Fabio Fuschino

Nuclear astrophysics, and particularly nuclear emission line diagnostics from a variety of cosmic sites, has remained one of the least developed fields in experimental astronomy, despite its central role in addressing a number of outstanding questions in modern astrophysics. Radioactive isotopes are co-produced with stable isotopes in the fusion reactions of nucleosynthesis in supernova explosions and other violent events, such as neutron star mergers. The origin of the 511 keV positron annihilation line observed in the direction of the Galactic Center is a 50-year-long mystery. In fact, we still do not understand whether its diffuse large-scale emission is entirely due to a population of discrete sources, which are unresolved with current poor angular resolution instruments at these energies, or whether dark matter annihilation could contribute to it. From the results obtained in the pioneering decades of this experimentally-challenging window, it has become clear that some of the most pressing issues in high-energy astrophysics and astro-particle physics would greatly benefit from significant progress in the observational capabilities in the keV-to-MeV energy band. Current instrumentation is in fact not sensitive enough to detect radioactive and annihilation lines from a wide variety of phenomena in our and nearby galaxies, let alone study the spatial distribution of their emission. In this White Paper (WP), we discuss how unprecedented studies in this field will become possible with a new low-energy gamma-ray space experiment, called ASTENA (Advanced Surveyor of Transient Events and Nuclear Astrophysics), which combines new imaging, spectroscopic and polarization capabilities. In a separate WP (Guidorzi et al. 39), we discuss how the same mission concept will enable new groundbreaking studies of the physics of Gamma–Ray Bursts and other high-energy transient phenomena over the next decades.

Unique spaceborne measurements of the three‐dimensional structure of convective clouds producing terrestrial gamma ray flashes (TGFs) were performed using both active and passive microwave sensors on board the Global Precipitation Measurement (GPM)‐Core Observatory satellite, finding coherent features for nine TGF‐producing storms. The delineation of cloud structure using the radar reflectivity factor shows convective cells with significant vertical development and thick layers with high ice content. Compared to other cumulonimbus clouds in the tropics, the TGFs counterparts have higher reflectivity values above 3 and 8 km altitude showing in all cases a cumulonimbus tower and the TGFs locations are very close, or coincident, to these high Z columns, where reflectivity exceeds 50 dBz. Using the GPM Microwave Imager radiometer, most thunderstorms show a very strong depression of polarization corrected temperature (PCT) at channel 89 GHz, indicating a strong scattering signal by ice in the upper cloud layers. At channel 166 GHZ, the difference between vertical and horizontal brightness temperature signal always returns positive values, from 0.2 up to 13.7 K indicating a complex structure with randomly/vertically oriented ice particles. The PCT was used to characterize the analyzed storms in terms of hydrometeor types, confirming in 7/9 cases a high likelihood of hail/graupel presence. To perform analysis on the TGFs parent flashes, radio atmospherics data from the Earth Networks Total Lightning Network lightning network were used. Waveform data indicate that all cases are intra‐cloud events and TGFs typically take place during the peak of flash rate production. Finally, the analysis of the most intense event is shown.

In this article, we report the first investigation over time of the atmospheric conditions around terrestrial gamma-ray flash (TGF) occurrences, using GPS sensors in combination with geostationary satellite observations and ERA5 reanalysis data. The goal is to understand which characteristics are favorable to the development of these events and to investigate if any precursor signals can be expected. A total of 9 TGFs, occurring at a distance lower than 45 km from a GPS sensor, were analyzed and two of them are shown here as an example analysis. Moreover, the lightning activity, collected by the World Wide Lightning Location Network (WWLLN), was used in order to identify any links and correlations with TGF occurrence and precipitable water vapor (PWV) trends. The combined use of GPS and the stroke rate trends identified, for all cases, a recurring pattern in which an increase in PWV is observed on a timescale of about two hours before the TGF occurrence that can be placed within the lightning peak. The temporal relation between the PWV trend and TGF occurrence is strictly related to the position of GPS sensors in relation to TGF coordinates. The life cycle of these storms observed by geostationary sensors described TGF-producing clouds as intense with a wide range of extensions and, in all cases, the TGF is located at the edge of the convective cell. Furthermore, the satellite data provide an added value in associating the GPS water vapor trend to the convective cell generating the TGF. The investigation with ERA5 reanalysis data showed that TGFs mainly occur in convective environments with unexceptional values with respect to the monthly average value of parameters measured at the same location. Moreover, the analysis showed the strong potential of the use of GPS data for the troposphere characterization in areas with complex territorial morphologies. This study provides indications on the dynamics of con-vective systems linked to TGFs and will certainly help refine our understanding of their production, as well as highlighting a potential approach through the use of GPS data to explore the lightning activity trend and TGF occurrences.

The response of the X and Gamma Imaging Spectrometer (XGIS) instrument onboard the Transient High Energy Sky and Early Universe Surveyor (THESEUS) mission, selected by ESA for an assessment phase in the framework of the Cosmic Vision M5 launch opportunity, has been extensively modeled with a Monte Carlo Geant-4 based software. In this paper, the expected sources of background in the Low Earth Orbit foreseen for THESEUS are described (e.g. diffuse photon backgrounds, cosmic-ray populations, Earth albedo emission) and the simulated on-board background environment and its effects on the instrumental performance is shown.

The XGIS (X and Gamma Imaging Spectrometer) is one of the three instruments onboard the THESEUS mission (ESA M5, currently in Phase-A). Thanks to its wide field of view and good imaging capabilities, it will efficiently detect and localize gamma-ray bursts and other transients in the 2-150 keV sky, and also provide spectroscopy up to 10 MeV. Its current design has been optimized by means of scientific simulations based on a Monte Carlo model of the instrument coupled to a state-of-the-art description of the populations of long and short GRBs extending to high redshifts. We describe the optimization process that led to the current design of the XGIS, based on two identical units with partially overlapping fields of view, and discuss the expected performance of the instrument.

The main goal of the FAMU experiment is the measurement of the hyperfine splitting (hfs) in the 1S state of muonic hydrogen ΔE_hfs (μ ^- p)1S. The physical process behind this experiment is the following: μp are formed in a mixture of hydrogen and a higher-Z gas. When absorbing a photon at resonance-energy ΔE_hfs ≈ 0.182 eV, in subsequent collisions with the surrounding H ₂ molecules, the μp is quickly de-excited and accelerated by ~ 2/3 of the excitation energy. The observable is the time distribution of the K-lines X-rays emitted from the μZ formed by muon transfer (μp) + Z → (μZ)* + p, a reaction whose rate depends on the μp kinetic energy. The maximal response, to the tuned laser wavelength, of the time distribution of X-ray from K-lines of the (μZ)* cascade indicate the resonance. During the preparatory phase of the FAMU experiment, several measurements have been performed both to validate the methodology and to prepare the best configuration of target and detectors for the spectroscopic measurement. We present here the crucial study of the energy dependence of the transfer rate from muonic hydrogen to oxygen (Λ _μp → _μ0 ), precisely measured for the first time.

Within the AHEAD consortium a mission concept named ASTENA (Advanced Surveyor of Transient Events and Nuclear Astrophysics) is proposed to address the top-priority themes identified by the AHEAD Science Advisory Group: Gamma-Ray Bursts and Nuclear Astrophysics. GRBs are among the most intriguing phenomena of the Universe, which thanks to their vast luminosities can be used to probe the first billion years of cosmic history, i.e. the era of first stars and black-holes. In spite of great advancements in the GRB astronomy since the BeppoSAX discovery of afterglows, several issues concerning both the prompt emission and the afterglow are still open. Concerning the prompt emission, for example, the emission mechanism of the radiation and the energy dissipation site (internal shocks? external shocks? photosphere?) are far from being understood. What is required is an accurate determination of the photon spectrum from few keV up to tens of MeV, and importantly, a measurement of the polarization of the radiation. The emission of the afterglow has been deeply investigated with Swift in the energy band from 0.5 to 10 keV, showing that an understanding of the origin of the emission mechanism requires spectral information extending to much higher energies, as already suggested by a few studies at 1 sr). b) a narrow field telescope (NFT), made of a broad-band Laue lens (50 – 600/700 keV) of a 20 m focal length, based on the exploitation of bent crystals, like those under development in Ferrara (FOV= 3.5 arcmin, angular resolution ≈20”). The NFT is coupled to a high efficiency (>80% above 600 keV) focal plane position sensitive detector, with 3D spatial resolution of at least 300 µm in the (X,Y) plane, fine spectroscopic response (1% @511 keV) and with polarization sensitivity. With the WFM/S, we expect to accurately determine the energy spectrum of GRB prompt emission in the broadest band ever achieved with a single instrument, to measure the gamma-ray polarization of, at least, the brightest GRBs and to search for electromagnetic counterparts of Gravitational Wave events. In addition, with adequate scintillator bars and fast electronics, the Lorentz invariance for the brightest events can be tested. With the NFT, which is >~100 times more sensitive at a few hundred keV than any other past or planned mission, we can carry out for the first time a long-sought study of the afterglow spectrum of GRBs up to high energies (600/700 keV), including its polarization level. We can also establish, thanks to its high angular resolution (about 20”), whether the 511 keV positron annihilation line is due to the superposition of emission from point-like sources. In addition, we can address many Legacy Science topics mentioned in the Call, such as the origin of the high energy emission from magnetars, the first determination of the spectrum of blazars out to z~8 in between the two Synchrotron and Compton bumps, the determination of the sources that give rise to...

The ASTENA mission, conceived within the AHEAD framework, consists of two coaligned instruments, a broad band Wide Field Monitor/Spectrometer WFM/S and a broad band Narrow Field Telescope (NFT). In the NFT a large geometric area Laue lens (3 m maximum diameter with a 20 m focal length) allows to focus the radiation of the 50 - 700 keV energy pass-band. Differently from other proposed Laue lenses in the past, the NFT is made of optimized thickness bent crystal tiles, made with Silicon (for the lower energy part of the lens pass-band) and Germanium (dedicated to the upper energy threshold). With these assumption we have optimized the NFT Field of View (FoV) to 3.5 arcmin with the angular resolution of 20”. The Laue lens is coupled with a high efficiency (>80% above 600 keV) focal plane position sensitive detector, with 3D spatial resolution of at least 300 µm in the (X,Y) plane and fine spectroscopic response (1% @511 keV) and with polarization sensitivity. In this SPIE contribution we will discuss the NFI geometry simulated with the MEGAlib toolkit and we will discuss its performances by simulating broad band and narrow energy typical sources, giving finally the instrument performances.