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B. Olmi

Publications and source records attributed to B. Olmi.

At least 19 recordsLinked to original sources
arXiv2026

A thick-shell formalism for pulsar wind nebulae based on energy conservation

One of the most powerful approaches to model the structural and spectral evolution of wind bubbles, and pulsar wind nebulae in particular, is based on the one-zone thin-shell formalism. By solving a set of simple equations, one can relate the spectral properties of these systems to the physical properties of the pulsar and supernova remnant. Due to its predictive power, this approach has been widely used both to fit existing objects and in population synthesis. However, there are inconsistencies when applied to the Crab nebula that have never been fully accounted for, casting doubts on its reliability. We introduce a new and more flexible formalism based on energy conservation that reconciles the observed structural and spectral properties of the Crab nebula, solving the inconsistencies, and provides a more realistic description of wind-bubble evolution. The equations of the formalism are presented and discussed, together with simplified solutions and more complex ones including radiation losses. Implications for the modeling of wind bubbles and pulsar wind nebulae are illustrated. We also introduce a new high-order upwind-implicit scheme for particle-spectrum evolution that ensures high accuracy in energy conservation, and an algebraic-vectorizable approach for synchrotron self-Compton emission in the full Klein-Nishina regime that avoids costly interpolations and integrations. We reproduce the Crab nebula structural properties and spectrum, accounting for the difference in convergence age between the optical filaments and the radio bubble, thereby removing the inconsistencies. The spectral accuracy of this approach is comparable to that of the standard one, but it is superior in reproducing structural properties and accounting for geometrical effects such as a thick layer of mixed material and the lack of efficient coupling between the wind bubble and the surrounding medium.

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arXiv2026

Population synthesis of Galactic middle-aged pulsar wind nebulae II. Observational signatures of superefficiency

Pulsar wind nebulae (PWNe) interacting with the host supernova remnants (SNRs) can enter the reverberation phase in which reverse-shock-driven compression amplifies the magnetic field and rapidly reprocesses particles, sometimes producing "superefficiency", where the radiative output in a given frequency band exceeds the pulsar's instantaneous spin-down power. We investigate the prevalence of this phenomenon in the Galactic population by modeling PWNe with the hybrid TIDE+L framework, which self-consistently follows dynamical evolution, particle spectra, and emission from radio to PeV energies. We track superefficiency across frequency bands and evolutionary stages, analyzing both individual objects and ensemble properties, including compression-resolved samples and population spectral energy distributions. Superefficiency is most common in the far-infrared, but emerges across frequencies and evolutionary phases. It is enhanced in systems where accumulated low-energy electrons radiate in magnetically amplified nebulae. We predict substantially more superefficient sources than a purely thin-shell model would, with differences ranging from factors of a few in FIR and GeV bands to more than an order of magnitude in several optical/UV/X-ray bands.

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arXiv2026

Old pulsar wind nebulae and the role of the thermal filaments

Old pulsar wind nebulae are among the foremost galactic high-energy gamma-ray sources. However we still lack a robust approach to their modeling, especially in the light of forthcoming high-energy observatories like Astri or CTAO. Part of the problem is due to the complex interaction that characterizes these systems. Understanding this complexity has then become mandatory for further advancements. We aim to develop a new approach to investigate the role the thermal thick layer of massive filaments, seen in objects like the Crab nebula and 3C 58, but likely present in all pulsar wind nebulae, can exert on the dynamics of the late reverberation phase, and compare results with standard approaches that neglect such a layer. A new formulation of the one-zone thin-shell plus Lagrangian formalism, developed in a series of previous papers, is here extended to the case of a thick layer of filamentary ejecta, complementing our former work, mostly focused on the initial free-expansion phase. We compare the dynamics of reverberation with and without the presence of filaments, and show that in the former case reverberation may be substantially anticipated and the following compression takes much longer. The total compression of the system does not seem to change much, and the qualitative behavior is preserved. Our results suggest that the presence or absence of an extended filamentary layer might affect the duration of both the free-expansion phase (shortening it) and the following compression phase during reverberation (lengthening it), but it does not change much the overall compression of the nebula. While this changes the relative number of systems in these two phases, and their contribution to high-energy emission, some peculiar radiative effects associated with the level of compression in old systems, like the "super-efficiency", might not be much affected.

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arXiv2026

Population synthesis of Galactic middle-aged pulsar wind nebulae I. Detection prospects for current and future instruments

Pulsar wind nebulae (PWNe) constitute the largest population of Galactic very-high-energy (VHE; $E > 100$ GeV) $\gamma$-ray sources and are key laboratories for studying particle acceleration and pulsar--supernova remnant (SNR) interactions. However, realistic population-level predictions have so far lacked any detailed treatment of the reverberation phase, when the nebula is compressed by the SNR reverse shock, significantly altering its dynamics and radiative spectrum. We employ the hybrid \texttt{TIDE+L} framework, which combines a thin-shell dynamical model with a Lagrangian treatment of the SNR structure during reverberation, allowing self-consistent evolution of thousands of PWNe across all stages up to $10^5$ yr. Each source is evolved under distributions of pulsar spin-down, SNR, and environmental properties, and the resulting $\gamma$-ray fluxes are used to estimate the detectability by current and next-generation $\gamma$-ray observatories while accounting for their sensitivity and sky coverage. The model predicts that the upcoming Cherenkov Telescope Array Observatory (CTAO) will detect an order of magnitude more PWNe than those firmly detected in the TeV range, confirming its dominant contribution to the forthcoming TeV population census. Our results demonstrate that realistic modeling of reverberation is important for predicting the Galactic TeV PWNe population.

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arXiv2023

Reverberation of pulsar wind nebulae (III): Modelling of the plasma interface empowering a long term radiative evolution

The vast majority of Pulsar Wind Nebulae (PWNe) present in the Galaxy is formed by middle-aged systems characterized by a strong interaction of the PWN itself with the supernova remnant (SNR). Unfortunately, modelling these systems can be quite complex and numerically expensive, due to the non-linearity of the PWN-SNR evolution even in the simple 1D / one-zone case when the reverse shock of the SNR reaches the PWN, and the two begin to interact (and reverberation starts). Here we introduce a new numerical technique that couples the numerical efficiency of the one-zone thin shell approach with the reliability of a full ``lagrangian'' evolution, able to correctly reproduce the PWN-SNR interaction during the reverberation and to consistently evolve the particle spectrum beyond. Based on our previous findings, we show that our novel strategy resolves many of the uncertainties present in previous approaches, as the arbitrariness in the SNR structure, and ensure a robust evolution, compatible with results that can be obtained with more complex 1D dynamical approaches. Our approach enable us for the first time to provide reliable spectral models of the later compression phases in the evolution of PWNe. While in general we found that the compression is less extreme than that obtained without such detailed dynamical considerations, leading to the formation of less structured spectral energy distributions, we still find that a non negligible fraction of PWNe might experience a super-efficient phase, with the optical and/or X-ray luminosity exceeding the spin-down one.

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arXiv2023

Reverberation of pulsar wind nebulae (II): Anatomy of the "thin-shell'' evolution

During its early evolution, a pulsar wind nebula (PWN) sweeps the inner part of the supernova ejecta and forms a thin massive shell. Later on, when the shell has been reached by the reverse shock of the supernova remnant, the evolution becomes more complex, in most cases reverting the expansion into a compression: this later phase is called "reverberation". Computations done so far to understand this phase have been mostly performed in the thin-shell approximation, where the evolution of the PWN radius is assimilated to that of the swept-up shell under the effect of both the inner pressure from the PWN, and the outer pressure from the supernova remnant. Despite the thin-shell approach seems rather justifiable, its implementations have so far been inaccurate, and its correctness, never tested. The outer pressure was naively assumed to be scaled according to the Sedov solution (or a constant fraction of it) along the entire evolution. The thin-shell assumption itself fails along the process, being the shell no longer thin in comparison with the size of the PWN. Here, through a combination of numerical models, dimensional arguments, and analytic approximations, we present a detailed analysis of the interaction of the PWN with the supernova remnant. We provide a new analytic approximation of the outer pressure, beyond the Sedov solution, and a revised "thin-shell" able to reproduce results from numerical simulations. Finally, we compute the efficiency by which the PWN is compressed during reverberation over a wide population of sources.

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arXiv2022

ASTRI Mini-Array Core Science at the Observatorio del Teide

The ASTRI (Astrofisica con Specchi a Tecnologia Replicante Italiana) Project led by the Italian National Institute for Astrophysics (INAF) is developing and will deploy at the Observatorio del Teide a mini-array (ASTRI Mini-Array) composed of nine telescopes similar to the small-size dual-mirror Schwarzschild-Couder telescope (ASTRI-Horn) currently operating on the slopes of Mt. Etna in Sicily. The ASTRI Mini-Array will surpass the current Cherenkov telescope array differential sensitivity above a few tera-electronvolt (TeV), extending the energy band well above hundreds of TeV. This will allow us to explore a new window of the electromagnetic spectrum, by convolving the sensitivity performance with excellent angular and energy resolution figures. In this paper we describe the Core Science that we will address during the first four years of operation, providing examples of the breakthrough results that we will obtain when dealing with current open questions, such as the acceleration of cosmic rays, cosmology and fundamental physics and the new window, for the TeV energy band, of the time-domain astrophysics.

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arXiv2022

Extragalactic Observatory Science with the ASTRI Mini-Array at the Observatorio del Teide

The ASTRI Mini-Array is a next-generation system of nine imaging atmospheric Cherenkov telescopes that is going to be built at the Observatorio del Teide site. After a first phase, in which the instrument will be operated as an experiment prioritizing a schedule of primary science cases, an observatory phase is foreseen in which other significant targets will be pointed. We focus on the observational feasibility of extragalactic sources and on astrophysical processes that best complement and expand the ASTRI Mini-Array core science, presenting the most relevant examples that are at reach of detection over long-term time scales and whose observation can provide breakthrough achievements in the very-high energy extragalactic science. Such examples cover a wide range of $\gamma$-ray emitters, including the study of AGN low states in the multi-TeV energy range, the possible detection of Seyfert galaxies with long exposures and the searches of dark matter lines above 10 TeV. Simulations of the presented objects show that the instrument performance will be competitive at multi-TeV energies with respect to current arrays of Cherenkov telescopes.

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arXiv2022

Galactic Observatory Science with the ASTRI Mini-Array at the Observatorio del Teide

The ASTRI (Astrofisica con Specchi a Tecnologia Replicante Italiana) Mini-Array will be composed of nine imaging atmospheric Cherenkov telescopes at the Observatorio del Teide site. The array will be best suited for astrophysical observations in the 0.3-200 TeV range with an angular resolution of few arc-minutes and an energy resolution of 10-15\%. A core-science programme in the first four years will be devoted to a limited number of key targets, addressing the most important open scientific questions in the very-high energy domain. At the same time, thanks to a wide field of view of about 10 degrees, ASTRI Mini-Array will observe many additional field sources, which will constitute the basis for the long-term observatory programme that will eventually cover all the accessible sky. In this paper, we review different astrophysical Galactic environments, e.g. pulsar wind nebulae, supernova remnants, and gamma-ray binaries, and show the results from a set of ASTRI Mini-Array simulations of some of these field sources made to highlight the expected performance of the array (even at large offset angles) and the important additional observatory science that will complement the core-science program.

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arXiv2022

Modeling the {\gamma}-ray Pulsar Wind Nebulae population in our Galaxy

Pulsar wind nebulae (PWNe) represent the largest class of sources that upcoming {\gamma}-ray surveys will detect. Therefore, accurate modelling of their global emission properties is one of the most urgent problems in high-energy astrophysics. Correctly characterizing these dominant objects is a needed step to allow {\gamma}-ray surveys to detect fainter sources, investigate the signatures of cosmic-ray propagation and estimate the diffuse emission in the Galaxy. In this paper we present an observationally motivated construction of the Galactic PWNe population. We made use of a modified one-zone model to evolve for a long period of time the entire population. The model provides, for every source, at any age, a simplified description of the dynamical and spectral evolution. The long term effects of the reverberation phase on the spectral evolution are described, for the first time, based on physically motivated prescriptions for the evolution of the nebular radius supported by numerical studies. This effort tries to solve one of the most critical aspects of one-zone modeling, namely the typical overcompression of the nebula during the reverberation phase, resulting in a strong modification of its spectral properties at all frequencies. We compare the emission properties of our synthetic Pulsar Wind Nebulae population with the most updated catalogues of TeV Galactic sources. We find that the firmly identified and candidate PWNe sum up to about 50% of the expected objects in this class above threshold for detection. Finally, we estimate that CTA will increase the number of TeV detected PWNe by a factor$\geq3$.

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arXiv2021

Revisiting the evolution of nonradiative supernova remnants: A hydrodynamical-informed parameterization of the shock positions

Understanding the evolution of a supernova remnant shell in time is fundamental. Such understanding is critical to build reliable models of the dynamics of the supernova remnant shell interaction with any pulsar wind nebula it might contain. Here, we perform a large study of the parameter space for the one-dimensional spherically symmetric evolution of a supernova remnant, accompanying it by analytical analysis. Assuming, as is usual, an ejecta density profile with a power-law core and an envelope, and a uniform ambient medium, we provide a set of highly-accurate approximations for the evolution of the main structural features of supernova remnants, such as the reverse and forward shocks and the contact discontinuity. We compare our results with previously adopted approximations, showing that existing simplified prescriptions can easily lead to large errors. In particular, in the context of pulsar wind nebulae modelling, an accurate description for the supernova remnant reverse shock is required. We also study in depth the self-similar solutions for the initial phase of evolution, when the reverse shock propagates through the envelope of the ejecta. Since these self-similar solutions are exact, but not fully analytical, we here provide highly-accurate approximations as well.

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arXiv2021

Survey of the Galactic Plane with the CherenkovTelescope Array

Observations with the current generation of very-high-energy gamma-ray telescopes have revealed an astonishing variety of particle accelerators in the Milky Way, such as supernova remnants, pulsar wind nebulae, and binary systems. The upcoming Cherenkov Telescope Array (CTA) will be the first instrument to enable a survey of the entire Galactic plane in the energy range from a few tens of GeV to 300 TeV with unprecedented sensitivity and improved angular resolution. In this contribution we will revisit the scientific motivations for the survey, proposed as a Key ScienceProject for CTA. We will highlight recent progress, including improved physically-motivated models for Galactic source populations and interstellar emission, advance on the optimization of the survey strategy, and the development of pipelines to derive source catalogues tested on simulated data. Based on this, we will provide a new forecast on the properties of the sources thatCTA will detect and discuss the expected scientific return from the study of gamma-ray source populations.

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arXiv2021

Origin and role of relativistic cosmic particles

This white paper briefly summarizes the importance of the study of relativistic cosmic rays, both as a constituent of our Universe, and through their impact on stellar and galactic evolution. The focus is on what can be learned over the coming decade through ground-based gamma-ray observations over the 20 GeV to 300 TeV range. The majority of the material is drawn directly from "Science with the Cherenkov Telescope Array", which describes the overall science case for CTA. We request that authors wishing to cite results contained in this white paper cite the original work.

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arXiv2021

Probing extreme environments with the Cherenkov Telescope Array

The physics of the non-thermal Universe provides information on the acceleration mechanisms in extreme environments, such as black holes and relativistic jets, neutron stars, supernovae or clusters of galaxies. In the presence of magnetic fields, particles can be accelerated towards relativistic energies. As a consequence, radiation along the entire electromagnetic spectrum can be observed, and extreme environments are also the most likely sources of multi-messenger emission. The most energetic part of the electromagnetic spectrum corresponds to the very-high-energy (VHE, E>100 GeV) gamma-ray regime, which can be extensively studied with ground based Imaging Atmospheric Cherenkov Telescopes (IACTs). The results obtained by the current generation of IACTs, such as H.E.S.S., MAGIC, and VERITAS, demonstrate the crucial importance of the VHE band in understanding the non-thermal emission of extreme environments in our Universe. In some objects, the energy output in gamma rays can even outshine the rest of the broadband spectrum. The Cherenkov Telescope Array (CTA) is the next generation of IACTs, which, with cutting edge technology and a strategic configuration of ~100 telescopes distributed in two observing sites, in the northern and southern hemispheres, will reach better sensitivity, angular and energy resolution, and broader energy coverage than currently operational IACTs. With CTA we can probe the most extreme environments and considerably boost our knowledge of the non-thermal Universe.

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arXiv2020

Reverberation of pulsar wind nebulae (I): Impact of the medium properties and other parameters upon the extent of the compression

The standard approach to the long term evolution of pulsar wind nebulae (PWNe) is based on one-zone models treating the nebula as a uniform system. In particular for the late phase of evolved systems, many of the generally used prescriptions are based on educated guesses for which a proper assessment lacks. Using an advanced radiative code we evaluate the systematic impact of various parameters, like the properties of the supernova ejecta, of the inner pulsar, as well of the ambient medium, upon the extent of the reverberation phase of PWNe. We investigate how different prescriptions shift the starting time of the reverberation phase, how this affects the amount of the compression, and how much of this can be ascribable to the radiation processes. Some critical aspects are the description of the reverse shock evolution, the efficiency by which at later times material from the ejecta accretes onto the swept-up shell around the PWN, and finally the density, velocity and pressure profiles in the surrounding supernova remnant. We have explicitly treated the cases of the Crab Nebula, and of J1834.9--0846, taken to be representatives of the more and the less energetic pulsars, respectively. Especially for the latter object the prediction of large compression factors is confirmed, even larger in the presence of radiative losses, also confirming our former prediction of periods of super-efficiency during the reverberation phase of some PWNe.

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arXiv2020

3D Relativistic MHD Simulations of Pulsar Bow Shock Nebulae

Pulsars out of their parent SNR directly interact with the ISM producing so called Bow-Shock Pulsar Wind Nebulae, the relativistic equivalents of the heliosphere/heliotail system. These have been directly observed from Radio to X-ray, and are found also associated to TeV halos, with a large variety of morphologies. They offer a unique environment where the pulsar wind can be studied by modelling its interaction with the surrounding ambient medium, in a fashion that is different/complementary from the canonical Plerions. These systems have also been suggested as the possible origin of the positron excess detected by AMS and PAMELA, in contrast to dark matter. I will present results from 3D Relativistic MHD simulations of such nebulae. On top of these simulations we computed the expected emission signatures, the properties of high energy particle escape, the role of current sheets in channeling cosmic rays, the level of turbulence and magnetic amplification, and how they depend on the wind structure and magnetisation.

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arXiv2020

Laboratory disruption of scaled astrophysical outflows by a misaligned magnetic field

The shaping of astrophysical outflows into bright, dense and collimated jets due to magnetic pressure is here investigated using laboratory experiments. We notably look at the impact on jet collimation of a misalignment between the outflow, as it stems from the source, and the magnetic field. For small misalignments, a magnetic nozzle forms and redirects the outflow in a collimated jet. For growing misalignments, this nozzle becomes increasingly asymmetric, disrupting jet formation. Our results thus suggest outflow/magnetic field misalignment to be a plausible key process regulating jet collimation in a variety of objects from our Sun's outflows to extragalatic jets. Furthermore, they provide a possible interpretation for the observed structuring of astrophysical jets. Jet modulation could be interpreted as the signature of changes over time in the outflow/ambient field angle, and the change in the direction of the jet could be the signature of changes in the direction of the ambient field.

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