Marco Tavani

The “Fermi bubbles” are two giant gamma-ray emitting configurations in the form of bubbles that extend above and below the Galactic plane. They are characterized by a smooth surface, sharp edges, and an almost flat intensity distribution. They were discovered in 2010 by the Fermi telescope and detected in the energy range 1–100 GeV. We propose a novel theoretical model to explain the process involved in the “Fermi bubbles” phenomenon. The model is based on the “in situ” generation of an energetic electron population powered by unstable electromagnetic waves triggered by a nonrelativistic outflow of matter escaping from the Galactic center region. A hadronic outflow crossing the magnetic field in the Galactic halo of low-density plasma can induce an electromagnetic instability in the “lower hybrid” frequency range (between the ion and electron cyclotron frequencies). These lower hybrid waves propagate along the magnetic field lines in the halo and can accelerate, via resonant Landau damping, the thermal electrons up to energies on the order of hundreds of GeV. This energetic electron population, localized in a thin boundary layer of the halo, radiates by synchrotron emission and by inverse Compton scattering. An evaluation of the radiation emission and a comparison with spectral data is presented. This model successfully explains in a natural way the GeV spectral properties of the “Fermi bubbles” as well as their microwave emission.

Recent detections of gamma-ray bursts (GRBs) at TeV energies opened new prospects for investigating radiative environments and particle acceleration mechanisms under extreme conditions. In this paper, we study the afterglows of these GRBs—namely GRB 180720B, GRB 190114C, GRB 190829A, GRB 201216C, and GRB 221009A—modeling their synchrotron and inverse Compton emission within the framework of an optimized relativistic fireball model. We constrain the model parameters and their temporal evolution by applying our theoretical model to the high-energy emission in the X-ray and GeV–TeV energy bands observed at intermediate and late times. Our results reveal interesting differences among the TeV-detected GRBs, potentially reflecting a variety of underlying physical processes that lead to different maximum energies E max = γ max m e c 2 of the accelerated particles responsible for the GRB high-energy emission. We indeed obtain different behaviors of the late TeV afterglows that ultimately depend on γ max . We discuss how late afterglow observations—on timescales of hours and days—of X-ray and GeV–TeV emissions are crucial for providing diagnostics of the physical processes behind GRBs, and we emphasize the theoretical expectations for future TeV observations.

Fast radio bursts (FRBs) are millisecond-duration bursts originating from distant sources. They are classified into two categories: non-repeating FRBs, which manifest as singular events, and repeater FRBs (R-FRBs), which emit multiple bursts over time. In this work, we report a search for X- and γ-ray counterparts to a selected sample of R-FRBs using data from the AGILE satellite. The sample focused on sources with an excess dispersion measure (DM) below 300 pc cm⁻³. The analysis focused on the bursts covered by AGILE Mini-Calorimeter (MCAL) high-resolution data. No astrophysical signals were identified, and we derived upper limits (ULs) on the flux above 400 keV for the associated sources, adopting a spectral magnetar model, one of the leading models for FRB emission. Moreover, for a single burst of FRB 20200120E we estimated the flux ULs from the SuperAGILE detector data in the 18–60 keV energy range. We also performed a check of the GRID coverage for each burst in the 0.03–10 GeV energy band on short timescales, from 10 to 10³ s, and on longer ones including the complete ∼17 yr AGILE/GRID archive. We then considered the famous event FRB 200428 from the Galactic magnetar SGR 1935+2154 as a reference to extrapolate a possible X-ray emission in MCAL and SuperAGILE bands from the radio energies of R-FRBs using the E _X/E _radio of FRB 200428 as a fixed parameter. We compared these energies with historical magnetar X-ray bursts rescaled in the same bands. Our observations set useful constraints on the FRB magnetar model in particular, the MCAL ULs are currently the most stringent in the 0.4–30 MeV band.

The extraordinary gamma-ray burst GRB 221009A provides a great opportunity to investigate the enigmatic origin and evolution of gamma-ray bursts (GRBs). However, the complexity of the observations associated with this GRB provides significant challenges to developing a theoretical modeling in a coherent framework. In this paper, we present a theoretical interpretation of the GRB 221009A afterglow within the relativistic fireball scenario, aiming to describe the broadband data set with a consistent model evolution. We find that the adiabatic fireball evolution in the slow-cooling regime provides a viable scenario in good agreement with observations. Crucial to our analysis is the set of simultaneous GeV and TeV gamma-ray data obtained by AGILE and LHAASO during the early afterglow phases. Having successfully modeled as inverse Compton emission the high-energy spectral and lightcurve properties of the afterglow up to 10⁴ s, we extend our model to later times when also optical and X-ray data are available. This approach results in a coherent physical framework that successfully describes all observed properties of the afterglow up to very late times, approximately 10⁶ s. Our model requires time-variable microphysical parameters, with a moderately increasing efficiency ε _e of a few percent for transferring the shock energy to radiating particles and a decreasing efficiency for magnetic field generation ε _B in the range 10⁻⁵–10⁻⁷. Fitting the detailed multifrequency spectral data across the afterglow provides a unique test of our model.

The AGILE space mission was launched in 2007 to study X-ray and gamma-ray astrophysics. AGILE operated in spinning mode from 2009 until 2024 February 14, when it re-entered the Earth’s atmosphere. This work uses data acquired from the AGILE anticoincidence system (ACS) from 2019 January 1 to 2022 December 31. The ACS is designed to reject charged background particles. It also detects X-ray photons in the 50–200 KeV energy range and saves each panel count rate in the telemetry as ratemeter data, a time series with a resolution of 1.024 s. We developed a method that uses a deep learning model to predict the background count rates of the AGILE ACS top panel (perpendicular to the pointing direction of the payload detectors) using the satellite’s orbital parameters as input. Then, we use the difference between predicted and acquired count rates to detect gamma-ray bursts (GRB). We trained the model with a background-only data set. After the training, the model can predict the ACS count rates with a mean reconstruction error of 3.8%. We used the GRBs listed in the GRBweb catalog to search for significant anomalies in the ACS data. We extracted light curves of 140 bins of 1.024 s for each GRB from the AGILE ACS to cover the trigger time of the GRBs. The model detected 39 GRBs with a significance of σ ≥ 3. The results contain four GRBs detected for the first time in the AGILE data.