arXiv subjects

Paola Rossi

Publications and source records attributed to Paola Rossi.

16 recordsLinked to original sources

arXiv2026

Numerical Simulations of Restarted Jets -- I. Dynamics

We performed high-resolution three-dimensional relativistic magnetohydrodynamic (RMHD) simulations of restarted jets evolving within the cavity of a previous jet episode, using the PLUTO code. The simulations cover a spatial domain of $50\;\mbox{kpc}$ with a resolution of $40\;\mbox{pc}$. Three suites of simulations were performed to understand the impact of jet power, magnetic fields and quiescence time on the evolution of restarted jets. The restarted jets undergo a complex, multi-stage evolution, with the remnant cocoon relaxing from an initially conical structure to a mushroom-shaped morphology via an intermediate cylindrical phase. As the cocoon of the initial jet expands, dense material entrained by fluid instabilities such as Kelvin-Helmholtz and Rayleigh-Taylor significantly alter the conditions within it. As a result, the interaction of the restarted jet with the cocoon is markedly different from that of the initial jet with the ambient medium. In particular, we find that the restarted jet propagates ballistically through the rarefied remnant cocoon, without creating prominent backflows. Deceleration of the jet and associated backflows are observed only when it encounters dense structures. The structure and strength of the shocks in restarted jets are affected by jet power, magnetic field strength, and quiescence time. Finally, we discuss the implications of the dynamics for observed properties of radio galaxies.

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arXiv2026

Galactic-scale evolution of classical and complex radio galaxies. Impact of ambient morphology and jet geometry

Extragalactic jets exhibit a wide range of propagation orientations relative to the host galaxy's principal axis. This study investigate the spatiotemporal evolution of jets as a function of their propagation direction within their triaxial hosts-introducing varying degrees of environmental hindrance-and as a function of internal jet properties (while maintaining identical jet power)-introducing varying collimation and thrust. Observational data on extended radio sources are re-analyzed to identify key traits arising from variations in jet orientation and intrinsic properties. These findings are then systematically tested using a suite of 3D RMHD simulations. When a jet propagates along host's major axis (path of maximal environmental resistance), it produces an X-shaped morphology with secondary lobe aligns along the minor axis, co-evolving actively alongside the active jet. At intermediate angles to the major axis, the jet morphology transitions into a double-boomerang structure with notably curved lobes. Such lobes are interestingly regenerative through both backflow and jet precession mechanisms, making it difficult to disentangle their origin. Jets propagating along the minor axis (path of minimal resistance) exhibit faster propagation, forming classical double-lobed sources. With increased thrust and improved collimation (keeping jet power constant), these jets advance even more rapidly, potentially evolving into giant radio galaxy candidates. Counterexample sources that deviate from these traits were also modeled. The spatial variation of internal turbulence shows significant fluctuations below 1 kpc, with stronger magnetic fields further suppressing these irregularities. Magnetic field plays a key role in the radiative appearance of these sources, modulating features like missing or one-sided (wing) lobe emission, filamentary structures, and warmspot versus hotspot formation.

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arXiv2026

Automatic computation of the glycemic index: data driven analysis of the glucose standard

The Glycemic Index (GI) is a tool for classifying carbohydrates based on their impact on postprandial glycemia, useful for diabetes prevention and management. This study applies a mathematical model for a data driven simulation of the glycemic response following glucose ingestion. The analysis is performed on a dataset of 35 healthy subjects undergone a standard 50 g oral glucose test. The results reveal a direct correlation between glucose response profiles and parameters describing glucose absorption, enabling the classification of subjects into three groups based on the timing of their glycemic peak: <30 min, 30-50 min, >50 min. These findings highlight the ability of a physiology-based mathematical model to capture inter-individual variability in postprandial glucose dynamics and represent a step toward simulation-based approaches for GI estimation.

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arXiv2024

A comparative study of radio signatures from winds and jets: Modelling synchrotron emission and polarization

Outflows driven by active galactic nuclei (AGN) are seen in numerous compact sources; however, it has remained unclear how to distinguish between the driving mechanisms, such as winds and jets. Therefore, our study aims to offer observational insights from simulations to aid in this distinction. Specifically, in this paper, we investigate the evolution of wide-angled, moderately relativistic, magnetized winds and analyze their non-thermal radio emission and polarization properties. We find that the evolution of winds varies depending on factors such as power, density, and opening angle, which in turn influence their observable characteristics. Additionally, different viewing angles can lead to varying observations. Furthermore, we note distinctions in the evolution of winds compared to jets, resulting in disparities in their observable features. Jets typically exhibit a thin spine and hotspot(s). Winds manifest broader spines or an "hourglass-shaped" bright emission in the cocoon, which are capped by bright arcs. Both display high polarization coinciding with the bright spine and hotspots/arcs, although these regions are relatively compact and localized in jets when compared to winds. We emphasize the importance of high resolution, as we demonstrate that emission features from both jets and winds can become indistinguishable at lower resolutions. The distribution of polarization is largely unaffected by resolution, though lower polarization becomes more noticeable when the resolution is decreased.

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arXiv2024

X-Shaped Radio Galaxies: Probing Jet Evolution, Ambient Medium Dynamics, and Their Intricate Interconnection

This review explores the field of X-shaped radio galaxies (XRGs), a distinctive subset of winged radio sources that are identified by two pairs of jetted lobes which aligned by a significant angle, resulting in an inversion-symmetric structure. These lobes, encompassing active (primary) and passive (secondary) phases, exhibit a diverse range of properties across the multiple frequency bands, posing challenges in discerning their formation mechanism. The proposed mechanisms can broadly be categorized into those related either to a triaxial ambient medium, into which the jet propagates, or to a complex, central AGN mechanism, where the jet is generated. The observed characteristics of XRGs as discovered in the most substantial sample to date, challenge the idea that there is universal process at work that produces the individual sources of XRGs. Instead, the observational and numerical results rather imply the absence of an universal model and infer that distinct mechanisms may be at play for the specific sources. By scrutinizing salient and confounding properties, this review intends to propose the potential direction for future research to constrain and constrict individual models applicable to XRGs.

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arXiv2024

The different flavors of extragalactic jets: Magnetized relativistic flows

We perform three-dimensional numerical simulations of magnetized relativistic jets propagating in a uniform density environment in order to study the effect of the entrainment and the consequent deceleration, extending a previous work in which magnetic effects were not present. As in previous papers, our aim is to understand the connection between the jet properties and the resulting Fanaroff-Riley classification. We consider jets with different low densities, and therefore low power, and different magnetizations. We find that lower magnetization jets effectively decelerate to sub-relativistic velocities and may then result in an FR~I morphology on larger scales. At the opposite, in the higher magnetization cases, the entrainment and consequent deceleration are substantially reduced. }

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arXiv2023

A polarization study of jets interacting with turbulent magnetic fields

We investigate the effect of the jet's immediate surroundings on the non-thermal synchrotron emission and its polarization properties. The ambient medium is equipped with a turbulent magnetic field, which is compressed and amplified by the jets as they progress. This leads to high polarization at the forward shock surface. The randomness in the magnetic polarities of the external fields in the shocked ambient medium (SAM) results in vector cancellation of the polarized components from the jet, thereby causing depolarization of the radiation from the cocoon. We find that due to the slow decay of the fields in the SAM, such depolarization by the fields with large correlation lengths is more prominent when compared to the small-scale fields. Also, the low-power jets, which have magnetic fields comparable in strength to those in the SAM, are more severely affected by the SAM's depolarizing effect, than the high-power ones. The turbulent backflows in the cocoon, as well as the shearing of fields near the contact discontinuity, strengthen the poloidal component in the jet. This causes internal depolarization due to the cancellation of the orthogonally polarized components along the Line of Sight as the field transitions from ordered toroidal to poloidal. The synchrotron maps display high-emission filaments in the cocoon with magnetic fields aligned along them. The kink instability leads to the wiggling motion of the jet's spine, resulting in hotspot complexes in low-power sources.

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arXiv2023

Magnetocentrifugal mechanism of pair creation in AGN

In the manuscript, we study the efficiency of pair creation by means of the centrifugal mechanism. The strong magnetic field and the effects of rotation, which always take place in Kerr-type black holes, guarantee the frozen-in condition, leading to the generation of an exponentially amplifying electrostatic field. This field, when reaching the Schwinger threshold, leads to efficient pair production. The process has been studied for a wide range of AGN luminosities and black hole masses, and it was found that the mechanism is very efficient, indicating that for AGNs where centrifugal effects are significant, the annihilation lines in the MeV range will be very strong.

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arXiv2022

Modelling X-shaped Radio Galaxies: Dynamical and Emission Signatures from the Back-flow model

Some of the radio galaxies show distortion in their jets, forming tailed or winged sources. X-shaped radio galaxies are a subclass of winged sources formation mechanism of which is still unclear. The focus of this work is to understand hydro-dynamical back-flows and their role in dynamics and non-thermal emission signatures (in presence of radiative losses and diffusive shock acceleration) during the initial phase of these galaxies. We have performed relativistic MHD simulations of an under-dense jet travelling in a tri-axial ambient using a hybrid Eulerian-Lagrangian framework to incorporate effects of micro-physical processes. We have demonstrated the dominant role played by pressure gradient in shaping XRGs in thermally dominated cases. We show that the prominence of the formed structure decreases as the jet deviates from the major axis of the ambient. The wing evolution is mainly governed by re-energized particles due to shocks that keep the structure active during the evolution time. The synthetic intensity maps of the radio galaxy show similarities with morphologies that are typically found in observed XRGs. This includes the cases with wider wings than the active lobes. The characteristic emission signatures in terms of its synchrotron spectra and implication of equipartition condition in age estimation are also discussed here. Additionally, we show that discrepancy of age can be attributed to mixing of different aged particle populations. Further, the effect of viewing angle on the difference of spectral index of the active lobes and the wings $(Δα)$ shows a large variation and degenerate behaviour. We have demonstrated the role of diffusive shocks in the obtained variation and have concluded that the spread of $(Δα)$ is not a dependable characteristic in determining the formation model of XRGs.

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arXiv2021

Simulating the dynamics and synchrotron emission from relativistic jets II. Evolution of non-thermal electrons

We have simulated the evolution of non-thermal cosmic ray electrons (CREs) in 3D relativistic magneto hydrodynamic (MHD) jets evolved up to a height of 9 kpc. The CREs have been evolved in space and in energy concurrently with the relativistic jet fluid, duly accounting for radiative losses and acceleration at shocks. We show that jets stable to MHD instabilities show expected trends of regular flow of CREs in the jet spine and acceleration at a hotspot followed by a settling backflow. However, unstable jets create complex shock structures at the jet-head (kink instability), the jet spine-cocoon interface and the cocoon itself (Kelvin-Helmholtz modes). CREs after exiting jet-head undergo further shock crossings in such scenarios and are re-accelerated in the cocoon. CREs with different trajectories in turbulent cocoons have different evolutionary history with different spectral parameters. Thus at the same spatial location, there is mixing of different CRE populations, resulting in a complex total CRE spectrum when averaged over a given area. Cocoons of unstable jets can have an excess build up of energetic electrons due to re-acceleration at turbulence driven shocks and slowed expansion of the decelerated jet. This will add to the non-thermal energy budget of the cocoon.

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arXiv2021

Numerical study of Kelvin-Helmholtz instability and its impact on synthetic emission from magnetized jets

Non-thermal emission from Active Galactic Nuclei (AGN) jets extends up-to large scales in-spite of them being prone to a slew of magneto-hydrodynamic instabilities. The main focus of this study is to understand the impact of MHD instabilities on the non-thermal emission from large-scale AGN jets. We perform high-resolution three-dimensional numerical magneto-hydrodynamic simulations of a plasma column to investigate the dynamical and emission properties of jet configurations at kilo-parsec scales with different magnetic field profiles, jet speeds, and density contrast. We also obtain synthetic non-thermal emission signatures for different viewing angles using an approach that assumes static particle spectra and that obtained by evolving the particle spectra using Lagrangian macro-particles incorporating the effects of shock acceleration and radiative losses. We find that the shocks due to Kelvin-Helmholtz (KH) instability in the axial magnetic field configurations can strongly affect the jet dynamics. Additionally, we also find the presence of weak biconical shocks in the under-dense jet columns. The inclusion of a helical magnetic field hinders the vortex growth at the shear surface thereby stabilizing the jet column. With the evolving particle spectra approach, the synthetic SEDs obtained for cases with strong KH instability show the presence of multiple humps ranging from radio to TeV gamma-ray band. We conclude that the high-energy electrons accelerated in the vicinity of freshly formed shocks due to KH instability, result in high X-ray emission.

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arXiv2020

Simulating the dynamics and non-thermal emission of relativistic magnetised jets I. Dynamics

We have performed magneto-hydrodynamic simulations of relativistic jets from supermassive blackholes over a few tens of kpc for a range of jet parameters. One of the primary aims were to investigate the effect of different MHD instabilities on the jet dynamics and their dependence on the choice of jet parameters. We find that two dominant MHD instabilities affect the dynamics of the jet, small scale Kelvin- Helmholtz (KH) modes and large scale kink modes, whose evolution depend on internal jet parameters like the Lorentz factor, the ratio of the density and pressure to the external medium and the magnetisation and hence consequently on the jet power. Low power jets are susceptible to both instabilities, kink modes for jets with higher central magnetic field and KH modes for lower magnetisation. Moderate power jets do not show appreciable growth of kink modes, but KH modes develop for lower magnetisation. Higher power jets are generally stable to both instabilities. Such instabilities decelerate and decollimate the jet while inducing turbulence in the cocoon, with consequences on the magnetic field structure. We model the dynamics of the jets following a generalised treatment of the Begelman-Cioffi relations which we present here. We find that the dynamics of stable jets match well with simplified analytic models of expansion of non self-similar FRII jets, whereas jets with prominent MHD instabilities show a nearly self-similar evolution of the morphology as the energy is more evenly distributed between the jet head and the cocoon.

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arXiv2019

Linear stability analysis of magnetized relativistic rotating jets

We carry out a linear stability analysis of a magnetized relativistic rotating cylindrical jet flow using the approximation of zero thermal pressure. We identify several modes of instability in the jet: Kelvin-Helmholtz, current driven and two kinds of centrifugal-buoyancy modes -- toroidal and poloidal. The Kelvin-Helmholtz mode is found at low magnetization and its growth rate depends very weakly on the pitch parameter of the background magnetic field and on rotation. The current driven mode is found at high magnetization, the values of its growth rate and the wavenumber, corresponding to the maximum growth, increase as we decrease the pitch parameter of the background magnetic field. This mode is stabilized by rotation, especially, at high magnetization. The centrifugal-buoyancy modes, arising due to rotation, tend also to be more stable when magnetization is increased. Overall, relativistic jet flows appear to be more stable with respect to their non-relativistic counterpart.

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arXiv2018

A Particle Module for the PLUTO code: II - Hybrid Framework for Modeling Non-thermal emission from Relativistic Magnetized flows

We describe a new hybrid framework to model non-thermal spectral signatures from highly energetic particles embedded in a large-scale classical or relativistic MHD flow. Our method makes use of \textit{Lagrangian} particles moving through an Eulerian grid where the (relativistic) MHD equations are solved concurrently. Lagrangian particles follow fluid streamlines and represent ensembles of (real) relativistic particles with a finite energy distribution. The spectral distribution of each particle is updated in time by solving the relativistic cosmic ray transport equation based on local fluid conditions. This enables us to account for a number of physical processes, such as adiabatic expansion, synchrotron and inverse Compton emission. An accurate semi-analytically numerical scheme that combines the method of characteristics with a Lagrangian discretization in the energy coordinate is described. In presence of (relativistic) magnetized shocks, a novel approach to consistently model particle energization due to diffusive shock acceleration has been presented. Our approach relies on a refined shock-detection algorithm and updates the particle energy distribution based on the shock compression ratio, magnetic field orientation and amount of (parameterized) turbulence. The evolved distribution from each \textit{Lagrangian} particle is further used to produce observational signatures like emission maps and polarization signals accounting for proper relativistic corrections. We further demonstrate the validity of this hybrid framework using standard numerical benchmarks and evaluate the applicability of such a tool to study high energy emission from extra-galactic jets.

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arXiv2015

Origin of X-shaped radio-sources: further insights from the properties of their host galaxies

We analyze the properties of a sample of X-shaped radio-sources (XRSs). These objects show, in addition to the main lobes, a pair of wings producing their peculiar radio morphology. We obtain our sample by selecting from the initial list of Cheung (2007, AJ, 133, 2097) the 53 galaxies with the better defined wings and with available SDSS images. We identified the host galaxies and measured their optical position angle, obtaining a positive result in 22 cases. The orientation of the secondary radio structures shows a strong connection with the optical axis, with all (but one) wing forming a angle larger than 40 degrees with the host major axis. The probability that this is compatible with a uniform distribution is P = 0.9 10E-4. Spectra are available from the SDSS for 28 XRSs. We modeled them to extract information on their emission lines and stellar population properties. The sample is formed by approximately the same number of high and low excitation galaxies (HEG and LEG); this classification is essential for a proper comparison with non-winged radio-galaxies. XRSs follow the same relations between radio and line luminosity defined by radio-galaxies in the 3C sample. While in HEGs a young stellar population is often present, this is not detected in the 13 LEGs, again in agreement with the properties of non XRSs. The lack of young stars in LEGs support the idea that they did not experiences a recent gas rich merger. The connection between the optical axis and the wings orientation, as well as the stellar population and emission lines properties, provide further support for an hydro-dynamic origin of the radio-wings (for example associated with the expansion of the radio cocoon in an asymmetric external medium) rather than with a change of orientation of the jet axis.

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arXiv2011

Symmetries, scaling laws and convergence in shearing-box simulations of MRI driven turbulence

We consider the problem of convergence in homogeneous shearing box simula- tions of magneto-rotationally driven turbulence. When there is no mean magnetic flux, if the equations are non dimensionalized with respect to the diffusive scale, the only free parameter in the problem is the size of the computational domain. The problem of convergence then relates to the asymptotic form of the solutions as the computational box size becomes large. By using a numerical code with a high order of accuracy we show that the solutions become asymptotically inde- pendent of domain size. We also show that cases with weak magnetic flux join smoothly to the zero flux cases as the flux vanishes. These results are consistent with the operation of a subcritical small-scale dynamo driving the turbulence. We conclude that for this type of turbulence the angular momentum transport is a proportional to the diffusive flux and therefore has limited relevance in as- trophysical situations.

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