The accurate simulation of sub-GeV particle detectors is essential for interpreting experimental data and optimizing detector design. This work identifies and addresses several critical aspects in modeling such detectors, taking as a case study the High-Energy Particle Detector (HEPD-02), a space-borne instrument developed within the CSES-02 mission to measure electrons in the ∼3–100 MeV range, protons and light nuclei in the ∼30–200 MeV/n. The HEPD-02 instrument consists of a silicon tracker, plastic and LYSO scintillator calorimeters, and anticoincidence systems, making it a representative example of a complex low-energy particle detector operating in Low Earth Orbit. Key challenges arise from replicating intricate detector geometries derived from CAD models, selecting appropriate hadronic physics lists for low-energy interactions, and accurately describing the detector response—particularly quenching effects in scintillators and digitization in solid-state tracking planes. Particular attention is given to three critical aspects: the precise CAD-level geometry implementation, the impact of hadronic physics models on the detector response, and the parameterization of scintillation quenching. In this study, we present original solutions to these challenges and provide data–MC comparisons using data from HEPD-02 beam tests.

3C 403 is a well-known FRII radio galaxy with jets extending up to kiloparsec scales. We report its identification as the second most significant candidate among more than 150 sources examined using the 15-year neutrino dataset from the ANTARES collaboration, making it one of the most promising radio-galaxy candidates for high-energy neutrino emission. Motivated by previous associations between blazars and neutrino events, we investigated the jet properties of 3C 403 and their possible role in neutrino production. Multi-scale radio observations, from parsec to kiloparsec scales, reveal a stable, twosided jet lying close to the plane of the sky, with no evidence of strong Doppler boosting, while X-ray data indicate a dominant, heavily absorbed accretion-related component. We also examined the recently proposed correlation between neutrino and hard X-ray fluxes - originally identified in blazars and Seyfert galaxies - and find that 3C 403 occupies an intermediate location in the L nu-LhX plane between jet-dominated and corona-dominated systems. However, the current upper limit on its neutrino flux prevents a firm assessment of whether it follows the proposed relation. With radiatively efficient accretion (lambda Edd similar to 10-2), strong hard X-ray emission, and a powerful but misaligned jet, 3C 403 provides a physically motivated laboratory for exploring the interplay between coronal activity and jet environments in multimessenger scenarios of neutrino production in active galaxies.

The China Seismo-Electromagnetic Satellite (CSES) mission is a joint China-Italy initiative focused on investigating Earth’s geophysical environment through non-imaging remote sensing from space. Its primary objective is to establish a constellation of satellites capable of continuously monitoring global electromagnetic fields, particle fluxes, and plasma parameters within the iono-magnetospheric system. Goals of this space program are the investigation of possible lithosphere-atmosphere-ionosphere-magnetosphere coupling mechanisms and their role in inducing perturbations in the upper ionosphere and the lower boundary of the radiation belts. Additionally, CSES contributes to space weather studies, including investigations of the magnetosphere, heliosphere, and galactic cosmic rays. Each satellite in the constellation carries multiple instruments to measure charged particles, electromagnetic fields, and plasma properties. CSES-01, launched in February 2018, remains operational. The second satellite, CSES-02, was launched on June 14, 2025, marking the transition to a multi-point observation capability. Italy contributed the High-Energy Particle Detector to both missions—HEPD-01 for CSES-01 and the upgraded HEPD-02 for CSES-02. These detectors are designed for precise measurements of electrons, protons, light nuclei, and transient gamma rays in the multi-MeV range. HEPD-02 significantly enhances energy resolution and extends the detection range when combined with the lower-energy instruments aboard CSES-02, enabling continuous coverage from 100 keV to 200 MeV. This article presents the scientific goals of the CSES program, with a focus on the role of HEPD-02 in energetic particle studies, both as a standalone detector and in synergy with the mission’s multi-instrument, multi-satellite framework.

This paper introduces TROPix, a parametric simulation tool designed to reproduce the output of silicon pixel detectors. Developed for the Direction Detector of the HEPD-02 detector on the second China Seismo-Electromagnetic Satellite (CSES-02), TROPix utilizes the GEANT4 toolkit for energy deposition and a parametric approach for charge generation and diffusion. The simulation process incorporates thermal noise and digitization, with all parameters fine-tuned using experimental beam-test data. Although TROPix is tuned for HEPD-02 data, it can also be adapted for use with other detectors employing pixel sensors, providing a versatile and computationally efficient solution for numerous applications. Results for Monolithic Active Pixel Sensors (MAPS) based on ALTAI chips are presented, highlighting TROPix’s ability to replicate the property of pixel clusters, defined as the number of pixels fired by the particle passage. These characteristics comprehend the dependencies on charged particle deposited energy and incident angle, as well as the typical shapes of clusters. Validation using beam test data acquired with ALTAI sensors demonstrates that TROPix reproduces the cluster size dependence on deposited energy, with agreement within 20%. For cluster shape occurrences, the simulated results align with the data within 25% of the total uncertainty.

We report the results on the short gamma-ray burst GRB 241107A, obtained with the IBIS instrument on board the INTEGRAL satellite. The burst had a duration of about 0.2 s, a fluence of 8 × 10 −7 erg cm −2 in the 20 keV–10 MeV range, and a hard spectrum, characterized by a peak energy of 680 keV. The position of GRB 241107A has been precisely determined because it fell inside the imaging field of view of the IBIS coded mask instrument. The presence of the nearby galaxy PGC 86046 in the 3′ radius error region suggests that GRB 241107A might be a giant flare from a magnetar rather than a canonical short gamma-ray burst. For the 4.1 Mpc distance of PGC 86046, the isotropic energy of 1.6 × 10 45 erg is in agreement with this hypothesis, which is also supported by the time-resolved spectral properties similar to those of the few other extragalactic magnetars giant flares detected so far.

Ionospheric electric field monitoring has an important role in the understanding of the terrestrial environment and in the study of the various couplings with both the outer space and the ground level. The China Seismo-Electromagnetic Satellite (CSES) mission consists of a constellation of spacecrafts equipped with several instruments able to provide a large set of information, especially for applications ranging from Space Weather to Ionosphere-Lithosphere coupling. In this work we present the main features of the new Electric Field Detector (EFD-02), developed for the CSES-02 satellite, together with its measurement concept and peculiarities. The characteristics of each subsystem are reported to show the state of the art of the instrument. Particular emphasis is given to the high-level performance and novelties of EFD-02, which significantly improve the scientific information about plasma dynamics. The main scientific objectives of ionospheric electric field monitoring for both the low and high latitudes relevant to EFD-02 capabilities are finally described.

It is well known that electromagnetic (EM) processes can affect the trapped population of ionized particles in the Earth’s radiation belts and induce particle precipitations that can be measured by satellites. Moreover, in the last decades, several studies have suggested the concurrent occurrence of energetic particle flux variations (the so-called Particle Bursts, PBs) and ionospheric ELF-VLF electromagnetic activity in correspondence to (or even before) large earthquakes. However, to date, the underlying mechanisms connecting seismic-related electromagnetic processes to satellite-detected particle precipitation events remain elusive. In addition, a comprehensive model capable of explaining observed EM perturbations and PBs is still missing, especially during seismo-related phenomena. The lack of detailed investigation into these processes introduces uncertainties regarding the expected time delay between the two phenomena, which hinders the reproducibility and confirmation of reported findings across different studies, even when employing identical methodologies. Consequently, the temporal distribution of claimed seismo-related phenomena exhibits significant variability.To address these challenges, we present novel numerical simulations investigating wave-particle interactions within a realistic topside ionospheric plasma environment. A hybrid code was successfully employed to simulate the topside ionosphere, incorporating realistic plasma parameters, including plasma beta and species composition. Simulation results demonstrate some modifications in the ion velocity distribution function, including the emergence of fast ion beams capable of inducing particle precipitation. These simulations provide, for the first time, an estimate of the time delay between the onset of EM waves and the resulting plasma modifications.

The Athena mission entered a redefinition phase in July 2022, driven by the imperative to reduce the mission cost at completion for the European Space Agency below an acceptable target, while maintaining the flagship nature of its science return. This notably called for a complete redesign of the X-ray Integral Field Unit (X-IFU) cryogenic architecture towards a simpler active cooling chain. Passive cooling via successive radiative panels at spacecraft level is now used to provide a 50 K thermal environment to an X-IFU owned cryostat. 4.5 K cooling is achieved via a single remote active cryocooler unit, while a multi-stage Adiabatic Demagnetization Refrigerator ensures heat lift down to the 50 mK required by the detectors. Amidst these changes, the core concept of the readout chain remains robust, employing Transition Edge Sensor microcalorimeters and a SQUID-based Time-Division Multiplexing scheme. Noteworthy is the introduction of a slower pixel. This enables an increase in the multiplexing factor (from 34 to 48) without compromising the instrument energy resolution, hence keeping significant system margins to the new 4 eV resolution requirement. This allows reducing the number of channels by more than a factor two, and thus the resource demands on the system, while keeping a 4' field of view (compared to 5' before). In this article, we will give an overview of this new architecture, before detailing its anticipated performances. Finally, we will present the new X-IFU schedule, with its short term focus on demonstration activities towards a mission adoption in early 2027.

The South Atlantic Anomaly (SAA) is a region where the Earth's magnetic field is lower by a factor of 2-3 with respect to the mean field value, resulting in a remarkably higher density of trapped charged particles. Updated surveys of this unique region are of capital importance to track the evolution of the geomagnetic field and improve models of the Earth's ionosphere. The High-Energy Particle Detector (HEPD-01) on board the China Seismo-Electromagnetic Satellite ( CSES-01 ), operational between the 24th and 25th Solar cycles, is one of the few instruments able to probe the radiation environment of the SAA at high energies. Sensitive to > 1 MeV electrons and > 10 MeV protons, HEPD-01 counters can reliably measure rates as large as several kHz/cm(2), overcoming SAA saturation issues that often affect spaceborne detectors. By mapping the total particle flux observed within the SAA, we report here on the geographical shift of its center during the 2018-2022 period and deliver HEPD-01's estimate of the associated integral particle flux, which validates NASA's AP9-AE9 model. The present assessment of the SAA drift, besides featuring a notable accuracy, is unique in its time coverage, altitude, and energy range. In addition, our flux estimate covers the proton energy range above similar to 200 MeV where the AP9 model relies on extrapolated data. The present results enrich the available observations of the radiation environment within the SAA, which are crucial for the investigation of the temporal evolution of the geomagnetic field, for models of geomagnetically trapped particles, and for the investigation of perturbations generated by space weather events. Knowledge of the SAA radiation environment is also relevant for spacecraft system design and to safeguard human crew health.

This article describes the innovative system performing the trigger and the readout of the photomultiplier tubes (PMTs) for the high-energy particle detector (HEPD) onboard the second satellite of the China Seismo Electromagnetic Satellite (CSES) mission. The second HEPD is designed to measure cosmic rays covering an energy spectrum ranging from a few megaelectronvolts to several hundreds of megaelectronvolts. This high-precision detector consists of different subsystems: a solid-state tracker, a segmented trigger, a calorimeter composed of a tower of plastic scintillators, and two layers of lutetium-yttrium oxyorthosilicate (LYSO) crystals, along with a containment detector. The data acquisition (DAQ) process for the trigger, calorimeter, and containment detector is carried out by a single electronic board, relying on two Weeroc CITIROC application-specific integrated circuits (ASICs), which are being utilized in space for the first time. This board also generates and manages trigger signals for the entire detector. It effectively captures signals with distinct timing characteristics from plastic scintillators and inorganic crystals. Given the wide range of particle fluxes encountered during CSES's orbit, adaptability of the trigger generation system becomes crucial, and DAQ is optimized to ensure consistent energy spectra measurement with a substantial duty cycle. The trigger system of the second HEPD implements concurrent trigger patterns and the ability to select DAQ strategies based on orbital zones and the presence of impulsive events. Each trigger configuration is designed to meet scientific demands concerning the field of view and the characteristics of the particles reaching the detector, with prescaling settings fine-tuned accordingly. In addition to monitoring particle bursts, trigger configurations specific to gamma rays will be tracked in 5-ms intervals to measure photon fluxes in the energy range from 2 to 20 MeV and provide sensitivity for impulsive events, such as gamma-ray bursts (GRBs). This article provides a comprehensive account of the design criteria, the architectural choices specifically adapted for space applications, and the original trigger management strategy. Additionally, this article presents an in-depth analysis of the performance of the trigger system and comprehensive results from laboratory and beam tests conducted on the qualification and flight models of second HEPD.

In the last decades, the scientific community has been focused on searching earthquake signatures in the Earth’s atmosphere, ionosphere, and magnetosphere. This work investigates an offshore Mw 5.5 earthquake that struck off the Marche region’s coast (Italy) on 9 November 2022, with a focus on the potential coupling between the Earth’s lithosphere, atmosphere, and magnetosphere triggered by the seismic event. Analysis of atmospheric temperature data from ERA5 reveals a significant increase in potential energy (Ep) at the earthquake’s epicenter, consistent with the generation of Atmospheric Gravity Waves (AGWs). This finding is further corroborated by the MILC analytical model, which accurately simulates the observed Ep trends (within 5%), supporting the theory of Lithosphere–Atmosphere–Ionosphere–Magnetosphere coupling. The study also examines the vertical Total Electron Content (vTEC) and finds notable fluctuations at the epicenter, exhibiting periodicities (7–12 min) characteristic of AGWs and traveling ionospheric disturbances. The correlation between ERA5 observations and MILC model predictions, particularly in temperature deviations and Ep distributions, strengthens the hypothesis that earthquake-generated AGWs impact atmospheric conditions at high altitudes, leading to observable ionospheric perturbations. This research contributes to a deeper understanding of Lithosphere–Atmosphere–Ionosphere–Magnetosphere coupling mechanisms and the potential for developing reliable earthquake prediction tools.

GRS 1758–258 and 1E 1740.7–2942 are two long-known persistent black hole binaries in the Galactic center region. Using the International Gamma-Ray Astrophysics Laboratory (INTEGRAL)'s extensive monitoring of the Galactic center and bulge, we studied their temporal and spectral evolutions in the 30–610 keV energy range from 2003 March through 2022 April with the Imager on Board INTEGRAL/INTEGRAL Soft Gamma-ray Imager gamma-ray telescope. Our analyses found that the sources typically had Comptonized spectra, though not always with the same parameters. The spectral states with more than 8 Ms of observation time show deviations from a Comptonized spectrum above ∼200 keV or a “hard tail” that extends up to at least 600 keV. The origin of this component remains debated, with the most popular scenarios being synchrotron emission from the jet or Comptonization in a hybrid thermal/nonthermal plasma. Anyway, the GRS 1758–258 and 1E 1740.7–2942 spectra are acceptably described by CompTT+po (jet) and Eqpair (hybrid Comptonization) scenarios. To differentiate between the two scenarios, we calculated the Spearman correlation coefficient comparing 30–50 keV count rates with those in higher energy bands (50–100, 100–300, and 300–600 keV). The count rates below 300 keV are strongly correlated, indicating those photons arise from the same physical process. Above 300 keV the count rates are either anticorrelated or not correlated with the 30–50 keV count rates for GRS 1758–258, which suggests that the photons originate from a different physical process. For 1E 1740.7–2942, the level of correlation is unclear due to scatter in the data points. However, the 300–600 keV count rates are consistent with a constant value. This disfavors the hybrid Comptonization scenario for both sources.

The High-Energy Particle Detector-02 (HEPD-02), onboard the second China Seismo-Electromagnetic Satellite (CSES-02), introduces-for the first time in a space application-a tracker designed entirely using monolithic active pixel sensors (MAPS). The MAPS selected for the HEPD-02 tracker were developed as part of CERN's R&D efforts for the realization of the ALICE Inner Tracking System (ITS). The readout system has been optimized, significantly reducing power consumption. The performance of the MAPS has been validated for HEPD-02's scientific objectives through beam tests using particle and ion beams in the tens to hundreds of MeV energy range. This article details the design principles and structure of the tracker. The integration procedure is outlined step by step, with a discussion of the challenges encountered at each stage and the strategies adopted to address them. The description concludes with an analysis of production yields and the criteria used for selecting components for the flight and qualification models. In addition, an overview of the space qualification activities is provided. The launch of CSES-02 is scheduled for mid-2025. Following an initial in-orbit commissioning phase, the technology will achieve technology readiness level 9, making it available for future space missions.

The High-Energy Particle Detector (HEPD-02) is one of the scientific payloads of the China Seismo-Electromagnetic Satellite (CSES-02). The HEPD-02’s main purpose is to characterize the particle environment in the Earth’s vicinity, identifying sudden changes in the fluxes and correlating them with solar and terrestrial phenomena. Additionally, HEPD-02 also has capabilities in detecting Gamma-Ray Bursts. At the core of HEPD-02, a tower of scintillation counters made of plastic and LYSO crystals is able to recognize electrons in the range between 3 and 100 MeV, protons and nuclei between 30 and 200 MeV/n. Plastic scintillators covering the calorimeter on five sides allow to reject particles entering from the top and not completely absorbed within its volume. In this work, the design of the HEPD-02 is reviewed in comparison to its predecessor, HEPD-01, highlighting the innovations of the new design. The design of each scintillation counter type has been fully validated through a campaign of prototype realization, testing, and characterization. The production of the scintillation counters, including the PMT selection process, is also discussed. Finally, the performance of the counters is compared with simulations, showing an agreement of within 20% with the expected performance, thereby meeting expectations.

The intricate behavior of particle acceleration and transport mechanisms complicates the overall efforts in formulating a comprehensive understanding of solar energetic particle (SEP) events; these efforts include observations of low-energy particles (from tens of keV to hundreds of MeV) by space-borne instruments and measurements by the ground-based neutron monitors of the secondary particles generated in the Earth atmosphere by SEPs in the GeV range. Numerous space-borne missions provided good data on the nature/characteristics of these solar particles in past solar cycles, but more recently-concurrently with the rise toward the maximum of solar cycle 25-the High-Energy Particle Detector (HEPD-01) proved to be well suited for the study of solar physics and space weather. Its nominal 30-300 MeV energy range for protons can enlarge the detection capabilities of solar particles at low Earth orbit, closer to the injection limit of many SEP events. In this work, we characterize three SEP events within the first six months of 2022 through spectral and velocity dispersion analysis, assessing the response of HEPD-01 to >M1 events.

Gamma-ray telescopes in space are bombarded by large fluxes of charged particles, photons and secondary neutrons. These particles and radiation pose a threat to the nominal operation of satellites and limit the detection sensitivity of gamma-ray instruments. The background noise generated in gamma-ray space detectors by impinging particles is always much higher than the astrophysical signal to be detected. In this chapter, we present the different types of orbits suitable for gamma-ray missions, discussing their advantages and disadvantages, as well as the value of experiments embarked in stratospheric balloons. We then review the physical properties of all the background components in the different orbits and the stratosphere.

The gamma-ray burst GRB 221009A is among the most luminous of its kind and its proximity to Earth has made it an exceptionally rare observational event. The International Gamma-ray Astrophysics Laboratory (INTEGRAL) was in an optimal aspect position to use its all-sky instruments for recording the prompt emission and early gamma-ray afterglow in unprecedented detail. Following the initial detection, a swiftly scheduled follow-up observation allowed for the hard X-ray afterglow time and spectral evolution to be observed for up to almost a week. The INTEGRAL hard X-ray and soft gamma-ray observations have started to bridge the energy gap between the traditionally well-studied soft X-ray afterglow and the high-energy afterglow observed by Fermi/LAT. We discuss the possible implications of these observations for follow-ups of multi-messenger transients with hard X-ray and gamma-ray telescopes.

Giant flares, short explosive events releasing up to 10$^{47}$ erg of energy in the gamma-ray band in less than one second, are the most spectacular manifestation of magnetars, young neutron stars powered by a very strong magnetic field, 10$^{14-15}$ G in the magnetosphere and possibly higher in the star interior. The rate of occurrence of these rare flares is poorly constrained, as only three have been seen from three different magnetars in the Milky Way and in the Large Magellanic Cloud in about 50 years since the beginning of gamma-ray astronomy. This sample can be enlarged by the discovery of extragalactic events, since for a fraction of a second giant flares reach peak luminosities above 10$^{46}$ erg/s, which makes them visible by current instruments up to a few tens of Mpc. However, at these distances they appear similar to, and difficult to distinguish from, regular short gamma-ray bursts (GRBs). The latter are much more energetic events, 10$^{50-53}$ erg, produced by compact binary mergers and originating at much larger distances. Indeed, only a few short GRBs have been proposed, with different levels of confidence, as magnetar giant flare candidates in nearby galaxies. Here we report the discovery of a short GRB positionally coincident with the central region of the starburst galaxy M82. Its spectral and timing properties, together with the limits on its X-ray and optical counterparts obtained a few hours after the event and the lack of an associated gravitational wave signal, qualify with high confidence this event as a giant flare from a magnetar in M82.

Cosmic rays' interactions with the residual atmosphere surrounding the Earth produce a variety of particles, like electrons, positrons, protons, anti-protons, and Helium nuclei that can be observed below the local geomagnetic cutoff. In this work, we present new measurements of downward-going, albedo proton fluxes with kinetic energy in the range ∼40–∼250 MeV, performed by the High-Energy Particle Detector (HEPD-01) on board of the China Seismo-Electromagnetic Satellite - CSES-01 - at an altitude of ∼500 km. Employing a dedicated trajectory-tracing simulation routine, the protons collected by HEPD-01 are classified into quasi-trapped (QT), long lifetime (≳10 s) particles concentrating in the equatorial region of the Earth, and un-trapped (UT), distributed at all latitudes; the latter includes both precipitating short lifetime particles (UTS) and pseudo-trapped long lifetime (UTL) populations, abundant in the so-called penumbra regions. The temporal trend of re-entrant protons between 2018 and 2022 is also reported, assessing the stability of such population during the data-taking period of HEPD-01; this highlights their independence from the long-term modulating effect of the solar activity.