloads; best use of COTS technologies in scientific missions. We finally discuss the prospects of applying these concepts for the creation of an all-sky, all-time monitor to search for the high-energy counterparts of gravitational wave events that Advanced LIGO/Virgo/Kagra will find at the end of this decade and the Einstein Telescope during the 2030s.