arXiv subjects

Giada Peron

Publications and source records attributed to Giada Peron.

At least 19 recordsLinked to original sources

arXiv2026

A cosmic-ray loaded nascent outflow driven by a massive star cluster

Cosmic rays are widely held to drive outflows from star-forming galaxies and profoundly influence galaxy evolution. Direct evidence for cosmic-ray carrying outflows is however lacking. At the same time there is increasing awareness of the importance of massive star clusters in the acceleration of cosmic rays in galaxies. Here we report on the discovery of a nascent outflow driven by the massive star cluster Westerlund 1. Giga-electronvolt gamma-ray emission coincident with a cavity visible in atomic hydrogen traces the emergence of a population of relativistic electrons out of the Galactic Disc. The emission is offset from tera-electronvolt gamma-ray radiation surrounding the cluster, but connects to it smoothly spectrally and spatially. The implied energy density of co-accelerated protons and nuclei, assuming standard non-thermal electron/proton injection efficiencies, is at least an order of magnitude higher than that in the general interstellar medium. These particles therefore have the potential to dynamically influence the outflow. This discovery suggests that cosmic-ray loaded outflows may be a common feature of young massive star clusters, with implications for the transport of cosmic rays into the halo of the Galaxy.

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arXiv2026

A broadband view of the thermal and non-thermal emission from the embedded massive star cluster RCW 38

Gamma-ray emission has now been detected from a variety of source classes in the Galaxy, including clusters of young massive stars. RCW 38, a very young embedded massive star cluster, is a case of particular interest: its gamma-ray emission detected up to hundreds of GeV, provided the first observational evidence of high-energy particle acceleration powered exclusively by stellar winds. In this work, we aim to characterize the emission mechanisms responsible for the gamma-ray flux in RCW 38 and to provide estimates of the acceleration efficiency, as well as the fraction of accelerated electrons compared to protons, $K_{ep}$. We present the most comprehensive multi-wavelength study of a single star cluster to date. Our analysis ranges from MHz radio observations obtained with the GLEAM-X survey from the Murchison Widefield Array (MWA) to GeV gamma-ray data from Fermi-LAT, and includes GHz and THz measurements from Parkes, Planck, and IRAS. We model the thermal and non-thermal emission of RCW 38 using an eight-parameter model constrained by the Markov chain Monte Carlo method. Our results support an interpretation in which the gamma-ray emission from RCW 38 is produced by hadronic interactions with the host molecular cloud. We derive robust constraints on the electron-to-proton ratio, with $K_{ep} \lesssim 10^{-3}$, and on the acceleration efficiency, estimated to be $\gtrsim$1%, consistent with the values required to explain the cosmic-ray composition, and in particular its $^{22}$Ne anomaly. These results strengthen the idea that stellar clusters play a significant role as contributors to cosmic-ray protons in our Galaxy at least up to energies of a few TeV. Future investigations with the next generation of ground-based detectors will determine whether they also play a relevant role at higher energies, particularly in the context of the cosmic-ray knee.

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arXiv2026

Modelling Galactic neutrino emission: contributions from massive star clusters and interstellar cosmic rays

The recent detection of Galactic neutrinos by the IceCube Observatory constitutes a remarkable achievement for neutrino astrophysics. By means of model dependent analyses based on spatial and spectral templates, a purely diffuse neutrino flux was measured in which no individual source was resolved. We present here a novel theoretical computation about the expected neutrino emission from the Galactic Plane that, differently from previous models, includes both the contributions from cosmic-ray (CR) sea and hadronic sources, represented by star clusters and supernova remnants therein, which are to date believed to be the dominant sources of Galactic CR protons. For the modelling of sources, diffusive particle acceleration is considered at both the collective wind termination shock blown by member stars and at the supernova shocks. The predicted flux of very-high energy neutrinos from individual star clusters is found to be marginally detectable even by cubic kilometer scale detectors, such that their cumulative contribution is expected to appear as an unresolved diffuse component, on top of that guaranteed by the CR sea interacting with the gas along the Plane. The overall neutrino production of the Milky Way star cluster population is computed, based on multiple synthetic realizations of the cluster population reproducing local stellar observations. As a result, we obtain novel neutrino template maps and provide them to the community, to be tested in future neutrino analyses in order to constrain the role of star clusters for extreme CR acceleration and neutrino production. The normalization of our models is consistent with the IceCube best-fit of existing Galactic templates, suggesting that the unresolved contribution from cluster emission may be non-negligible.

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arXiv2026

Wolf-Rayet stars as tracers of gamma-ray emission: Isolated stars and stellar clusters/associations

Context: Recent gamma-ray observations of young star clusters revealed that stellar wind termination shocks accelerate particles, with the energy reservoir provided by the mechanical power of massive-star winds. Aims: Our goal is to identify promising targets for future gamma-ray studies of stellar clusters and associations powered by massive stars. As the wind power of a single Wolf-Rayet (WR) star can rival the cumulative wind power of the most massive clusters, we also investigate isolated WR stars, many of which are indeed isolated. Methods: We ranked a large sample of stellar clusters and associations according to the number of member WR stars divided by the distance squared, a quantity proportional to the expected gamma-ray signal, and searched for spatial correlations with known gamma-ray sources. We repeated the same procedure for individual WR stars with known wind mechanical powers and distances. Results: We found a hint ($\lesssim 3 \sigma$ confidence) for a correlation between WR-hosting clusters and unidentified GeV gamma-ray sources, and identified new spatial associations for 11 clusters. We also found spatial coincidences between 4 isolated WR stars (WR110, WR114, WR111, and WR14) and unidentified gamma-ray sources. Although no significant correlation is found for isolated WR stars as a population, these 4 objects exhibit particularly large wind-power-to-distance-squared ratios, a necessary condition for detectability with current instruments. Assuming the gamma-ray emission is powered by WR winds, it can be interpreted as arising from interactions between particles accelerated at the wind termination shock and ambient matter or radiation fields. Conclusions: Since the wind power of an individual WR star can rival that of an entire stellar cluster, we provide a ranking of stellar clusters and isolated WR stars that may constitute potential gamma-ray emitters.

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arXiv2026

Cosmic rays: constraints from future MeV detectors

Cosmic rays are charged energetic particles that permeate the interstellar medium. Their sizeable energy share and penetration power makes them essential players in the dynamical and chemical processes that rule Galactic evolution, such as the launching of outflows and the formation of star and planets. For these processes low-energy (MeV-GeV) CRs are particularly important, both because they are the most abundant and because they have the largest cross-section for ionization. The study of cosmic rays naturally connects with gamma-ray astronomy, as high-energy photons are the principal products of their interaction with the interstellar plasma. In this article, after reviewing our current understanding of Galactic cosmic rays as derived from direct measurements, we present the state of the art regarding Galactic cosmic rays covering their direct observables, their acceleration processes and models for their propagation in the Galactic Disk. We present then an excursus on the current state of gamma-ray observations, and propose new prospects for investigating the physical properties of Galactic cosmic rays, exploiting the observational capability of future MeV missions.

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arXiv2025

Star clusters in the gamma-ray sky

Massive Star Clusters (SCs) have been proposed as additional contributors to Galactic Cosmic rays (CRs), to overcome the limitations of supernova remnants (SNRs) to reach the highest energy end of the CR spectrum. Thanks to fast mass losses due to the collective stellar winds, the environment around SCs is potentially suitable for particle acceleration up to PeV energies, and their energetics is enough to account for a non-negligible fraction of the Galactic CRs. Anyhow, the theoretical expectations need to be corroborated by clear observations. Despite the increasing number of detections at different energies, the contamination of other sources often makes it difficult to constrain the contribution arising from stellar winds only, unless one selects objects younger than a few million years, namely before stars start to explode inside clusters. I will review the results obtained with gamma-ray data towards a few massive young star clusters and discuss what implications these result have, especially concerning their contribution to the bulk of Galactic CRs.

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arXiv2025

Unveiling TeV halos among unidentified extended TeV sources

In recent years, the number of known sources emitting very- and ultra-high-energy gamma-rays has increased significantly thanks to facilities such as LHAASO and HAWC. Many of the observed sources are still unidentified or poorly constrained due to the limited angular resolution of these instruments; however, it is now ascertained that approximately half of them have a pulsar in coincidence. Some of these unidentified extended sources may be the result of the diffusion of leptons accelerated by the pulsar itself or in its nebula to energies exceeding 50 TeV. This new class of sources, called TeV halos, is characterized by a peculiar radial profile that, if properly resolved, is key to distinguishing them from other TeV sources that are associated with a pulsar, such as supernova remnants and pulsar wind nebulae. In this contribution, we consider all the pulsars which are spatially coincident with an unidentified extended TeV source, in order to quantify whether its spin-down power, age and distance allow the pulsar to produce a TeV halo with the observed flux and extension. We also investigate how the next generation of Imaging Atmospheric Cherenkov Telescopes (IACTs), namely the Cherenkov Telescope Array Observatory (CTAO) and the ASTRI Mini-Array, will observe and characterize these TeV halos. We present a set of simulated sources with the expected morphology and spectrum, and we show for which of them we can distinguish between TeV halos and other classes of extended sources.

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arXiv2025

Hadronic acceleration in the young star cluster NGC 6611 inside the M16 region unveiled by Fermi-LAT: constraints on the acceleration efficiency

Context. Young Massive Star Clusters, long considered as potentially important sources of galactic cosmic rays, have recently emerged as gamma-ray emitters up to very high energies. Aims. In order to quantify the contribution of this source class to the pool of Galactic CRs, we need to estimate the typical acceleration efficiency of these systems. Methods. We search for emission in the GeV band, as most of the energy is emitted in this band. We perform an analysis of Fermi-LAT data collected towards the M16 region, a star-forming region also known as the Eagle Nebula, which hosts the Young Massive Star Cluster NGC 6611. We model the acceleration at the stellar wind termination shock and the propagation through the wind-blown bubble to derive the energetics of the process and interpret the GeV observations. Results. We find significant GeV emission in correspondence of a molecular cloud associated to the Young Massive Star Cluster NGC 6611. We interpret this as hadronic emission associated to particle accelerated at the cluster wind termination shock and propagated through the low-density wind-excavated bubble to the cloud. Our modeling allows us to put firm constraints on the acceleration efficiency in NGC 6611, assessing it between $\sim$ 1 % and $\sim$ 4 %.

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arXiv2025

Massive star clusters in the gamma-ray sky: the role of HII regions

Massive Star Clusters (SCs) have been proposed as important CR sources, with the potential of explaining the high-energy end of the Galactic cosmic-ray (CR) spectrum, that Supernova Remnants (SNRs) seem unable to account for. Thanks to fast mass losses due to the collective stellar winds, the environment around SCs is potentially suitable for particle acceleration up to PeV energies and the energetics is enough to account for a large fraction of the Galactic CRs, if the system is efficient enough. A handful of star clusters have been detected in gamma-rays confirming the idea that particle acceleration is taking place in this environment. However, contamination by other sources often makes it difficult to constrain the contribution arising from SCs only. Here we present a new analysis of Fermi-LAT data collected towards a few massive young star clusters. The young age (< 3 Myr) of the clusters guarantees that no SN has exploded in the region, allowing us to determine the power contributed by the stellar component alone, and to quantify the contribution of this type of sources to the bulk of CRs. Moreover, we will present a recent statistical investigation that quantifies the degree of correlation between gamma-ray sources and these astrophysical objects and briefly discuss the observational prospect for ASTRI and CTAO.

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arXiv2025

Gamma ray emission from embedded young massive star clusters unveiled by Fermi-LAT

Massive star clusters (SCs) have been proposed as additional contributors to Galactic Cosmic rays (CRs), to overcome the limitations of supernova remnants (SNR) to reach the highest energy end of the Galactic CR spectrum. Thanks to fast mass losses through collective stellar winds, the environment around SCs is potentially suitable for particle acceleration up to PeV energies. A handful of star clusters has been detected in gamma-rays confirming the idea that particle acceleration is taking place in these environments. Here we present a new analysis of Fermi-LAT data collected towards a few massive young star clusters and estimate the contribution of these types of sources to the bulk of CRs. We then briefly discuss the observational prospects for ASTRI and CTAO.

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arXiv2024

Interpreting the LHAASO Galactic diffuse emission data

Recently, the Large High-Altitude Air Shower Observatory (LHAASO) collaboration has obtained a measurement of the gamma-ray diffuse emission in the ultra-high energy range, $10-10^3$ TeV after masking the contribution of known sources. The measurement is provided in two Galactic regions and appears to be 2-3 times higher than the gamma-ray signal expected from the hadronic interactions of diffuse cosmic rays with the interstellar medium, potentially implying that either additional emission sources exist or cosmic ray intensities have spatial variations. In this work, we calculate the hadronic gamma-ray diffuse emission outside the masks, considering a realistic gas distribution. We present a comprehensive calculation of the emission, which includes systematic uncertainties in the gas content of the Galactic disk, in the energy and spatial distribution of cosmic rays, as well as in the hadronic interaction cross-sections. Our results show that these factors mitigate the tension between data and predictions. The LHAASO data appear compatible with our baseline model in the outer Galactic region. In the inner region, the data show an excess with respect to the predictions below $\sim 50$ TeV, while at higher energies they are well described by our model. We argue that two plausible explanations for enhanced gamma-ray emission-unresolved sources and CR spectral hardening in the inner Galaxy-are likely suppressed by the LHAASO masking strategy, which excludes regions where both effects are expected to be most prominent.

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arXiv2024

On the correlation between young massive star clusters and gamma-ray unassociated sources

Star clusters (SCs) are potential cosmic-ray (CR) accelerators and therefore are expected to emit high-energy radiation. However, a clear detection of gamma-ray emission from this source class has only been possible for a handful of cases. This could in principle result from two different reasons: either detectable SCs are limited to a small fraction of the total number of Galactic SCs, or gamma-ray-emitting SCs are not recognized as such and therefore are listed in the ensemble of unidentified sources. In this Letter we investigate this latter scenario, by comparing available catalogs of SCs and HII regions, obtained from Gaia and WISE observations, to the gamma-ray GeV and TeV catalogs built from Fermi-LAT, H.E.S.S. and LHAASO data. The significance of the correlation between catalogs is evaluated by comparing the results with simulations of synthetic populations. A strong correlation emerges between Fermi-LAT unidentified sources and HII regions which trace massive SCs in the earliest (< 1-2 Myr) phase of their life, where no supernova explosions have happened yet, confirming that winds of massive stars can alone accelerate particles and produce gamma-ray emission at least up to GeV energies. The association with TeV-energies sources is less evident. Similarly, no significant association is found between Gaia SCs and GeV nor TeV sources. We ascribe this fact to the larger extension of these objects, but also to an intrinsic bias in the Gaia selection towards SCs surrounded by a lower target gas density, that would otherwise hinder the detection in the optical waveband.

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arXiv2024

The contribution of winds of star clusters to the Galactic cosmic-ray population

Cosmic rays are energetic nuclei that permeate the entire Galactic disk. Their existence requires the presence of powerful particle accelerators. While Galactic supernova explosions may supply the required energy, there is growing evidence that they cannot explain all of the observed properties of cosmic rays, such as their maximum particle energy and isotopic composition. Among Galactic objects, winds from stellar clusters meet the energetic requirement and provide a suitable environment for particle acceleration. The recent detection of some of these objects in gamma rays confirms that they indeed harbor high-energy particles.However, as most supernovae explode inside stellar clusters, it is difficult to distinguish the contribution of winds to particle acceleration. Here we report the detection of young star clusters in the nearby Vela molecular ridge star forming region. The young age of the systems guarantees an unbiased estimate of the stellar CR luminosity free from any supernova or pulsar contamination and allows us to draw conclusions on the acceleration efficiency and the total power supplied by these objects. We demonstrate that much more than 1% of the wind mechanical power is converted into CRs and consequently conclude that a small but non-negligible fraction ~ 1-10% of the CR population is contributed by stellar clusters.

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arXiv2024

Contribution of young massive star clusters to Galactic diffuse $\gamma$-ray emission

Context: Young massive stellar clusters (YMSCs) have emerged as potential $\gamma$-ray sources, after the recent association of a dozen YMSCs with extended $\gamma$-ray emission. The large size of the detected halos, comparable to that of the wind-blown bubble expected around YMSCs, makes the $\gamma$-ray detection of individual YMSCs rather challenging. As a result, the emission from most of the Galactic YMSCs could be unresolved, thus contributing to the diffuse $\gamma$-ray radiation observed along the Galactic Plane. Aims: In this study, we estimate the possible contribution to the Galactic diffuse $\gamma$-ray emission from a synthetic population of YMSCs, and we compare it with observations obtained with different experiments, from 1 GeV to hundreds of TeV, in two regions of the Galactic Plane. Methods: As the population of galactic YMSCs is only known locally, we evaluate the contribution of $\gamma$-ray emission relying on the simulation of synthetic populations of YMSCs based on the observed properties of local clusters. We compute the $\gamma$-ray emission from each cluster assuming that the radiation is purely hadronic in nature and produced by cosmic rays accelerated at the cluster's collective wind termination shock. Results: We find that the $\gamma$-ray emission from unresolved YMSCs can significantly contribute to the observed Galactic diffuse flux, especially in the inner part of the Galaxy, and that an important role is played by the power injected by Wolf-Rayet stellar winds. The predicted $\gamma$-ray flux should be considered as a lower limit, given that our calculation neglects the contribution of supernovae exploding in YMSCs.

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arXiv2024

Detection prospects of very and ultra high-energy gamma rays from extended sources with ASTRI, CTA, and LHAASO

Context. The recent discovery of several ultra high-energy gamma-ray emitters in our Galaxy represents a significant advancement towards the characterisation of its most powerful accelerators. Nonetheless, in order to unambiguously locate the regions where the highest energy particles are produced and understand the responsible physical mechanisms, detailed spectral and morphological studies are required, especially given that most of the observed sources were found to be significantly extended. Aims. In these regards, pointing observations with the next-generation Imaging Atmospheric Cherenkov Telescopes, such as the Cherenkov Telescope Array (CTA) Observatory and the ASTRI Mini-Array (ASTRI), are expected to provide significant improvements. Here we aim to identify the most promising sources to target in future observations. Methods. For this purpose, we performed a comparative analysis of the expected performance of ASTRI and CTA, computing their differential sensitivities towards extended sources, and further explored their capabilities with respect to specific case studies, including follow-ups of existing gamma-ray source catalogues. Results. We find that almost all of the sources thus far detected by LHAASO-WCDA and in the H.E.S.S. Galactic Plane Survey will be in the reach of ASTRI and CTA with about 300 and 50 hours of exposure, respectively. For the highest energy emitters detected by LHAASO-KM2A, in turn, we provide a list of the most promising objects that would require further investigation. We additionally examined specific classes of sources in order to identify potentially detectable gamma-ray emitters, such as passive molecular clouds (i.e. illuminated by the cosmic-ray sea) and pulsars surrounded by a halo of runaway particles.

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arXiv2023

Gamma-ray emission from the Seyfert galaxy NGC 4151: Multi-messenger implications for ultra-fast outflows

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.

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arXiv2021

Probing the galactic cosmic-ray density with current and future $\gamma$-ray instruments

Cosmic Rays (CRs) propagating through dense molecular clouds (MCs) produce gamma rays which carry direct information about the CR distribution throughout the Galaxy. Observations of gamma rays in different energy bands allow exploration of the average CR density in the Galactic Disk, the so-called level of the "CR Sea". Fermi-LAT observations have demonstrated the method's feasibility based on two dozen MCs in our Galaxy. However, the potential of Fermi-LAT is limited by the most massive and relatively nearby MCs; thus, the current observations cover only a tiny fraction of the Milky Way. In this paper, we study the prospects of expanding the CR measurements to very and ultra-high energies and remote parts of the Galaxy with the current and next-generation detectors. Based on calculations of fluxes expected from MCs, we formulate the requirements to the sensitivity of the post-Fermi-LAT detectors to map GeV-TeV CRs in the Galactic Disk. We also explore the potential of the current and future air-shower and atmospheric Cherenkov telescope arrays for the extension of CR studies to multi-TeV and PeV energy bands. We demonstrate that the improvement of the Fermi-LAT sensitivity by a factor of a few would allow a dramatic increase in the number of detectable MCs covering almost the entire Galaxy. The recently completed LHAASO should be able to take the first CR probes at PeV energies in the coming five years or so.

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arXiv2021

Probing the Cosmic Ray density in the inner Galaxy

The galactic diffuse $\gamma$-ray emission, as seen by Fermi Large Area Telescope (LAT), shows a sharp peak in the region around 4 kpc from the Galactic center, which can be interpreted either as due to an enhanced density of cosmic-ray accelerators or to a modification of the particle diffusion in that region. Observations of $\gamma$-rays originating in molecular clouds are a unique tool to infer the cosmic-ray density point by point, in distant regions of the Galaxy. We report here the analysis of 11 yr Fermi-LAT data, obtained in the direction of nine molecular clouds located in the 1.5--4.5 kpc region. The cosmic-ray density measured at the locations of these clouds is compatible with the locally measured one. We demonstrate that the cosmic-ray density gradient inferred from the diffuse gamma-ray emission is the result of the presence of cosmic-ray accelerators rather than a global change of the sea of Galactic cosmic rays due to their propagation.

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