PRIN MUR 2022

Unveiling the footprints of the cosmic ray journey through the Galaxy and beyond

PRIN: PROGETTI DI RICERCA DI RILEVANTE INTERESSE NAZIONALE – Bando 2022 - Prot. 2022TJW4EJ

Research Unit 1: Istituto Nazionale di Astrofisica (INAF) Elena Amato -- Research Unit 2: Gran Sasso Science Institute (GSSI) Pasquale Blasi

Project Abstract

The propagation of non-thermal particles in the acceleration region, in the vicinity of sources, through the Galaxy and even when escaping the host galaxies is affected by the particles themselves, through the excitation of instabilities that amplify the turbulence level in the ambient medium. The importance of such non-linear process is well established in the context of particle acceleration, where its effectiveness is key to explain the levels of magnetisation and the particle energies deduced from X-ray observations of powerful accelerators, as for example young Supernova Remnant (SNR) shocks. In the past few years, the idea that cosmic rays (CRs) modify the ambient plasma to ultimately rule their own transport has found support in a number of diverse observations. Examples are the breaks detected by PAMELA, AMS-02 and DAMPE in the spectra of galactic CRs, likely hints of a change in their diffusion properties, or the extended haloes of gamma-ray emission detected by HAWC around evolved Pulsar Wind Nebulae (PWNe), but also SNRs and star clusters. 

Building on our previous work exploring non-linear CR transport in various contexts, and on the expertise in computational astrophysics within our team, this project aims at a quantitative assessment of the phenomenon, by means of numerical tools appropriate to deal with the multi-scale nature of the problem. We plan to investigate non-linear CR transport primarily by means of MHD+PIC simulations, in which the background fluid is treated within the MHD approximation while CRs are evolved according to kinetic equations. Different from the acceleration process that does not allow a separation of scales between CRs and thermal particles, unless a recipe for particle injection is a-priori assumed, the process of propagation entails a natural separation of scales, being the initial distribution in energy and space of the non-thermal particles determined by the source, and hence playing the role of a predefined injection term: the usage of a MHD+PIC approach is then well justified.

The specific physical problems we plan to investigate are: 1) escape of CRs from SNRs and star forming regions; 2) escape of electron-positron pairs from PWNe and their connection with the mysterious TeV halos and with the CR positron excess; 3) escape of CRs from the Galaxy into the intergalactic medium, where CR driven instabilities may confine particles in a turbulent bubble, possible source of multiple non-thermal radiations including gamma-rays and neutrinos; 4) escape of ultra-high energy CRs from their sources, including possible imprints of non-linear transport on their spectrum and on intergalactic magnetic fields. The project has a large potential of major  breakthroughsin the field and its outcome will certainly affect our understanding of current observations and the planning of future observational campaigns, especially with reference to the upcoming Cherenkov Telescope Array and neutrino telescopes.

List of acknowledging works (being updated, for the ADS version click here)

• de Oña Wilhelmi E., Lòpez-Coto R., Aharonian F., Amato E., Cao Z., Gabici S., Hinton J. Nature Astronomy, Volume 8, p. 425-431, April 2024, “The hunt for PeVatrons as the origin of the most energetic photons observed in the Galaxy”, https://ui.adsabs.harvard.edu/abs/2024NatAs...8..425D/abstract

• Amato E. & Recchia S., La Rivista del Nuovo Cimento, Volume 47, Issue 7, pp. 399-452, July 2024, “Gamma-ray halos around pulsars: impact on pulsar wind physics and galactic cosmic ray transport”, https://ui.adsabs.harvard.edu/abs/2024NCimR..47..399A/abstract

• Bandiera R. & Petruk O., Astronomy & Astrophysics, Volume 689, id.A137, 14 pp., September 2024, "Synchrotron polarization with a partially random magnetic field: General approach and application to X-ray polarization from supernova remnants" https://ui.adsabs.harvard.edu/abs/2024A%26A...689A.137B/abstract

• Blasi P., Morlino G., Monthly Notices of the Royal Astronomical Society, Volume 533, Issue 1, pp.561-571, September 2024, “Different spectra of cosmic ray H, He and heavier nuclei escaping compact star clusters”, https://ui.adsabs.harvard.edu/abs/2024MNRAS.533..561B/abstract

• Menchiari S., Morlino G., Amato E., Bucciantini N., Beltrán M.T., Astronomy & Astrophysics, Volume 686, id.A242, 18 pp., July 2024,  “Cygnus OB2 as a test case for particle acceleration in young massive star clusters”,  https://ui.adsabs.harvard.edu/abs/2024A%26A...686A.242M/abstract

• Olmi B., Amato E., Bandiera R., Blasi P., Astronomy & Astrophysics, Volume 684, id.L1, 7 pp., April 2024, “The nature of X-ray filaments around bow shock pulsar wind nebulae”,  https://ui.adsabs.harvard.edu/abs/2024A%26A...686A.242M/abstract

• Peron G., Morlino G., Gabici S., Amato E., Purushothaman A., Brusa M., The Astrophysical Journal Letters, Volume 972, Issue 2, id.L22, 11 pp., September 2024, “On the correlation between Young Massive Star Clusters and Gamma-Ray unassociated sources”,  https://ui.adsabs.harvard.edu/abs/2024ApJ...972L..22P/abstract

• Ambrosone A. et al., Astronomy & Astrophysics, Volume 698, id.L18, 7 pp., June 2025, "The origin of very high-energy diffuse gamma-ray emission: The case for galactic source cocoons", https://ui.adsabs.harvard.edu/abs/2025A%26A...698L..18A/abstract

• Blasi P., Astronomy & Astrophysics, Volume 694, id.A244, 6 pp., February 2025, "Gamma rays from star clusters and implications for the origin of Galactic cosmic rays", https://ui.adsabs.harvard.edu/abs/2025A%26A...694A.244B/abstract

• Blasi P., Astronomy & Astrophysics, Volume 702, id.A24, 9 pp., October 2025, "Gamma radiation from cosmic rays escaping a young supernova remnant: The case of Cas A", https://ui.adsabs.harvard.edu/abs/2025A%26A...702A..24B/abstract

• Bourguinat L.-M., Evoli C., Martin P., Recchia S., Astronomy & Astrophysics, Volume 706, id.A140, 12 pp., February 2026, "The environment of TeV halo progenitors", https://ui.adsabs.harvard.edu/abs/2025arXiv250701495B/abstract

• Menchiari S., Morlino G., Amato E., Bucciantini N., Peron G., Sacco G., Astronomy & Astrophysics, Volume 695, id.A175, 13 pp., March 2025, “Contribution of young massive stellar clusters to the Galactic diffuse gamma-ray emission”, https://ui.adsabs.harvard.edu/abs/2025A%26A...695A.175M/abstract

• Peron G., Menchiari S., Morlino G., Amato E., Astronomy & Astrophysics, Volume 703, id.L8, 9 pp., November 2025, “Hadronic acceleration in the young star cluster NGC 6611 inside the M16 region unveiled by Ferm-LAT: Constraints on the acceleration efficiency,
  https://ui.adsabs.harvard.edu/abs/2025A%26A...703L...8P/abstract

• Sushch I., Blasi P., Brose R., Astronomy & Astrophysics, Volume 700, id.A37, 11 pp., August 2025, "Supernova remnants in super bubbles acting as cosmic ray accelerators", https://ui.adsabs.harvard.edu/abs/2025A%26A...700A..37S/abstract

• Schroer B., Evoli C., Blasi P., ​​Physical Review D, Volume 111, Issue 12, id.123003, 13 pp., June 2025, “Critical examination of the nested leaky box model for Galactic cosmic ray transport”, https://ui.adsabs.harvard.edu/abs/2025PhRvD.111l3003S/abstract

• Schroer B., Caprioli D., Blasi P., Physical Review Letters, Volume 134, Issue 4, id.045201, 6 pp., January 2025, “Role of Nonlinear Landau Damping for Cosmic Ray Transport”, https://ui.adsabs.harvard.edu/abs/2025PhRvL.134d5201S/abstract

• Vecchiotti V., Peron G., Amato E., Menchiari S., Morlino G., Pagliaroli G., Villante F., Journal of Cosmology and Astroparticle Physics, Volume 2025, Issue 09, id.041, 22 pp. , September 2025, "Interpreting the LHAASO Galactic diffuse emission data", https://ui.adsabs.harvard.edu/abs/2025JCAP...09..041V/abstract

• Del Zanna L., Bucciantini N., Landi S., Astronomy & Astrophysics, Volume 702, id.A171, 13 pp., October 2025, "Polarization properties of synchrotron sources from simulations of relativistic magnetohydrodynamic turbulence", https://ui.adsabs.harvard.edu/abs/2025A%26A...702A.171D/abstract

• Cermenati A., Aloisio R., Blasi P., Evoli C., Astronomy & Astrophysics, Volume 707, id.A19, 11 pp.,  February 2026, “Excitation of the nonresonant streaming instability around sources of ultrahigh-energy cosmic rays”, https://ui.adsabs.harvard.edu/abs/2026A%26A...707A..19C/abstract

• Schroer B., Caprioli D., Blasi P., Physical Review D, Volume 113, Issue 2, id.023025, 10 pp., January 2026, “Investigating nonlinear Landau damping in hybrid simulations” https://ui.adsabs.harvard.edu/abs/2026PhRvD.113b3025S/abstract

Articles Submitted for Publication in Peer-Reviewed Journals:

• Martin P., Coriat M., Olmi B., Amato E., Bucciantini N., Marcowith A., Recchia S., 2026, “Radio streaks in the Lighthouse nebula discovered with MeerKAT – Particles escaping from the tail and illuminating the ambiente magnetic field” https://ui.adsabs.harvard.edu/abs/2026arXiv260118596M/abstract

• Peretti E., Amato E., Cerri S.S., Morlino G., Pullano L.P., Recchia S., 2026, “Particle acceleration at recollimation shocks in sub-relativistic jets A model for jets in Seyfert Galaxies, Microquasars and protostellar Systems”, https://ui.adsabs.harvard.edu/abs/2026arXiv260316647P/abstract

• Thanh L. H., Dörner J., Fichtner H., Becker Tjus J., Amato E., 2026, in press, “Revisiting the role of the streaming instability for the cosmic-ray spectrum in the GeV to TeV range” https://ui.adsabs.harvard.edu/abs/2025arXiv251213128T/abstract

Brief summary of the content of the papers:

• “The hunt for PeVatrons as the origin of the most energetic photons observed in the Galaxy”, Oña Wilhelmi E., Lòpez-Coto R., Aharonian F., Amato E., Cao Z., Gabici S., Hinton J. Nature Astronomy 2024. 

The article reviews the challenge of identifying astrophysical accelerators capable of reaching petaelectronvolt (PeV) energies—so‑called PeVatrons—which are thought to be the origin of the most energetic cosmic rays in the Milky Way. It outlines the theoretical conditions for plasma acceleration to PeV energies and discusses which Galactic sources might satisfy these conditions. The authors compare these theoretical predictions with the latest experimental results on high‑energy gamma‑ray observations. They summarize the current state of knowledge about potential PeVatrons and how they might produce ultrahigh‑energy photons. Finally, the review describes prospects for future work to better understand and identify these elusive accelerators.

• “Gamma-ray halos around pulsars: impact on pulsar wind physics and galactic cosmic ray transport”, Amato E. & Recchia S., La Rivista del Nuovo Cimento (2024).

The paper reviews the recently discovered class of TeV gamma‑ray halos—extended multi‑TeV emission regions around middle‑aged pulsars such as Geminga and PSR B0656+14—interpreted as inverse Compton emission from electrons/positrons escaping pulsar wind nebulae into the interstellar medium. It outlines current observations, theoretical models for halo formation, and how these findings impact our understanding of pulsar wind physics and particle escape mechanisms. The authors discuss implications for galactic cosmic‑ray transport, noting that inferred slow diffusion around these objects challenges standard cosmic‑ray propagation models. They also address how pulsar halos affect interpretations of the cosmic‑ray positron excess and broader diffuse gamma‑ray emission. The review concludes with prospects for future observations to better understand the phenomenon. 

• “Synchrotron polarization with a partially random magnetic field: General approach and application to X-ray polarization from supernova remnants”, Bandiera R. & Petruk O., Astronomy & Astrophysics (2024).

The paper develops a general approach to model synchrotron polarization from relativistic particles in magnetic fields that combine ordered and random components, extending beyond simple power‑law energy distributions. It introduces an optimized numerical scheme suited to handle power laws with exponential or super‑exponential cutoffs and anisotropic random magnetic field components. The authors show that previous analytic formulas for polarization degree remain good approximations even when a cutoff is present, improving interpretation of X‑ray polarization. They apply their method to young supernova remnants SN 1006, Tycho, and Cas A, finding consistency between radio and X‑ray polarization in SN 1006 that favors a predominantly random anisotropic field. The work enables more effective use of radio and X‑ray synchrotron polarization data from current and future observations.

• “Different spectra of cosmic ray H, He and heavier nuclei escaping compact star clusters”, Blasi P., Morlino G., Monthly Notices of the Royal Astronomical Society (2024).The paper examines acceleration of cosmic‑ray nuclei at the termination shock of compact star clusters and how energy losses and spallation reactions downstream shape the spectra of particles escaping the wind‑blown bubble. It finds that for plausible mean gas densities the spectrum of helium nuclei escaping the bubble is systematically harder than that of protons, consistent with observed discrepant hardening in cosmic‑ray data. In contrast, spallation reactions strongly suppress heavier nuclei, producing very hard spectra with low normalization and implying they likely contribute little to the cosmic‑ray flux at Earth. The work highlights that these effects depend sensitively on conditions inside the cluster wind cavity. Limitations and implications of this scenario for star clusters as sources of Galactic cosmic rays are discussed.

• “Cygnus OB2 as a test case for particle acceleration in young massive star clusters”, Menchiari S., Morlino G., Amato E., Bucciantini N., Beltrán M.T., Astronomy & Astrophysics (2024).

This paper models the γ‑ray emission from the young massive star cluster Cygnus OB2, assuming cosmic rays are accelerated at the termination shock of its collective wind and produce hadronic γ‑rays. The authors compare simulations with Fermi‑LAT and HAWC observations, finding that the emission’s spectral and spatial properties are very sensitive to the turbulent magnetic field’s nature. Their scenario is incompatible with Kolmogorov turbulence, while Kraichnan‑ or Bohm‑type turbulence can fit very high‑energy data but not the centrally peaked morphology at lower energy. This suggests additional effects are important for lower‑energy γ‑ray emission and that more detailed observational characterization is needed.

• “The nature of X-ray filaments around bow shock pulsar wind nebulae”, Olmi B., Amato E., Bandiera R., Blasi P., Astronomy & Astrophysics (2024).

The authors propose that X‑ray filaments seen around some bow shock pulsar wind nebulae arise from charge‑separated beams of electrons/positrons escaping into the interstellar magnetic field. They argue that these particles, released collimated from reconnection regions, excite a nonresonant streaming instability that accounts for filament brightness, length, and thickness. They successfully test this idea on the Guitar Nebula filament and discuss other cases. These filaments provide a strong diagnostic of particle escape from evolved pulsar wind nebulae, which is key for assessing contributions to cosmic‑ray positrons. The same phenomena might relate to TeV halos and cosmic‑ray transport.

• “On the correlation between Young Massive Star Clusters and Gamma-Ray unassociated sources”, Peron G., Morlino G., Gabici S., Amato E., Purushothaman A., Brusa M., The Astrophysical Journal Letters (2024).

This Letter investigates whether gamma‑ray unassociated sources arise from young massive star clusters (SCs) that are not yet identified as gamma‑ray emitters by comparing catalogs of SCs and H II regions from Gaia and WISE with GeV/TeV gamma‑ray catalogs from Fermi‑LAT, H.E.S.S., and LHAASO. A significant correlation is found between Fermi‑LAT unidentified sources and HII regions that trace massive SCs in very early (<1–2 Myr) stages, implying that stellar winds alone can accelerate particles and produce gamma rays at least up to GeV energies. The correlation with TeV sources is less evident, suggesting that such high‑energy associations are weaker or more complex. No significant association is found between Gaia SCs and either GeV or TeV sources, likely due to selection biases and the large spatial extents of these clusters. The results support the idea that very young massive SCs contribute to gamma‑ray emission via particle acceleration driven by stellar winds.

• “The origin of very high-energy diffuse gamma-ray emission: The case for galactic source cocoons”, Ambrosone A. et al., Astronomy & Astrophysics (2025).

Using TeV–PeV gamma-ray data from LHAASO and high-energy neutrinos from IceCube, the paper shows that cosmic rays accumulate a roughly energy-independent grammage (~0.4 g cm⁻²) near their sources. This occurs in “cocoons”—regions like star-cluster wind bubbles—where particles remain for ~0.3 Myr before diffusing into the Galaxy. Including this cocoon grammage explains both the diffuse gamma-ray background and cosmic-ray secondary-to-primary ratios, producing hard gamma-ray spectra without varying the Galactic diffusion coefficient.

• “Gamma rays from star clusters and implications for the origin of Galactic cosmic rays”, Blasi P., Astronomy & Astrophysics (2025).

The paper examines gamma‑ray emission (E ≳ 1 TeV) observed from star clusters such as the Cygnus cocoon and Westerlund 1 and its implications for their role as sources of Galactic cosmic rays. It shows that if this emission is hadronic in origin, cosmic rays escaping clusters at ≳10 TeV must traverse a grammage inside the cluster that exceeds the Galactic grammage. Analytic models of cosmic‑ray injection and transport are compared with observed gamma‑ray fluxes and consistently require large grammage before escape. If gamma rays are hadronic, either star clusters contribute only a small fraction of the Earth’s cosmic‑ray flux or the standard paradigm of Galactic cosmic‑ray transport must be revised to include source grammage. Alternatively, if the gamma rays are leptonic, these conclusions do not apply.

• “Gamma radiation from cosmic rays escaping a young supernova remnant: The case of Cas A”,  Blasi P., Astronomy & Astrophysics (2025).

The paper uses LHAASO’s gamma‑ray observations of the region around the young supernova remnant Cassiopeia A to constrain how accelerated particles escape this SNR and contribute to high‑energy emission. It pays special attention to the effects of shock evolution and near‑source propagation on the resulting gamma‑ray signal. The non‑detection of ≳100 TeV gamma rays from Cas A indicates that the remnant is not currently acting as a PeVatron. Although the instantaneous maximum energy may have reached ~PeV early in its history, very few such high‑energy particles are present in Cas A’s surroundings. These results place limits on the role of young SNRs in producing cosmic rays at the knee of the Galactic spectrum.

• “The environment of TeV halo progenitors”, Bourguinat L.-M., Evoli C., Martin P., Recchia S., Astronomy & Astrophysics (2026).

The paper investigates the environments of pulsars from birth until they enter the interstellar medium to understand conditions that lead to TeV halos, extended gamma-ray sources around middle-aged pulsars. Using Monte Carlo sampling of progenitor star properties, it models the surrounding medium for both isolated stars and star clusters, and follows supernova remnant evolution in these environments. Accounting for neutron-star kick velocities, the study finds that pulsars typically escape their birth environment around 200–300 kyr, later than commonly assumed. As a result, many confirmed TeV-halo pulsars likely remain in progenitor-shaped media, which can influence the confinement and transport of high-energy electrons and positrons responsible for the observed gamma-ray emission. This work emphasizes the role of the local medium shaped by progenitors in forming TeV halos.

• “Contribution of young massive stellar clusters to the Galactic diffuse gamma-ray emission”, Menchiari S., Morlino G., Amato E., Bucciantini N., Peron G., Sacco G., Astronomy & Astrophysics (2025).

This study estimates how much unresolved young massive stellar clusters (YMSCs) contribute to the Galactic diffuse gamma‑ray emission by simulating a synthetic Galactic population of these clusters and computing their hadronic gamma‑ray output from cosmic rays accelerated at their wind termination shocks. It finds that gamma‑ray emission from unresolved YMSCs can significantly contribute to the observed diffuse flux, especially toward the inner Galaxy. The results highlight that Wolf‑Rayet stellar winds’ kinetic power plays an important role in the total emission. The predicted flux is considered a lower limit because supernova contributions within YMSCs are not included. This work suggests that unresolved YMSCs should be accounted for in models of the Galactic diffuse gamma‑ray background.

• “Hadronic acceleration in the young star cluster NGC 6611 inside the M16 region unveiled by Ferm-LAT: Constraints on the acceleration efficiency”, Peron G., Menchiari S., Morlino G., Amato E., Astronomy & Astrophysics (2025).

The paper analyzes Fermi‑LAT GeV gamma‑ray data toward the M16/Eagle Nebula region that hosts the young massive star cluster NGC 6611, searching for evidence of particle acceleration. It identifies significant GeV emission correlated with a molecular cloud near the cluster, interpreted as hadronic gamma rays from cosmic rays accelerated at the stellar wind termination shock and propagating through the wind‑blown bubble to the cloud. By modeling acceleration and propagation, the authors constrain the acceleration efficiency in NGC 6611 to be around ~1 %–4 % of the wind kinetic power. This provides a quantitative estimate of how efficiently young star clusters convert wind energy into relativistic particles. The results help assess the role of such clusters in contributing to the Galactic cosmic‑ray population. 

• “Supernova remnants in super bubbles acting as cosmic ray accelerators”, Sushch I., Blasi P., Brose R., Astronomy & Astrophysics (2025).

The paper studies cosmic‑ray acceleration at shocks of supernova remnants (SNRs) expanding into the collective wind and turbulent environment of a star cluster super bubble, extending beyond standard interstellar or progenitor wind scenarios. It analyzes how SNRs in such environments evolve over time and how this affects the spectrum and maximum energy of accelerated particles using both analytic estimates and numerical simulations. The authors find that, similar to isolated SNRs, acceleration up to PeV energies is plausible only under extreme conditions achievable in a small subset of remnants. For typical cases with pre‑existing turbulence, maximum energies remain well below PeV, with only very energetic explosions approaching hundreds of TeV. These results suggest that SNRs in super bubbles face similar limitations in reaching the energies needed to explain the cosmic‑ray knee, requiring strong self‑generated turbulence or rare high‑energy events.

• “Critical examination of the nested leaky box model for Galactic cosmic ray transport”, Schroer B., Evoli C., Blasi P., ​​Physical Review D (2025).

The paper revisits the nested leaky box model for Galactic cosmic‑ray transport by comparing its predictions for primary and secondary spectra—including stable/unstable nuclei and antimatter—with observational data. It finds that the standard version of the model is in direct conflict with multiple observations and should be considered ruled out by current data in its vanilla form. The authors discuss how this challenges the idea that cosmic‑ray grammage is accumulated only during propagation through the interstellar medium. They speculate that some grammage could instead be acquired inside sources or around them, which may become increasingly relevant with higher‑precision data at high energies. The work highlights fundamental limitations of this toy model in explaining contemporary cosmic‑ray measurements.

• “Role of Nonlinear Landau Damping for Cosmic Ray Transport”, Schroer B., Caprioli D., Blasi P., Physical Review Letters (2025).

This Letter uses hybrid particle‑in‑cell (PIC) simulations to assess how nonlinear Landau damping affects self‑generated cosmic‑ray scattering in a high‑β plasma appropriate for the Galactic halo. The results show that damping reduces cosmic‑ray drift speed but it remains super‑Alfvénic, and that damping heats the background plasma. They also find that damping triggers an inverse cascade, producing nonresonant large‑scale modes with potential implications for cosmic‑ray phenomenology.

• “Interpreting the LHAASO Galactic diffuse emission data”, Vecchiotti V., Peron G., Amato E., Menchiari S., Morlino G., Pagliaroli G., Villante F., Journal of Cosmology and Astroparticle Physics (2025).

Recent LHAASO measurements of ultra‑high‑energy (∼10–1000 TeV) Galactic diffuse gamma‑ray emission appear higher than simple hadronic models predict, but this work shows that when uncertainties in interstellar gas, cosmic‑ray spatial/energy distribution, and hadronic cross sections are properly included, the LHAASO data above ∼30 TeV are consistent with standard hadronic diffuse models. As a result, no additional contributions from unresolved sources or cosmic‑ray spectral variations toward the Galactic center are required by current data.

• “Polarization properties of synchrotron sources from simulations of relativistic magnetohydrodynamic turbulence”, Del Zanna L., Bucciantini N., Landi S., Astronomy & Astrophysics (2025).

The paper uses 3D relativistic magnetohydrodynamic (MHD) turbulence simulations to model the synchrotron emission and linear polarization expected from relativistically hot astrophysical sources, especially pulsar wind nebulae. Turbulence develops into an Alfvénic equilibrium with a Kolmogorov‑like cascade, and dissipation occurs in thin current sheets with reconnection and intermittency. Synthetic synchrotron maps show that the degree of linear polarization depends on the ratio of turbulent fluctuations to the background magnetic field and spans the range observed in real sources. The results agree well with analytical estimates even when the turbulence is anisotropic, elucidating how magnetic fluctuations shape observed polarization. This work provides a theoretical link between MHD turbulence properties and polarization diagnostics in high‑energy synchrotron emitters.

• “Excitation of the nonresonant streaming instability around sources of ultrahigh-energy cosmic rays”, Cermenati A., Aloisio R., Blasi P., Evoli C., Astronomy & Astrophysics, (2026).

The paper analyses conditions near ultrahigh‑energy cosmic‑ray sources where the nonresonant streaming instability may be excited, studying how escaping high‑energy particles interact with ambient magnetic fields and generate turbulence. It finds that the nonresonant instability can be triggered with sufficient growth rates and energy density under certain source conditions, influencing cosmic‑ray confinement near sources and contributing to magnetic field amplification. This affects both the acceleration process and the propagation environment of ultrahigh‑energy particles.

• “Investigating nonlinear Landau damping in hybrid simulations”, Schroer B., Caprioli D., Blasi P., Physical Review D (2026).

This paper performs a detailed hybrid‑PIC simulation study of nonlinear Landau damping, which is often assumed crucial for cosmic‑ray self‑confinement. It shows that the damping rate at a given scale depends on the power in magnetic fields at larger scales, and that an inverse cascade develops, generating magnetic perturbations beyond resonant scales. The presence of pre‑existing large‑scale turbulence can greatly affect the growth and damping of streaming instability, potentially either confining cosmic rays at all energies or not at all depending on turbulence strength. These results improve understanding of damping effects on cosmic‑ray self‑confinement.

• “Radio streaks in the Lighthouse nebula discovered with MeerKAT – Particles escaping from the tail and illuminating the ambiente magnetic field”, Martin P., Coriat M., Olmi B., Amato E., Bucciantini N., Marcowith A., Recchia S. (2026).

MeerKAT radio observations of the Lighthouse Nebula (PSR J1101‑6101) reveal a structured synchrotron nebula with a cometary tail extending beyond ~5 pc and, for the first time, multiple transverse two‑sided radio streaks. These streaks are interpreted as occasional, charge‑independent release of energetic electrons/positrons from instabilities in the pulsar wind tail into the surrounding medium. Most of the nebula’s particle content appears to be discharged into the ambient medium within several parsecs, lighting up the ambient magnetic field with a coherence length of at least a few parsecs. The observed streaks’ length and persistence imply a low level of magnetic turbulence, slightly enhanced compared to average Galactic cosmic‑ray transport conditions. This structure may result from self‑generated turbulence via resonant streaming instability or past activity of the pulsar’s progenitor star.

• “Particle acceleration at recollimation shocks in sub-relativistic jets A model for jets in Seyfert Galaxies, Microquasars and protostellar Systems”, Peretti E., Amato E., Cerri S.S., Morlino G., Pullano L.P., Recchia S. (2026).

The paper develops a semi‑analytic model of particle acceleration at recollimation shocks that form when sub‑relativistic astrophysical jets expand into dense environments, producing stationary, strong shocks mediated by cocoon pressure. Extending previous jet hydrodynamics to this regime, the authors identify the locations of recollimation shocks and compute particle acceleration via diffusive shock acceleration, solving a space‑dependent transport equation for particle distributions and spectra along the jet. They derive maximum achievable energies as functions of system properties, finding that in Seyfert galaxies particles can be accelerated from PeV up to EeV energies, in microquasars up to tens of PeV, and in protostellar jets up to TeV. Escaping protons may then diffuse through the surrounding cocoon, potentially producing hadronic signatures. Their results indicate that recollimation shocks can play a central role in particle acceleration across a range of astrophysical jet systems.

• “Revisiting the role of the streaming instability for the cosmic-ray spectrum in the GeV to TeV range”,Thanh L. H., Dörner J., Fichtner H., Becker Tjus J., Amato E.,  (2026).

The paper re‑examines how the streaming instability—where cosmic rays generate their own scattering waves—affects the cosmic‑ray energy spectrum from GeV to TeV energies by revisiting the model of Blasi et al. (2012) with a self‑consistent treatment of diffusion and self‑generated waves. It performs an extensive parameter study to identify the range of conditions that best fit cosmic‑ray spectrum data, including the observed spectral hardening around ~300 GeV. The authors conclude that when cosmic‑ray transport is treated self‑consistently with streaming instability effects, this mechanism remains a competitive explanation for the spectral hardening in the GeV–TeV range.