arXiv subjects

Alberto Sciaccaluga

Publications and source records attributed to Alberto Sciaccaluga.

6 recordsLinked to original sources

arXiv2026

Detectability of Polarized Gamma-ray Emission from Blazar Flares with COSI

We investigate the detectability of polarized gamma-ray emission from blazar flares with the Compton Spectrometer and Imager (COSI). Using 17 years of Fermi Large Area Telescope observations, we analyze light curves for 1413 blazars and identify a maximum of 787 sources with flaring episodes through Bayesian block analysis. For each flare, we estimate the minimum detectable polarization MDP99 in the COSI energy band (0.2-5 MeV) using instrument response functions under a range of spectral assumptions and background conditions. Under baseline background levels (1 counts/s), and assuming that blazar flare statistics in the MeV band are comparable to those observed at GeV energies, we find that COSI can realistically detect polarization in up to ~6 flares with MDP99<50% over its two-year prime mission depending on different spectral and flare identification assumptions, with only a few most powerful ones reaching MDP99<20%. These expectations are shown to improve when shorter intervals around bright peaks within long flares are considered. We provide a ranked list of the most promising targets, finding that flat-spectrum radio quasars dominate the population of polarization-detectable events. Through its continuous all-sky monitoring in the largely unexplored MeV band, COSI will open a new observational window on blazar variability and deliver the first direct measurements of MeV polarization, offering unique insights into jet geometry and high-energy emission processes.

astro-ph.HE↗

arXiv2026

The IXPE and multifrequency polarimetric view of the extreme blazars 1ES 1101-232 and RGB J0710+591

Multiwavelength polarimetry is a powerful tool to probe magnetic field and flow geometries in the relativistic jets of blazars. In this respect, particularly interesting are the sources whose synchrotron emission covers a broad range of frequencies, from radio to X-rays, such as the BL Lac objects of the HSP type. Previous measurements including radio, optical and X-ray data show a clear trend, with the degree of polarization increasing with frequency. Here we report radio, optical and X-ray observations ($Swift$, $Nustar$ and $IXPE$) of 1ES 1101-232 and RGB J0710+591, two blazars belonging to the puzzling subclass of extreme BL Lacs (EHBL). For 1ES 1101-232 we found a strong frequency-dependency of the degree of polarization (with a ratio $Π_X/Π_O\simeq 5.2$). For RGB J0710+591, IXPE derived a 1$σ$ upper limit $Π_X<11.6\%$, comparable with the measured optical degree of polarization (average $Π_O\sim 12\%$). We discuss the results in the framework of current interpretations and, in particular, we report an improved version of the stratified shock model that is able to reproduce the observed data of both sources.

astro-ph.HE↗

arXiv2025

Instabilities at recollimation shocks in MHD jets

AGN jet structure and stability remain uncertain; recollimation shocks are linked to morphology and variability, but the role of downstream instabilities is still unclear. We aim to investigate how jet magnetization and other physical parameters influence the development of instabilities beyond the first recollimation shock. In particular, we focus on identifying the conditions under which the centrifugal instability (CFI) is effective. We perform high-resolution 2D and 3D simulations using the relativistic magnetohydrodynamics code PLUTO. The jets are initialized with a conical geometry and propagate into an ambient medium, and we follow by axisymmetric simulations how they evolve towards a steady-state. In 2D we explore a range of magnetizations (from 0 to 1), pressure contrasts, and inertia ratios to characterize the formation and evolution of recollimation shocks. The results are further evaluated using linear stability analysis to assess the growth and suppression of CFI. Finally, we perform 3D simulations of unstable and stable jets. We discuss how the different parameters of the axisymmetric steady solutions influence the location and strength of recollimation. We find that, even in moderately magnetized jets, $σ$=0.1, the CFI can still develop under suitable local conditions and disrupt the jet structure. This instability is governed by the jet radius, curvature, Lorentz factor, and magnetization, and is not always predictable from injection conditions. While magnetization can delay or locally suppress instability growth, it does not guarantee long-term jet stability. Our 3D results highlight the limitations of 2D models in capturing non-axisymmetric and nonlinear effects, and underline the complex interplay between magnetic confinement and destabilizing mechanisms. These findings have implications for interpreting variability, and polarization structure in AGN jets.

astro-ph.HE↗

arXiv2025

The polarization of the synchrotron radiation from a recollimated jet: application to high-energy BL Lacs

Multifrequency polarimetry, recently extended to the X-ray band thanks to the Imaging X-ray Polarimetry Explorer (IXPE) satellite, is an essential tool for understanding blazar jets. High-frequency-peaked BL Lacs (HBLs) and extreme high-frequency-peaked BL Lacs (EHBLs) are especially interesting because the polarimetric properties of their synchrotron emission, extending up to the X-ray band, can be fully tracked by sensitive polarimetric measurements. We investigated the polarization properties of the synchrotron emission of these sources, starting directly from relativistic magnetohydrodynamic simulations of recollimated relativistic jets. To bridge the gap between fluid and kinetic scales, we elaborated a post-processing code based on the Lagrangian macroparticle approach, which models the spectral evolution and emission of nonthermal particles within the jet given the local fluid conditions. When comparing our results with early particle-in-cell (PIC) simulations, we find that shocks formed through jet recollimation are primarily superluminal, limiting particle acceleration in a laminar flow. However, recent PIC simulations suggest that acceleration can occur in the presence of small-scale turbulence or inhomogeneities even in a superluminal configuration. In this case, we reproduce the observed polarization chromaticity (i.e., the polarization degree increases with frequency), along with a stable polarization angle between the X-ray and optical bands. This study sheds light on the role of recollimation shocks in blazar jets and supports the energy-stratified shock model as a plausible explanation for IXPE observations.

astro-ph.HE↗

arXiv2024

Stochastic acceleration in Extreme TeV BL Lacs through MCMC

Extreme TeV BL Lacs are a class of blazars with unique spectral and temporal features, not easily reproducible using standard one-zone models based on single shock acceleration. To account for their peculiar properties we elaborated a two-steps acceleration model where a recollimation shock and the subsequent downstream turbulence energize non-thermal electrons. We apply the model to a sample of Extreme TeV BL Lacs with well characterized spectral energy distributions. Since we are considering several sources, we automatized the exploration of the parameter space. This also allows us to derive the parameter distributions and to study the correlations among them. We numerically solved a system of two coupled non-linear differential equations to obtain the non-thermal particles and turbulence spectra. We calculated the spectral energy distribution via the Synchrotron Self-Compton emission model. The automatization of the parameter space exploration is possible through a Markov Chain Monte Carlo (MCMC) ensemble sampler, in our case emcee. We derived well defined posterior distributions for the parameters, showing that the model is well constrained by available data and supporting the suitability of our method. The cross-correlations among some of the physical parameters are not trivial, therefore we conclude that MCMC sampling is a key instrument to characterize the complexity of our multi-parametric phenomenological model.

astro-ph.HE↗

arXiv2022

Extreme TeV BL Lacs: a self-consistent stochastic acceleration model

Lately a specific kind of blazars drew the attention of the gamma-ray astronomy community: the extreme TeV BL Lacs, blazars that present an extremely energetic and hard emission at very high-energy. Explaining their features is still an open challenge, in fact the most used phenomenological models have difficulties to satisfactorily reproduce their SED. Based on a scenario we have recently proposed, we suppose that the non-thermal particles are firstly accelerated by a jet recollimation shock, which induces turbulence in the rest of the jet. Non-thermal particles are further accelerated by the turbulence, which hardens the particle spectra and accordingly the radiative emission. Given the physical properties of the plasma, as inferred by emission models, we expect a strong impact of the accelerating particles on the turbulence. Assuming isotropy and homogeneity, the interaction between non-thermal particles and turbulence and their spectra is modeled solving a system of two non-linear, coupled Fokker-Planck equations, while the radiative emission is calculated through the Synchrotron Self Compton model. The emission predicted by our model is then compared with the prototype extreme TeV BL Lac object 1ES 0229+200 and the parameters obtained to reproduce its SED are in line with the expectations.

astro-ph.HE↗