arXiv subjects

Carolina Casadio

Publications and source records attributed to Carolina Casadio.

At least 19 recordsLinked to original sources

arXiv2026

Spectroscopic redshifts of selected flat-spectrum radio sources I

We present the first results of a spectroscopic campaign carried out as part of the Search for Milli-Lenses (SMILE) program, which aims to constrain the prevalence of gravitational lens systems on milli-arcsecond angular scales (milli-lenses) using high-resolution Very Long Baseline Interferometry (VLBI) imaging. The SMILE parent sample contains ~ 5000 radio-loud active galaxies, selected as a flux-limited, complete subsample of CLASS (The Cosmic Lens All-Sky Survey) sources. We compiled redshift information for the full sample from multiple literature and catalog sources and found that 491 sources have no available redshift estimate, either spectroscopic or photometric. A further 948 sources have only photometric redshifts, many of which show substantial discrepancies between catalogs. Reliable redshifts are essential for VLBI radio-source studies because they convert angular measurements into physical linear scales, enable estimates of intrinsic luminosities and jet kinematics, and allow robust cosmological and population studies. To address this key limitation for lensing and population studies, we initiated a dedicated spectroscopic campaign to secure reliable redshifts for as many targets as possible. This paper focuses on the brightest sources in the SMILE sample. We report newly determined spectroscopic redshifts for 6 targets out of 15 observed with the Skinakas 1.3 m telescope.

astro-ph.CO↗

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↗

arXiv2026

Where within the 3C 84 jet are $γ$-rays produced?

The location of $γ$-ray creation and emission within extra-galactic jets is a matter of active debate. One particularly well-suited source to pinpoint the location is the nearby, bright radio galaxy 3C 84, harbouring a powerful jet. Here we investigate the origin of $γ$-rays measured during a recent $γ$-ray flare, by analysing the linear polarisation signal of close-in-time very long baseline interferometry (VLBI) observations at centimetre and millimetre wavelengths. While 3C 84 is overall almost unpolarised, we find that close-in-time to the $γ$-ray flare peak regions at parsec-scale distances from the central engine shows a fractional linear polarisation increase. Under the physically well-motivated assumption of a causal relation between this polarisation enhancement and the $γ$-ray flare, and combined with insights from concurrent X-ray polarisation measurements, the $γ$-rays being created in this region is a physically motivated scenario, in a process consistent with synchrotron self-Compton.

astro-ph.HE↗

arXiv2025

VIPCALs: A fully automated calibration pipeline for very long baseline interferometry data

Very long baseline interferometry (VLBI) is a powerful technique that can achieve sub-milliarcsecond resolution. However, it requires complex and often manual post-correlation calibration to correct for instrumental, geometric, and propagation-related errors. Unlike connected-element interferometers, VLBI arrays typically provide raw visibilities rather than science-ready data, and existing pipelines are largely semi-automated and reliant on user supervision. We present VIPCALs, a fully automated, end-to-end calibration pipeline for continuum VLBI data that operates without human intervention or prior knowledge of the dataset. Designed for scalability to thousands of sources and heterogeneous archival observations, VIPCALs addresses the needs of initiatives such as the Search for Milli-Lenses (SMILE) project. Implemented in Python using ParselTongue, VIPCALs reproduces the standard AIPS calibration workflow in a fully unsupervised mode. Besides the usual calibration tasks, the pipeline also performs automatic reference antenna selection, calibrator identification, and generates diagnostic outputs for inspection. We validated it on a representative sample of Very Long Baseline Array (VLBA) data corresponding to 1000 sources from the SMILE project. VIPCALs successfully calibrated observations of 955 of the test sources across multiple frequency bands. Over 91% of the calibrated datasets achieved successful fringe fitting on target in at least half of the solutions attempted. The median ratio of calibrated visibilities to initial total visibilities was 0.87. The average processing time was below 10 minutes per dataset, demonstrating both efficiency and scalability. VIPCALs enables robust, reproducible, and fully automated calibration of VLBI continuum data, significantly lowering the entry barrier for VLBI science and making large-scale projects like SMILE feasible.

astro-ph.IM↗

arXiv2025

Detection of Compton scattering in the jet of 3C 84

3C 84 is the brightest cluster galaxy in the Perseus Cluster. It is among the closest radio-loud active galaxies and among the very few that can be detected from low frequency radio up to TeV $γ$-rays. Here we report on the first X-ray polarization observation of 3C~84 with the Imaging X-ray Polarimetry Explorer, for a total of 2.2 Msec that coincides with a flare in $γ$-rays. This is the longest observation for a radio-loud active galaxy that allowed us to reach unprecedented sensitivity, leading to the detection of an X-ray polarization degree of $\rmΠ_X=4.2\pm1.3\%$ ($\sim3.2σ$ confidence) at an X-ray electric vector polarization angle of $\rm ψ_X=163^{\circ}\pm9^{\circ}$, that is aligned with the radio jet direction on the sky. Optical polarization observations show fast variability about the jet axis as well. Our results strongly favor models in which X-rays are produced by Compton scattering from relativistic electrons -- specifically Synchrotron Self-Compton -- that takes places downstream, away from the supermassive black hole.

astro-ph.HE↗

arXiv2025

Monitoring of 3C 286 with ALMA, IRAM, and SMA from 2006 to 2025: Stability, Synchrotron Ages, and Frequency-Dependent Polarization Attributed to Core-Shift

We present the results of multi-frequency monitoring of the radio quasar 3C 286, conducted using three instruments: ALMA at 91.5, 103.5, 233.0, and 343.4 GHz, the IRAM 30-m Telescope at 86 and 229 GHz, and SMA at 225 GHz. The IRAM measurements from 2006 to 2024 show that the total flux of 3C 286 is stable within measurement uncertainties, indicating long-term stability up to 229 GHz, when applying a fixed Kelvin-to-Jansky conversion factor throughout its dataset. ALMA data from 2018 to 2024 exhibit a decrease in flux, which up to 4% could be attributed to an apparent increase in the absolute brightness of Uranus, the primary flux calibrator for ALMA with the ESA4 model. Taken together, these results suggest that the intrinsic total flux of 3C 286 has remained stable up to 229 GHz over the monitoring period. The polarization properties of 3C 286 are stable across all observing frequencies. The electric vector position angle (EVPA) gradually rotates as a function of wavelength squared, which is well described by a single power-law over the full frequency range. We therefore propose using the theoretical EVPA values from this model curve for absolute EVPA calibration between 5 and 343.4 GHz. The Faraday rotation measure increases as a function of frequency up to (3.2+/-1.5)x10^4 rad m^-2, following RM proportional to nu^alpha with alpha = 2.05+/-0.06. This trend is consistent with the core-shift effect expected in a conical jet.

astro-ph.GA↗

arXiv2025

Unveiling blazar synchrotron emission: a multiwavelength polarimetric study of HSP and LSP populations

Polarimetric properties of blazars allow us to put constraints on the acceleration mechanisms that fuel their powerful jets. By studying the multiwavelength polarimetric behaviour of high synchrotron peaked (HSP) and low synchrotron peaked (LSP) blazars, we aim to explore differences in their emission mechanisms and magnetic field structure in the acceleration region. In this study, we take advantage of several X-ray polarisation observations of HSP by the IXPE, including four new observations of Mrk 501, and optical polarisation observations of LSP from RoboPol and many others. We find that the polarisation degree (PD) distribution of HSP in X-rays is systematically higher than in optical and mm-radio wavelengths, as reported in previous IXPE publications. The distribution of the X-ray electric vector position angles (PA) is centered around the jet axis with most of the observations consistent with zero difference within uncertainties. In fact, the distribution of the offset of the PA from the jet axis is consistent between the LSP and HSP populations (with PA measured in optical for the first, X-ray for the latter), suggesting a common magnetic field structure close to the acceleration region, in strong support of the emerging energy stratified picture of particle acceleration followed by energy loss in blazar jets.

astro-ph.HE↗

arXiv2025

High optical to X-ray polarization ratio reveals Compton scattering in BL Lacertae's jet

Blazars, supermassive black hole systems (SMBHs) with highly relativistic jets aligned with the line of sight, are the most powerful long-lived emitters of electromagnetic emission in the Universe. We report here on a radio to gamma-ray multiwavelength campaign on the blazar BL Lacertae with unprecedented polarimetric coverage from radio to X-ray wavelengths. The observations caught an extraordinary event on 2023 November 10-18, when the degree of linear polarization of optical synchrotron radiation reached a record value of 47.5%. In stark contrast, the Imaging X-ray Polarimetry Explorer (IXPE) found that the X-ray (Compton scattering or hadron-induced) emission was polarized at less than 7.4% (3sigma confidence level). We argue here that this observational result rules out a hadronic origin of the high energy emission, and strongly favors a leptonic (Compton scattering) origin, thereby breaking the degeneracy between hadronic and leptonic emission models for BL Lacertae and demonstrating the power of multiwavelength polarimetry to address this question. Furthermore, the multiwavelength flux and polarization variability, featuring an extremely prominent rise and decay of the optical polarization degree, is interpreted for the first time by the relaxation of a magnetic "spring" embedded in the newly injected plasma. This suggests that the plasma jet can maintain a predominant toroidal magnetic field component parsecs away from the central engine.

astro-ph.HE↗

arXiv2025

Gamma-ray flares from the jet of the blazar CTA 102 in 2016-2018

CTA 102 is a $γ$-ray bright blazar that exhibited multiple flares in observations by the Large Area Telescope on board the Fermi Gamma-Ray Space Telescope during the period of 2016-2018. We present results from the analysis of multi-wavelength light curves aiming at revealing the nature of $γ$-ray flares from the relativistic jet in the blazar. We analyse radio, optical, X-ray, and $γ$-ray data obtained in a period from 2012 September 29 to 2018 October 8. We identify six flares in the $γ$-ray light curve, showing a harder-when-brighter-trend in the $γ$-ray spectra. We perform a cross-correlation analysis of the multi-wavelength light curves. We find nearly zero time lags between the $γ$-ray and optical and X-ray light curves, implying a common spatial origin for the emission in these bands. We find significant correlations between the $γ$-ray and radio light curves as well as negative/positive time lags with the $γ$-ray emission lagging/leading the radio during different flaring periods. The time lags between $γ$-ray and radio emission propose the presence of multiple $γ$-ray emission sites in the source. As seen in 43 GHz images from the Very Long Baseline Array, two moving disturbances (or shocks) were newly ejected from the radio core. The $γ$-ray flares from 2016 to 2017 are temporally coincident with the interaction between a traveling shock and a quasi-stationary one at $\sim$0.1 mas from the core. The other shock is found to emerge from the core nearly simultaneous with the $γ$-ray flare in 2018. Our results suggest that the $γ$-ray flares originated from shock-shock interactions.

astro-ph.HE↗

arXiv2024

X-ray and multiwavelength polarization of Mrk 501 from 2022 to 2023

We present multiwavelength polarization measurements of the luminous blazar Mrk~501 over a 14-month period. The 2--8 keV X-ray polarization was measured with the Imaging X-ray Polarimetry Explorer (IXPE) with six 100-ks observations spanning from 2022 March to 2023 April. Each IXPE observation was accompanied by simultaneous X-ray data from NuSTAR, Swift/XRT, and/or XMM-Newton. Complementary optical-infrared polarization measurements were also available in the B, V, R, I, and J bands, as were radio polarization measurements from 4.85 GHz to 225.5 GHz. Among the first five IXPE observations, we did not find significant variability in the X-ray polarization degree and angle with IXPE. However, the most recent sixth observation found an elevated polarization degree at $>3σ$ above the average of the other five observations. The optical and radio measurements show no apparent correlations with the X-ray polarization properties. Throughout the six IXPE observations, the X-ray polarization degree remained higher than, or similar to, the R-band optical polarization degree, which remained higher than the radio value. This is consistent with the energy-stratified shock scenario proposed to explain the first two IXPE observations, in which the polarized X-ray, optical, and radio emission arises from different regions.

astro-ph.HE↗

arXiv2024

Testing particle acceleration in blazar jets with continuous high-cadence optical polarization observations

Variability can be the pathway to understanding the physical processes in astrophysical jets, however, the high-cadence observations required to test particle acceleration models are still missing. Here we report on the first attempt to produce continuous, >24 hour polarization light curves of blazars using telescopes distributed across the globe and the rotation of the Earth to avoid the rising Sun. Our campaign involved 16 telescopes in Asia, Europe, and North America. We observed BL Lacertae and CGRaBS J0211+1051 for a combined 685 telescope hours. We find large variations in the polarization degree and angle for both sources in sub-hour timescales as well as a ~180 degree rotation of the polarization angle in CGRaBS J0211+1051 in less than two days. We compared our high-cadence observations to Particle-In-Cell magnetic reconnection and turbulent plasma simulations. We find that although the state of the art simulation frameworks can produce a large fraction of the polarization properties, they do not account for the entirety of the observed polarization behavior in blazar jets.

astro-ph.HE↗

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.

astro-ph.HE↗

arXiv2024

Detection of X-ray Polarization from the Blazar 1ES 1959+650 with the Imaging X-ray Polarimetry Explorer

Observations of linear polarization in the 2-8 keV energy range with the Imaging X-ray Polarimetry Explorer (IXPE) explore the magnetic field geometry and dynamics of the regions generating non-thermal radiation in relativistic jets of blazars. These jets, particularly in blazars whose spectral energy distribution peaks at X-ray energies, emit X-rays via synchrotron radiation from high-energy particles within the jet. IXPE observations of the X-ray selected BL Lac-type blazar 1ES 1959+650 in 2022 May 3-4 showed a significant linear polarization degree of $Π_\mathrm{x} = 8.0\% \pm 2.3\%$ at an electric-vector position angle $ψ_\mathrm{x} = 123^\circ \pm 8^\circ$. However, in 2022 June 9-12, only an upper limit of $Π_\mathrm{x} \leq 5.1\%$ could be derived (at the 99% confidence level). The degree of optical polarization at that time $Π_\mathrm{O} \sim 5\%$ is comparable to the X-ray measurement. We investigate possible scenarios for these findings, including temporal and geometrical depolarization effects. Unlike some other X-ray selected BL Lac objects, there is no significant chromatic dependence of the measured polarization in 1ES 1959+650, and its low X-ray polarization may be attributed to turbulence in the jet flow with dynamical timescales shorter than 1 day.

astro-ph.HE↗

arXiv2023

Lost in the curve: Investigating the disappearing knots in the blazar 3C 454.3

One of the most well-known extragalactic sources in the sky, quasar 3C 454.3, shows a curved parsec-scale jet that has been exhaustively monitored with very-long-baseline interferometry (VLBI) over the recent years. In this work, we present a comprehensive analysis of four years of high-frequency VLBI observations at 43 GHz and 86 GHz, between 2013-2017, in total intensity and linear polarization. The images obtained from these observations enabled us to study the jet structure and the magnetic field topology of the source on spatial scales down to 4.6 parsec in projected distance. The kinematic analysis reveals the abrupt vanishing of at least four new superluminal jet features in a characteristic jet region (i.e., region C), which is located at an approximate distance of 0.6 milliarcseconds from the VLBI core. Our results support a model in which the jet bends, directing the relativistic plasma flow almost perfectly toward our line of sight, co-spatially with the region where components appear to stop.

astro-ph.HE↗

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.

astro-ph.HE↗

arXiv2023

A Near Magnetic-to-kinetic Energy Equipartition Flare from the Relativistic Jet in AO 0235+164 during 2013-2019

We present the multiwavelength flaring activity of the blazar AO 0235+164 during its recent active period from 2013 to 2019. From a discrete correlation function (DCF) analysis, we find a significant (>95%) correlation between radio and $γ$-ray light curves with flares at longer wavelengths following flares at shorter wavelengths. We identify a new jet component in 43 GHz VLBA data that was ejected from the radio core on MJD $57246^{+26}_{-30}$ (2015 August 12), during the peak of the 2015 radio flare. From the analysis of the jet component, we derived a Doppler factor of $δ_{\rm var}=28.5\pm8.4$, a bulk Lorentz factor of $Γ=16.8^{+3.6}_{-3.1}$, and an intrinsic viewing angle of $θ_{\rm v}=1.42^{+1.07}_{-0.52}\textrm{ degrees}$. Investigation of the quasi-simultaneous radio data revealed a partially absorbed spectrum with the turnover frequency varying in the range of $10-70$ GHz and the peak flux density varying in the range of $0.7-4$ Jy. We find the synchrotron self-absorption magnetic field strength to be $B_{\rm SSA}=15.3^{+12.6}_{-14.0}$ mG at the peak of the 2015 radio flare, which is comparable to the equipartition magnetic field strength of $B_{\rm EQ}=43.6^{+10.6}_{-10.4}$ mG calculated for the same epoch. Additional analysis of the radio emission region in the relativistic jet of AO 0235+164 suggests that it did not significantly deviate from equipartition during its recent flaring activity.

astro-ph.HE↗

arXiv2023

Observations of Low and Intermediate Spectral Peak Blazars with the Imaging X-ray Polarimetry Explorer

We present X-ray polarimetry observations from the Imaging X-ray Polarimetry Explorer (IXPE) of three low spectral peak and one intermediate spectral peak blazars, namely 3C 273, 3C 279, 3C 454.3, and S5 0716+714. For none of these objects was IXPE able to detect X-ray polarization at the 3$σ$ level. However, we placed upper limits on the polarization degree at $\sim$10-30\%. The undetected polarizations favor models where the X-ray band is dominated by unpolarized photons upscattered by relativistic electrons in the jets of blazars, although hadronic models are not completely eliminated. We discuss the X-ray polarization upper limits in the context of our contemporaneous multiwavelength polarization campaigns.

astro-ph.HE↗

arXiv2023

Polarized Blazar X-rays imply particle acceleration in shocks

Most of the light from blazars, active galactic nuclei with jets of magnetized plasma that point nearly along the line of sight, is produced by high-energy particles, up to $\sim 1$ TeV. Although the jets are known to be ultimately powered by a supermassive black hole, how the particles are accelerated to such high energies has been an unanswered question. The process must be related to the magnetic field, which can be probed by observations of the polarization of light from the jets. Measurements of the radio to optical polarization - the only range available until now - probe extended regions of the jet containing particles that left the acceleration site days to years earlier (Jorstad et al., 2005; Marin et al., 2018; Blinov et al., 2021), and hence do not directly explore the acceleration mechanism, as could X-ray measurements. Here we report the detection of X-ray polarization from the blazar Markarian~501 (Mrk~501). We measure an X-ray linear polarization degree $Π_X \sim10\%$, a factor of $\sim2$ higher than the value at optical wavelengths, with a polarization angle parallel to the radio jet. This points to a shock front as the source of particle acceleration, and also implies that the plasma becomes increasingly turbulent with distance from the shock.

astro-ph.HE↗