Enrico Peretti

Context. Growing observational evidence suggests that subrelativistic astrophysical jets may accelerate particles at slowly evolving standing shocks. Recollimation shocks are expected to develop when jets expand in dense environments; their formation may be mediated by the pressure of the cocoon surrounding the jet, while remaining compatible with a quasi-stationary behavior. Despite their high inclination relative to the jet axis, such shocks can be strong and can enable efficient particle acceleration. Aims. The aim of this work is to improve the general understanding of particle acceleration via diffusive shock acceleration at recollimation shocks by developing a versatile modeling framework applicable to different classes of astrophysical jets, including Seyfert galaxies, microquasars, and protostellar systems. Methods. We extended an analytic jet hydrodynamics model previously introduced in the literature to the subrelativistic regime and used it to identify the expected locations of the recollimation shock and the jet head. Within this framework, we formulated a semi-analytic acceleration and transport model for particles injected at the recollimation shock via diffusive shock acceleration. Results. By solving the space-dependent transport equation, we obtained particle distributions and spectra along the jet, as well as robust predictions for the maximum energies achievable as a function of the intrinsic properties of the system and the source class. Conclusions. Our results indicate that recollimation shocks may play a central role in particle acceleration in subrelativistic jets. In Seyfert galaxies such shocks may accelerate particles from PeV up to EeV energies, while in microquasars and protostellar jets maximum energies of respectively tens of PeV and up to TeV energies are expected. While leptonic emission may be associated with bright knots along the jet, accelerated protons are expected to escape the jet and interact with the surrounding cocoon. Depending on the properties of the system, proton interactions can give rise to extended hadronic emission morphologies.

The TeV neutrino signal observed by IceCube from the active galactic nucleus NGC 1068 can probe its innermost coronal regions. If these neutrinos originate from hadrons accelerated within a magnetized turbulent corona, their intensity and spectrum depend on the turbulent magnetic field strength and turbulence coherence scale. The gamma rays accompanying neutrino production are absorbed in this optically thick environment, in a way that depends sensitively on the size of the corona. By a joint fit of the IceCube and Fermi-LAT observations, we translate the multimessenger signal from NGC 1068 and the tentative signal from NGC 7469 into quantitative constraints on coronal properties. NGC 1068, with a significant TeV neutrino excess, favors a compact, strongly magnetized corona with a large turbulence coherence length relative to the coronal size. NGC 7469, with two ∼100 TeV neutrino events, points instead to a somewhat larger corona with much smaller coherence length and high magnetization, but a very small fraction of energy in nonthermal protons. We obtain the diffuse flux from a population of Seyfert galaxies identical to either NGC 1068 or NGC 7469. Finally, we consider a third scenario, motivated by the spectral break observed in the diffuse neutrino flux at tens of TeV, with coronal properties intermediate between the two point-source-inspired models. To enable detailed comparisons with the IceCube and electromagnetic observations, we release our model predictions in a GitHub repository.

Context. Ultra-fast outflows (UFOs) from active galactic nuclei (AGNs) are expected to lead to the formation of sub-relativistic strong shocks expanding in a dense, circumnuclear medium, and thus have the potential to be efficient particle accelerators and proficient sources of gamma rays and neutrinos. Aims. We aim to investigate the detectability of a sample of nearby identified UFOs in gamma rays and neutrinos with current and next-generation instruments. Methods. We modeled the acceleration of particles at the strong shocks of UFOs and estimated the associated gamma-ray and neutrino signal. We adopted our model to investigate the prospects for detection with current and next-generation observatories. Results. We find that several UFOs could be detectable in the very-high-energy (VHE) domain – for example, by the Cherenkov Telescope Array Observatory (CTAO) – even if they remain undetected by Fermi -LAT in the high-energy range. Detectability is favored for hard proton spectra (spectral index α ≲ 3.9), high acceleration efficiencies, and amplified magnetic fields. Our results suggest that next-generation VHE observatories could detect the first gamma-ray signatures of AGN UFOs, providing a new probe of particle acceleration in sub-relativistic shocks.

In 2021, RS Ophiuchi was the first nova to be detected in the very-high-energy (TeV) gamma-ray domain, directly testifying to efficient acceleration of charged particles up to at least the TeV range at the nova shock. Surprisingly, the TeV gamma-ray signal peaks ∼2 days after the GeV signal, and the origin of this delay has still not been clearly understood. We investigate the possibility that this delay is due to the effect of gamma-ray absorption resulting from interactions between gamma rays and optical photons copiously emitted during the outburst. We model particle acceleration at a nova shock to obtain the gamma-ray emission produced in interactions between the accelerated particles and the shocked gas. The effect of gamma-ray absorption is then included in detail using the radiative transfer equation. We find that this can naturally account for the delay between the peaks of GeV and TeV gamma-ray lightcurves. This result emphasizes the importance of gamma-ray absorption for interpreting gamma-ray observations of novae in the TeV range, which, in turn, demonstrates the necessity of a multiwavelength view for unraveling the underlying physics of particle acceleration in these systems.

Active galactic nuclei (AGN) can accelerate protons to energies of ∼10–100 TeV, with secondary production of high-energy neutrinos. If the acceleration is driven by magnetized turbulence, the main properties of the resulting proton and neutrino spectra can be deduced based on insights from particle-in-cell simulations of magnetized turbulence. We have previously shown that these properties are consistent with the TeV neutrino signal observed from the nearby active galaxy NGC 1068. In this work, we extend this result to a population study. We show that the produced neutrino flux depends mainly on the energetics of the corona—the relative fractions of X-ray, magnetic, and nonthermal proton energy—and on the spectral energy distribution of the AGN. We find that coronae with similar properties can explain neutrinos from the candidate AGN for which IceCube has reported an excess, albeit less significant than NGC 1068. Building on this framework, we show how the neutrino signal evolves with AGN luminosity, and use this AGN sequence to predict the diffuse neutrino flux from the extragalactic population, showing that it can account for the diffuse neutrino signal observed by IceCube in the ∼1–100 TeV energy range.

The nuclear activity typical of Seyfert galaxies can drive powerful winds where high-energy phenomena occur. In spite of their high power content, the number of such non-jetted active galactic nuclei (AGN) detected in gamma rays is very limited. 4FGL J1210.3+ 3928, a source recently discovered by the Fermi-LAT telescope, is spatially consistent with the blazar 1E 1217.9+3945 and NGC 4151, a Seyfert galaxy located at about 15.8 Mpc known for hosting ultra-fast outflows (UFOs) in its innermost core. We show that the localization of 4FGL J1210.3+3928 might be affected by fluctuations due to a superposition of the two nearby sources. We explore the possibility of NGC 4151 to be a high-energy source and we conclude that particle acceleration at the UFO wind termination shock can explain the luminosity and spectral shape of the observed gamma-ray flux, whereas the multiwavelength spectral energy distribution of 1E 1217.9+3945 disfavors it as the dominant GeV gamma-ray counterpart. Interestingly, NGC 4151 is also spatially coincident with a weak excess of neutrino events identified by the IceCube neutrino observatory. We compute the contribution of the UFO to such a neutrino excess and we discuss other possible emission regions such as the AGN nearest neighborhood.

Super-Eddington accretion onto stellar-mass compact objects powers fast outflows in ultra-luminous X-ray sources (ULXs). Such outflows, which can reach mildly relativistic velocities, are often observed forming bubble structures. Wind bubbles are expected to develop strong wind termination shocks, which are sites of great interest for diffusive shock acceleration. We developed a model of diffusive shock acceleration in the wind bubbles powered by ULXs. We find that the maximum energy in these objects can easily reach the PeV range, promoting winds from ULXs as a new class of PeVatrons. We specialized our model in the context of the Galactic source SS 433 and show that high-energy protons in the bubble might explain the highest energy photons ($>100$ TeV) and their morphology recently observed by LHAASO. In this paper, we discuss the detectability of such a source in neutrinos, and we analyze the possible radio counterpart of ULXs focusing on the case of W50, the nebula surrounding SS 433. Finally, we discuss the possible contribution of Galactic ULXs to the cosmic-ray flux at the knee, concluding that their role could be significant only if one of these sources, currently undetected, were sufficiently close.

In this paper, we present PyBlastAfterglow, a modular C++ code with a Python interface to model light curves and sky maps of gamma-ray burst afterglows. The code is open-source, modular, and sufficiently fast to perform parameter grid studies. PyBlastAfterglow is designed to be easily extendable and used as a testing bed for new physics and methods related to gamma-ray burst afterglows. For the dynamical evolution of relativistic ejecta, a thin-shell approximation is adopted, where both forward and reverse shocks are included self-consistently, as well as lateral structure, lateral spreading, and radiation losses. Several models of the shock microphysics are implemented, including fully numerical model of the downstream electron distribution evolution, synchrotron emission, self-absorption, and synchrotron self-Compton emission under the one-zone approximation. Thus, the code is designed to be able to model complex afterglows that include emission from reverse shock, very high energy emission, structured jets, and off-axis observations.

The cores of active galactic nuclei are potential accelerators of 10–100 TeV cosmic rays, in turn producing high-energy neutrinos. This picture was confirmed by the compelling evidence of a TeV neutrino signal from the nearby active galaxy NGC 1068, leaving open the question of what is the site and mechanism of cosmic-ray acceleration. One candidate is the magnetized turbulence surrounding the central supermassive black hole. Recent particle-in-cell simulations of magnetized turbulence indicate that stochastic cosmic-ray acceleration is nonresonant, in contrast to the assumptions of previous studies. We show that this has important consequences on a self-consistent theory of neutrino production in the corona, leading to a more rapid cosmic-ray acceleration than previously considered. The turbulent magnetic-field fluctuations needed to explain the neutrino signal are consistent with a magnetically powered corona. We find that strong turbulence, with turbulent magnetic energy density higher than 1% of the rest-mass energy density, naturally explains the normalization of the IceCube neutrino flux, in addition to the neutrino spectral shape. Only a fraction of the protons in the corona, which can be directly inferred from the neutrino signal, are accelerated to high energies. Thus, in this framework, the neutrino signal from NGC 1068 provides a testbed for particle acceleration in magnetized turbulence.