arXiv subjects

Victor Barbosa Martins

Publications and source records attributed to Victor Barbosa Martins.

6 recordsLinked to original sources

arXiv2026

Self-consistent modeling of energy-dependent synchrotron polarization in blazars with application to Mrk 421

Observations of black hole jets closely aligned with our line of sight, known as blazars, have revealed that their X-ray emission can be significantly more polarized than in the optical bands. This chromatic polarization is often attributed to an energy-stratified jet. In blazars with X-ray emission dominated by electron synchrotron, the pitch angle between the magnetic field lines and electron velocity might also affect the frequency dependence of the polarization degree, but this effect is generally neglected in state-of-the-art models. In this work, we model the polarization and spectral energy distribution of blazars, accounting for the system's temporal evolution and pitch-angle-dependent electron synchrotron cooling. Our analysis investigates the impact of the magnetic field structure and electron distribution on polarization and tests whether this can help explain its observed frequency dependence. By extending the numerical simulation code AM3, we incorporate a self-consistent calculation of electron synchrotron emission and simulate various magnetic-field structures. We apply these models to Mrk 421 and compare them with observations. Finally, we present a time-dependent shock-in-jet model to investigate frequency-dependent polarization angle swings. Our results show how different magnetic-field structures affect energy-dependent polarization signatures. In the case of Mrk 421, the data can be explained by a helical field in the jet. In the shock-in-jet scenario, the X-ray polarization angle changes by 70 degrees, while remaining almost constant in the optical/infrared. Our results demonstrate that a self-consistent treatment of the temporal evolution of electrons is essential to model the polarized synchrotron emission from blazars and can naturally explain chromatic polarization signatures without requiring an energy-stratified jet.

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arXiv2026

UHECR doublets and their conditional association with nearby radio galaxies

The origin of ultra-high-energy cosmic rays (UHECRs) remains a fundamental question in astroparticle physics. While localized 3 $σ$ correlations with active galactic nuclei and starburst galaxies have been reported using time-integrated analyses, we propose and implement a spatiotemporal multiplet search method utilizing a pre-defined fixed window of 3 degrees and 15 days, a kinematic filter designed to isolate high-rigidity particles and keep chance coincidences low. Applying this method to 16 years of Pierre Auger Observatory data, we identify 28 UHECR multiplets (doublets) above 32 EeV. We backtrack these trajectories using three Galactic magnetic field models across eight distinct nuclear species. Testing the backtracked directions against the ten nearest bright radio galaxies yields an overall post-trial significance of 5.8 $σ$, conditioned on the individual best-fit associations. Specifically, we find a 4.5 $σ$ conditional post-trial significance for the joint association of 8 of these multiplets with the Fornax A region alone. These results point to radio galaxies, with a strong contribution from Fornax A, as long-term accelerators of heavy (Z > 3) UHECRs, possibly within the mildly relativistic backflows of their extended radio lobes, detected at Earth primarily as independent secondary fragments above 32 EeV.

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arXiv2024

Chasing Gravitational Waves with the Cherenkov Telescope Array

The detection of gravitational waves from a binary neutron star merger by Advanced LIGO and Advanced Virgo (GW170817), along with the discovery of the electromagnetic counterparts of this gravitational wave event, ushered in a new era of multimessenger astronomy, providing the first direct evidence that BNS mergers are progenitors of short gamma-ray bursts (GRBs). Such events may also produce very-high-energy (VHE, > 100GeV) photons which have yet to be detected in coincidence with a gravitational wave signal. The Cherenkov Telescope Array (CTA) is a next-generation VHE observatory which aims to be indispensable in this search, with an unparalleled sensitivity and ability to slew anywhere on the sky within a few tens of seconds. New observing modes and follow-up strategies are being developed for CTA to rapidly cover localization areas of gravitational wave events that are typically larger than the CTA field of view. This work will evaluate and provide estimations on the expected number of of gravitational wave events that will be observable with CTA, considering both on- and off-axis emission. In addition, we will present and discuss the prospects of potential follow-up strategies with CTA.

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arXiv2020

The structure monitoring of the MST prototype of CTA

The Cherenkov Telescope Array (CTA) is the next generation of ground-based gamma-ray observatory. The observatory will consist of two arrays, one located in the southern hemisphere (Paranal,Chile) and the other in the northern hemisphere (Canary Island, Spain), covering the whole sky in the range of observation. More than 100 telescopes are planned to be in operation for as long as 30 years, which motivated the development of a continuous condition monitoring of the individual telescopes. The main goal of the monitoring is to detect degradation and failures before critical damages occur. Two approaches are considered: the structure monitoring system, in which the Eigenfrequencies of the telescope and their damping rates are measured and monitored; and the drive monitoring, in which the power spectra of rotating components are measured during telescope movements. The structure monitoring concept system was applied to the prototype Medium Size telescope (MST) prototype of CTA in Berlin during late 2018 and in 2019, and the first results are presented here. The system showed reasonable stability during periods, in which the telescope structure was unchanged. The system was also capable to detect mechanical changes, e.g. varying tension in the steel ropes of the camera support structure. The successful implementation of the structure monitoring system supports the decision of implementing the system in all future MSTs.

astro-ph.IM↗

arXiv2019

A Condition Monitoring Concept Studied at the MST Prototype for the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) is a future ground-based gamma-ray observatory that will provide unprecedented sensitivity and angular resolution for the detection of gamma rays with energies above a few tens of GeV. In comparison to existing instruments (like H.E.S.S., MAGIC, and VERITAS) the sensitivity will be improved by installing two extended arrays of telescopes in the northern and southern hemisphere, respectively. A large number of planned telescopes (>100 in total) motivates the application of predictive maintenance techniques to the individual telescopes. A constant and automatic condition monitoring of the mechanical telescope structure and of the drive system (motors, gears) is considered for this purpose. The condition monitoring system aims at detecting degradations well before critical errors occur; it should help to ensure long-term operation and to reduce the maintenance efforts of the observatory. We present approaches for the condition monitoring of the structure and the drive system of Medium-Sized Telescopes (MSTs), respectively. The overall concept has been developed and tested at the MST prototype for CTA in Berlin. The sensors used, the joint data acquisition system, possible analysis methods (like Operational Modal Analysis, OMA, and Experimental Modal Analysis, EMA) and first performance results are discussed.

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arXiv2017

Autonomous RPCs for a Cosmic Ray ground array

We report on the behaviour of Resistive Plate Chambers (RPC) developed for muon detection in ultra-high energy cosmic ray (UHECR) experiments. The RPCs were developed for the MARTA project and were tested on field conditions. These RPCs cover an area of $1.5 \times 1.2\,{m^2}$ and are instrumented with 64 pickup electrodes providing a segmentation better than $20\,$cm. By shielding the detector units with enough slant mass to absorb the electromagnetic component in the air showers, a clean measurement of the muon content is allowed, a concept to be implemented in a next generation of UHECR experiments. The operation of a ground array detector poses challenging demands, as the RPC must operate remotely under extreme environmental conditions, with limited budgets for power and minimal maintenance. The RPC, DAQ, High Voltage and monitoring systems are enclosed in an aluminium-sealed case, providing a compact and robust unit suited for outdoor environments, which can be easily deployed and connected. The RPCs developed at LIP-Coimbra are able to operate using a very low gas flux, which allows running them for few years with a small gas reservoir. Several prototypes have already been built and tested both in the laboratory and outdoors. We report on the most recent tests done in the field that show that the developed RPCs have operated in a stable way for more than 2 years in field conditions.

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