arXiv subjects

Andrea Tramacere

Publications and source records attributed to Andrea Tramacere.

At least 19 recordsLinked to original sources

arXiv2026

A long-term multiwavelength study of the flat spectrum radio quasar OP 313

The Flat Spectrum Radio Quasar OP 313 is a high-redshift (z = 0.997) blazar that entered an intense gamma-ray active phase from November 2023 to March 2024, as observed by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. We present a multiwavelength analysis covering 15 years of data, from August 2008 to March 2024, to contextualize this period of extreme gamma-ray activity within the long-term emission of the source. We analyzed a long-term, comprehensive, multiwavelength dataset from different facilities and projects from radio to gamma-rays. We identified the 7 most intense gamma-ray flaring periods and performed a kinematic analysis of Very Long Baseline Array (VLBA) data to determine whether new jet components emerged before or during these flares. For 2 of these flaring periods, we performed the modeling of the spectral energy distribution (SED). The VLBA-BU-BLAZAR and MOJAVE datasets reveal a new jet component appearing in both visibility datasets prior to the onset of one of the strongest gamma-ray flares. By comparing the timing of the VLBA-BU-BLAZAR knots ejection with the gamma-ray flaring periods, we constrained the setup of the SED modeling. We also found that the first gamma-ray flaring period is less Compton-dominated than the others. Our results suggest that the recent activity of OP 313 is triggered by new jet components emerging from the core and interacting with a standing shock. The γ-ray emission likely arises from dusty torus photons upscattered via Inverse Compton (IC) by relativistic jet electrons. The SED modeling indicates that this component is less dominant during the first γ-ray flaring period than the later ones.

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arXiv2025

On the statistical characterization of the synchrotron multi-zone polarization of blazars

Multiwavelength polarimetric observations of blazars reveal complex, energy-dependent polarization behavior, including a decrease in polarization fraction from X-rays to millimeter bands and significant variability in the electric vector position angle (EVPA). These trends challenge simple single-zone synchrotron models and suggest a more intricate, turbulent jet structure with multiple emission zones. We develop a statistical framework to model the observed energy-dependent polarization patterns in blazars, focusing on the behavior captured by IXPE in the X-ray band and RoboPol in the optical. The goal is to statistically characterize multi-zone models in terms of the distributions of cell size and the physical parameters of the electron energy distribution (EED). A Monte Carlo approach, implemented with the JetSeT code, is used to generate synthetic multi-zone synchrotron emission from a spherical region filled with turbulent cells with randomly distributed physical properties. Simulations explore scenarios ranging from identical cells to power-law distributions of cell sizes and EED parameters with variable cutoff and low-energy slopes. The results show that a purely turbulent, multi-zone model can reproduce the observed energy-dependent polarization without requiring correlations between cell size and EED parameters. The polarization degree is primarily determined by the effective, flux-weighted, number of emitting cells, modulated by the dispersion in cell properties, particularly the EED cutoff energy at high frequencies and the low-energy spectral index at low frequencies. With a fractional dispersion in cutoff energy of about 90% and a low-energy spectral index dispersion of ~0.5-1.5, the model reproduces the chromatic mm-to-X-ray polarization trends seen by IXPE and the optical polarization limiting envelope observed in the RoboPol dataset.

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arXiv2025

The High-redshift Blazar MG3 J163554+3629: Physical Properties and the Enigma of Its Unexpected Supermassive Black Hole Growth

There is general consensus that active galactic nuclei (AGNs) derive their radiating power from a supermassive black hole (SMBH) that accretes matter. Yet, their precise powering mechanisms and the resulting growth of the SMBH are poorly understood, especially for AGNs at high redshift. Blazars are AGNs pointing their jet toward the observer, thus being detectable from radio through gamma rays at high redshift due to Doppler boosting. The blazar MG3 J163554+3629 is located at redshift z=3.65 and it is a flat spectrum radio quasar (FSRQ). In this work, we show the results of the modeling of its spectral energy distribution (SED) from radio to gamma rays with a one-zone leptonic model. We estimate the uncertainties through a Markov Chain Monte Carlo approach. As a result, we infer the black hole mass M_BH = 1.1(+0.2,-0.1) x 10^9 Msun and a modest magnetic field of B = 6.56(+0.13,-0.09) x 10^-2 G in line with the Compton dominance observed in high-redshift FSRQs. The emitting region is outside the broad line region but within the region of the dust torus radius. The rather small accretion efficiency of eta=0.083 is not solely inferred through the SED modeling but also through the energetics. An evolution study suggests that in an Eddington-limited accretion process the SMBH did not have time enough to grow from an initial seed mass of ~10^6 Msun at z~30 into a mass of M_BH ~ 10^9 Msun at z=3.65. Faster mass growth might be obtained in a super-Eddington process throughout frequent episodes. Alternative scenarios propose that the existence of the jet itself can facilitate a more rapid growth.

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arXiv2025

Search for $γ$-ray emission from SNRs in the Large Magellanic Cloud: a new cluster analysis at energies above 4 GeV

A search for $γ$-ray emission from SNRs in the Large Magellanic Cloud (LMC) based on the detection of concentrations in the arrival direction Fermi-LAT images of photons at energies higher than 10 GeV found significant evidence for 9 of these sources. This analysis was based on data collected in the time window since August 4 2008 to August, 4 2020 (12 years). In the present contribution, we report results of a new search extended using a 15-year long (up to August, 4 2023) data set and to a broad energy range (higher than 4 GeV). The longer baseline and the softer energy lower limit are required to further understand the relation between the X-ray and gamma ray SNRs in the LMC, and to investigate the completness of the sample at low luminosities. Two different methods of clustering analysis were applied: Minimum Spanning Tree (MST), and the combination of Density-Based Spatial Clustering of Applications with Noise (DBSCAN) and DENsity-based CLUstEring (DENCLUE) algorithms. We confirm all previous detections and found positive indications for at least 8 new clusters with a spatial correspondence with other SNRs, increasing thus the number of remnants in LMC candidate or detected in the high energy $γ$-rays to 16 sources. This study extends previous analyses of $γ$-ray emission from SNRs in the LMC by incorporating a longer observational baseline and a broader energy range. The improved dataset and advanced clustering techniques enhance our understanding of the connection between X-ray and $γ$-ray SNRs, providing new insights into their high-energy properties, and contributing to assessing the completeness of the sample at lower luminosities.

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arXiv2024

Exploring the nature of the jetted hybrid AGNs: PKS 2004-447, 3C 286 and PKS 0440-00 through the SED modeling

In this work, we explore the connection of three jetted $γ-$loud AGNs classes: Compact Steep-Spectrum Sources (CSS), Narrow-Line Seyfert 1 (NLS1), and Flat-Spectrum Radio Quasars, through the modeling of the spectral energy distribution (SED). We selected two sources identified as CSS/NLS1 hybrids, PKS 2004-440 and 3C 286. Additionally, we included the source PKS 0440-00, initially classified as an FSRQ in the first Fermi-LAT catalog, but recently reclassified as an NLS1. We present the results of their broadband SED modeling using a one-zone leptonic synchrotron-self Compton (SSC)+external Compton (EC) model. By exploring the parameter space and investigating the disk-jet connection in these sources, we analyze their classification in a model-dependent way. Our findings reveal that modeling PKS 2004-447 at relatively large angles, as expected for CSS, results in an SSC-dominated inverse Compton emission. In contrast, at low observing angles, the inverse Compton emission is dominated by external photon fields. Both scenarios result in a jet with a low radiative power. For 3C 286 we found that using a one-zone model limits the jet viewing angle to $\sim7^{\circ}$, mainly due to its impact on the $γ$-ray emission. Our model results show a magnetically dominated jet, consistent with $γ$-CSS sources. Our results suggest that PKS 0440-00, can be classified as a powerful $γ-$NLS1, characterized by high accretion power and a jet dominated by bulk motion, similar to FSRQs.

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arXiv2024

The flaring activity of blazar AO 0235+164 during year 2021

Context. The blazar AO 0235+164, located at redshift $z=0.94$, has displayed interesting and repeating flaring activity in the past, the latest episodes occurring in 2008 and 2015. In 2020, the source brightened again, starting a new flaring episode that peaked in 2021. Aims. We study the origin and properties of the 2021 flare in relation to previous studies and the historical behavior of the source, in particular to the 2008 and 2015 flaring episodes. Methods. We analyze the multi-wavelength photo-polarimetric evolution of the source. From Very Long Baseline Array images, we derive the kinematic parameters of new components associated with the 2021 flare. We use this information to constrain a model for the spectral energy distribution of the emission during the flaring period. We propose an analytical geometric model to test whether the observed wobbling of the jet is consistent with precession. Results. We report the appearance of two new components that are ejected in a different direction than previously, confirming the wobbling of the jet. We find that the direction of ejection is consistent with that of a precessing jet.The derived period independently agrees with the values commonly found in the literature. Modeling of the spectral energy distribution further confirm that the differences between flares can be attributed to geometrical effects.

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arXiv2023

The Repeating Flaring Activity of Blazar AO 0235+164

Context. Blazar AO 0235+164, located at redshift z = 0.94, has undergone several sharp multi-spectral-range flaring episodes during the last decades. In particular, the episodes peaking in 2008 and 2015, that received extensive multi-wavelength coverage, exhibited interesting behavior. Aims. We study the actual origin of these two observed flares by constraining the properties of the observed photo-polarimetric variability, those of the broad-band spectral energy-distribution and the observed time-evolution behavior of the source as seen by ultra-high resolution total-flux and polarimetric Very-long-baseline interferometry (VLBI) imaging. Methods. The analysis of VLBI images allows us to constrain kinematic and geometrical parameters of the 7 mm jet. We use the Discrete Correlation Function to compute the statistical correlation and the delays between emission at different spectral ranges. Multi-epoch modeling of the spectral energy distributions allows us to propose specific models of emission; in particular for the unusual spectral features observed in this source in the X-ray region of the spectrum during strong multi spectral-range flares. Results. We find that these X-ray spectral features can be explained by an emission component originating in a separate particle distribution than the one responsible for the two standard blazar bumps. This is in agreement with the results of our correlation analysis that do not find a strong correlation between the X-rays and the remaining spectral ranges. We find that both external Compton dominated and synchrotron self-Compton dominated models can explain the observed spectral energy distributions. However, synchrotron self-Compton models are strongly favored by the delays and geometrical parameters inferred from the observations.

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arXiv2023

Hadronic processes at work in 5BZB J0630-2406

Recent observations are shedding light on the important role that active galactic nuclei (AGN) play in the production of high-energy neutrinos. In this study, we focus on one object, 5BZB J0630-2406, which is among the blazars recently proposed as associated with neutrino emission during the first 7-yr IceCube observations. Modelling the quasi-simultaneous, broad-band spectral energy distribution, we explore various scenarios from purely leptonic to lepto-hadronic models, testing the inclusion of external photon fields. This theoretical study provides a complementary testing ground for the proposed neutrino-blazar association. Despite being historically classified as a BL Lac, our study shows that 5BZB J0630-2406 belongs to the relatively rare sub-class of high-power flat-spectrum radio quasars (FSRQs). Our results indicate that interactions between protons and external radiation fields can produce a neutrino flux that is within the reach of the IceCube detector. Furthermore, the spectral shape of the X-ray emission suggests the imprint of hadronic processes related to very energetic protons.

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arXiv2023

Extragalactic neutrino factories

Identifying the astrophysical sources responsible for the high-energy cosmic neutrinos has been a longstanding challenge. In a previous work, we report evidence for a spatial correlation between blazars from the 5th Roma-BZCat catalog and neutrino data of the highest detectable energies, i.e. >0.1 PeV, collected by the IceCube Observatory in the southern celestial hemisphere. The statistical significance is found at the level of 2 x 10^{-6} post-trial. In this work we test whether a similar correlation exists in the northern hemisphere, were IceCube is mostly sensitive to <0.1 PeV energies. We find a consistent correlation between blazars and northern neutrino data at the pre-trial p-value of 5.12 x 10^{-4}, and a post-trial chance probability of 6.79 x 10^{-3}. Combining the post-trial probabilities observed for the southern and northern experiments yields a global post-trial chance probability of 2.59 x 10^{-7} for the genuineness of such correlation. This implies that the spatial correlation is highly unlikely to arise by chance. Our studies push forward an all-sky subset of 52 objects as highly likely PeVatron extragalactic accelerators.

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arXiv2022

Observation of the galactic PeVatron candidate LHAASO J2108+5157 with the Large-Sized Telescope for Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) Observatory will be the next-generation ground-based very-high-energy gamma-ray observatory, sensitive from 20 GeV up to 300 TeV. The Large-Sized Telescope prototype (LST-1), currently in the commissioning phase, was inaugurated in October 2018 on La Palma (Spain). It is the first of four LST telescopes for CTA, to be built in La Palma. In 2021, LST-1 performed observations of one of the Galactic PeVatron candidates, LHAASO J2108+5157, recently discovered by the LHAASO collaboration. We present results of our analysis of the LST-1 data, putting strong constraints on the emission of the source in the multi-TeV band. We also present results of multi-wavelength modeling using 12-years Fermi-LAT data and Target of Opportunity observations with XMM-Newton. We test different scenarios for the parent particles producing the high energy emission and put constraints on their spectra.

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arXiv2022

Open-source Radiative Modelling Tools for Extragalactic VHE gamma-ray Sources

In this review, we discuss various open-source software for modelling the broad-band emission of extragalactic sources from radio up to the highest gamma ray energies. As we provide an overview of the different tools available, we discuss the physical processes such tools implement and detail the computations they can perform. We also examine their conformity with modern good software practices. After considering the currently available software as a first generation of open-source modelling tools, we outline some desirable characteristics for the next generation.

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arXiv2022

Beginning a journey across the universe: the discovery of extragalactic neutrino factories

Neutrinos are the most elusive particles in the Universe, capable of traveling nearly unimpeded across it. Despite the vast amount of data collected, a long standing and unsolved issue is still the association of high-energy neutrinos with the astrophysical sources that originate them. Amongst the candidate sources of neutrinos there are blazars, a class of extragalactic sources powered by supermassive black holes that feed highly relativistic jets, pointed towards the Earth. Previous studies appear controversial, with several efforts claiming a tentative link between high-energy neutrino events and individual blazars, and others putting into question such relation. In this work we show that blazars are unambiguously associated with high-energy astrophysical neutrinos at unprecedented level of confidence, i.e. chance probability of 6 x 10^{-7}. Our statistical analysis provides the observational evidence that blazars are astrophysical neutrino factories and hence, extragalactic cosmic-ray accelerators.

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arXiv2021

Radio$-γ-$ray response in blazars as a signature of adiabatic blob expansion

Multiwavelength light curves in long-term campaigns have shown that, for several blazars, the radio emission occurs with a significant delay w.r.t. to $γ$-ray band, with timescales ranging from weeks to years. Such observational evidence has been a matter of debate for years and usually is interpreted as a signature of the $γ$-ray emission originating upstream in the jet, with the emitting region becoming radio transparent at larger scales. In this paper, we show, by means of self-consistent numerical modelling, that the adiabatic expansion of relativistic blob can explain these delays. We use the JetSeT framework to reproduce the numerical modelling of the radiative/accelerative processes, reproducing the temporal evolution, from the initial flaring activity, and the subsequent expansion. We follow the spectral evolution and the light curves, investigating the relations among the observed parameters, rise, time, and decay time, identifying the link with the physical parameters. We find that, when adiabatic expansion is active, lags due to the shift of the synchrotron frequency occur, with an offset equal to the distance in time between the flaring onset and the beginning of the expansion, whilst the rising and decaying timescales depend on the velocity of the expansion and on time required to the source to exhibit a synchrotron self-absorption frequency. We derive an inter-band response function, and we investigate the effects of the competitions between radiative and adiabatic cooling timescales on the response. We apply the response function to long-term radio and $γ-$ray light curves of Mrk 421, Mrk 501 and 3C 273, finding a satisfactory agreement on the log-term behaviour, and we use a Monte Carlo Markov Chain approach, to estimate some physical relevant parameters. We discuss the applications to polarization measurements, and to jets collimation profile kinematics.

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arXiv2021

A ring accelerator? Unusual jet dynamics in the IceCube candidate PKS 1502+106

On 2019/07/30.86853 UT, IceCube detected a high-energy astrophysical neutrino candidate. The Flat Spectrum Radio Quasar PKS 1502+106 is located within the 50 percent uncertainty region of the event. Our analysis of 15 GHz Very Long Baseline Array (VLBA) and astrometric 8 GHz VLBA data, in a time span prior and after the IceCube event, reveals evidence for a radio ring structure which develops with time. Several arc-structures evolve perpendicular to the jet ridge line. We find evidence for precession of a curved jet based on kinematic modelling and a periodicity analysis. An outflowing broad line region (BLR) based on the C IV line emission (Sloan Digital Sky Survey, SDSS) is found. We attribute the atypical ring to an interaction of the precessing jet with the outflowing material. We discuss our findings in the context of a spine-sheath scenario where the ring reveals the sheath and its interaction with the surroundings (narrow line region, NLR, clouds). We find that the radio emission is correlated with the $γ$-ray emission, with radio lagging the $γ$-rays. Based on the $γ$-ray variability timescale, we constrain the $γ$-ray emission zone to the BLR (30-200 $r_{\rm g}$) and within the jet launching region. We discuss that the outflowing BLR provides the external radiation field for $γ$-ray production via external Compton scattering. The neutrino is most likely produced by proton-proton interaction in the blazar zone (beyond the BLR), enabled by episodic encounters of the jet with dense clouds, i.e. some molecular cloud in the NLR.

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arXiv2019

Toward a Public MAGIC Gamma-Ray Telescope Legacy Data Portal

The MAGIC telescopes are one of the three major IACTs (Imaging Atmospheric Cherenkov Telescopes) for observation of gamma rays in the TeV regime currently operative. MAGIC functions since 2003, and has published data from more than 60 sources, mostly blazars. MAGIC already provides astronomical \texttt{.fits} files with basic final scientific products such as spectral energy distributions, light curves and skymaps from published results. In future, the format of the files can be complemented with further relevant information to the community: a) by including the full multi-wavelength dataset enclosed in a publication, b) providing data in alternative easy-to-use formats such as ASCII or ECSV, which are accessible with other commonly used packages such as \texttt{astropy} or \texttt{gammapy}. Finally, besides high level products, activities have started to provide photon event lists and instrument response functions in a format such that scientists within and outside the community are allowed to perform higher level analysis. A second aim is to provide a full legacy of MAGIC data. This contribution will illustrate the achievements and plans of this activity.

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arXiv2019

Photometry of high-redshift blended galaxies using deep learning

The new generation of deep photometric surveys requires unprecedentedly precise shape and photometry measurements of billions of galaxies to achieve their main science goals. At such depths, one major limiting factor is the blending of galaxies due to line-of-sight projection, with an expected fraction of blended galaxies of up to 50%. Current deblending approaches are in most cases either too slow or not accurate enough to reach the level of requirements. This work explores the use of deep neural networks to estimate the photometry of blended pairs of galaxies in monochrome space images, similar to the ones that will be delivered by the Euclid space telescope. Using a clean sample of isolated galaxies from the CANDELS survey, we artificially blend them and train two different network models to recover the photometry of the two galaxies. We show that our approach can recover the original photometry of the galaxies before being blended with $\sim$7% accuracy without any human intervention and without any assumption on the galaxy shape. This represents an improvement of at least a factor of 4 compared to the classical SExtractor approach. We also show that forcing the network to simultaneously estimate a binary segmentation map results in a slightly improved photometry. All data products and codes will be made public to ease the comparison with other approaches on a common data set.

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arXiv2018

The Powerful Jet and Gamma-Ray Flare of the Quasar PKS 0438$-$436

PKS 0438$-$436 at a redshift of $z=2.856$ has been previously recognized as possessing perhaps the most luminous known synchrotron jet. Little is known about this source since the maximum elevation above the horizon is low for the Very Large Array (VLA). We present the first VLA radio image that detects the radio lobes. We use both the 151 MHz luminosity, as a surrogate for the isotropic radio lobe luminosity, and the lobe flux density from the radio image to estimate a long term, time averaged, jet power, $\overline{Q} =1.5\pm 0.7 \times 10^{47} \rm{ergs~s^{-1}}$. We analyze two deep optical spectra with strong broad emission lines and estimate the thermal bolometric luminosity of the accretion flow, $L_{\rm{bol}} = 6.7 \pm 3.0 \times 10^{46} \rm{ergs~s^{-1}}$. The ratio, $\overline{Q}/L_{\rm{bol}} = 3.3 \pm 2.6 $, is at the limit of this empirical metric of jet dominance seen in radio loud quasars and this is the most luminous accretion flow to have this limiting behavior. Despite being a very luminous blazar, it previously had no $γ$-ray detections (EGRET, AGILE or FERMI) until December 11 - 13 2016 (54 hours) when FERMI detected a flare that we analyze here. The isotropic apparent luminosity from 100 MeV - 100 GeV rivals the most luminous detected blazar flares (averaged over 18 hours), $\sim 5-6 \times 10^{49} \rm{ergs~s^{-1}}$. The $γ$-ray luminosity varies over time by two orders of magnitude, highlighting the extreme role of Doppler abberation and geometric alignment in producing the inverse Compton emission.

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arXiv2017

The Euclid Data Processing Challenges

Euclid is a Europe-led cosmology space mission dedicated to a visible and near infrared survey of the entire extra-galactic sky. Its purpose is to deepen our knowledge of the dark content of our Universe. After an overview of the Euclid mission and science, this contribution describes how the community is getting organized to face the data analysis challenges, both in software development and in operational data processing matters. It ends with a more specific account of some of the main contributions of the Swiss Science Data Center (SDC-CH).

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