Context. Fast radio bursts (FRBs) are millisecond-long bursts uniquely detected at radio frequencies. FRB 131104 is the only case for which a γ-ray transient positionally and temporally consistent was claimed. This high-energy transient had a duration of ∼400 s and a 15–150 keV fluence S_γ ∼ 4 × 10⁻⁶ erg cm⁻². However, the association with the FRB is still debated. Aims. We aim at testing the systematic presence of an associated transient high-energy counterpart throughout a sample of the FRB population. Methods. We used an approach like that used in machine learning methodologies to accurately model the highly-variable Fermi/GBM instrumental background on a time interval comparable to the duration of the proposed γ-ray counterpart of FRB 131104. A possible γ-ray signal is then constrained considering sample average lightcurves. Results. We constrain the fluence of the possible γ-ray signal in the 8–1000 keV band down to 6.4 × 10⁻⁷ (7.1 × 10⁻⁸) erg cm⁻² for a 200-s (1-s) integration time. Furthermore, we found the radio-to-gamma fluence ratio to be η > 10⁸ Jy ms erg⁻¹ cm². Conclusions. Our fluence limits exclude ∼94% of Fermi/GBM detected long gamma-ray bursts and ∼96% of Fermi/GBM detected short gamma-ray bursts. In addition, our limits on the radio-to-gamma fluence ratio point to a different emission mechanism from that of magnetar giant flares. Finally, we exclude a γ-ray counterpart as fluent as the one possibly associated with FRB 131104 to be a common feature of FRBs.

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.

We will describe the LAUE project, supported by the Italian Space Agency, whose aim is to demonstrate the capability to build a focusing optics in the hard X-/soft gamma-ray domain (80{600 keV). To show the lens feasibility, the assembling of a Laue lens petal prototype with 20 m focal length is ongoing. Indeed, a feasibility study, within the LAUE project, has demonstrated that a Laue lens made of petals is feasible. Our goal is a lens in the 80-600 keV energy band. In addition to a detailed description of the new LARIX facility, in which the lens is being assembled, we will report the results of the project obtained so far.

No transient electromagnetic emission has yet been found in association to fast radio bursts (FRBs), the only possible exception (3σ confidence) being the putative γ-ray signal detected in Swift/Burst Alert Telescope data in the energy band 15–150 keV at the time and position of FRB 131104. Systematic searches for hard X/γ-ray counterparts to other FRBs ended up with just lower limits on the radio/γ-ray fluence ratios. In 2001, at the time of the earliest discovered FRBs, the BeppoSAX Gamma-Ray Burst Monitor (GRBM) was one of the most sensitive open-sky γ-ray monitors in the 40–700 keV energy band. During its lifetime, one of the FRBs with the highest radio fluence ever recorded, FRB 010724 (800 ± 400 Jy ms), also known as the “Lorimer burst,” was promptly visible to the GRBM. Upon an accurate modeling of the GRBM background, eased by its equatorial orbit, we searched for a possible γ-ray signal in the first 400 s following the FRB, similar to that claimed for FRB 131104 and found no significant emission down to a 5σ limit in the range (0.24–4.7) × 10⁻⁶ erg cm⁻² (corresponding to 1 and 400 s integration time, respectively), in the energy band 40–700 keV. This corresponds to Jy ms erg⁻¹ cm², i.e., the deepest limit on the ratio between radio and γ-ray fluence, which rules out a γ-ray counterpart similar to that of FRB 131104. We discuss the implications on the possible mechanisms and progenitors that have been proposed in the literature, also taking into account its relatively low dispersion measure (375 ± 3 pc cm⁻³) and an inferred redshift limit of z < 0.4.

In the context of the Laue project devoted to build a Laue lens prototype for focusing celestial hard x-/soft gamma-rays, a Laue lens made of bent crystal tiles, with 20-m focal length, is simulated. The focusing energy passband is assumed to be 90 to 600 keV. The distortion of the image produced by the lens on the focal plane, due to effects of crystal tile misalignment and radial distortion of the crystal curvature, is investigated. The corresponding effective area of the lens, its point spread function, and sensitivity are calculated and compared with those exhibited by a nominal Laue lens with no misalignment and/or distortion. Such analysis is crucial to estimate the optical properties of a real lens, in which the investigated shortcomings could be present.