arXiv subjects

J. Otero-Santos

Publications and source records attributed to J. Otero-Santos.

At least 19 recordsLinked to original sources

arXiv2026

Detection of optical quasi-periodic oscillation in the blazar 3C 454.3

We analyzed 19 years of $R$-band data of the blazar 3C 454.3 from the Whole Earth Blazar Telescope (WEBT) archive, along with new data from its members and from public archives such as those provided by the Small and Moderate Aperture Research Telescope System (SMARTS) and the Steward Observatory projects to search for quasi-periodic oscillations (QPOs). We detected a QPO of $\sim$ 433 days using Lomb-Scargle periodogram, which lasted from MJD 54980--58450 as detected by the weighted wavelet Z-transform technique, making it one of the most persistent QPOs ever detected in the optical regime. The phase dispersion minimization technique was also performed to further validate this QPO claim. We detected this signal at a global significance of $2.53σ$ across all methodologies. To explain the observed QPO, we have considered both models focused on the accretion disk around the super-massive black hole (SMBH), and those based purely on jet emissions. Plausible jet-based models involve a shock moving down the jet in a helical magnetic field, whereas the SMBH models could involve Lense-Thirring effect-induced jet precession or dual jets in a binary SMBH system. We introduce a novel approach to distinguish genuine QPOs from spurious signals arising from annual seasonal gaps, a common limitation of ground-based observations.

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arXiv2026

Optical spectral characterization of OP 313. Constraining the contribution of thermal and non-thermal optical emission

The quasar OP 313 was discovered in December 2023 in very-high-energy $γ$ rays above 100 GeV, enabling for the first time a complete characterization of its emission. However, the lack of updated measurements of its accretion disk, broad line region and dusty torus hampers a detailed interpretation of the role of accretion in the observed $γ$-ray production. We intend to characterize, during high-activity states, the external photon fields contributing to the IR-to-UV emission$-$namely dusty torus, broad line region and accretion disk$-$and investigate potential variability and blurring effects on the broad emission lines. We present new spectroscopic observations of OP 313 with the NOT and TNG telescopes to characterize its optical spectrum and variability with respect to archival observations from SDSS. We measure the luminosity of different broad emission lines, characterizing the broad line region, accretion disk and dusty torus. We measure the Mg II emission line, with an average flux of $F_{\mathrm{Mg \ II}} = (0.85 \pm 0.11)\times 10^{-14}$ erg cm$^{-2}$ s$^{-1}$. Its equivalent width and luminosity are consistent with a constant line with a variable non-thermal continuum. From the stable Mg II line we derive a constant luminosity of the thermal components, $\log(L_{\mathrm{BLR}} \ \mathrm{[erg \ s^{-1}]}) = 44.91 \pm 0.19$, $\log(L_{\mathrm{disk}} \ \mathrm{[erg \ s^{-1}]}) = 45.91 \pm 0.19$ and $\log(L_{\mathrm{torus}} \ \mathrm{[erg \ s^{-1}]}) = 44.70 \pm 0.16$, and estimated a BH mass of $\log(M_{BH}/M_{\odot})=8.36 \pm 0.18$, in line with with that derived from the C III] line. These characteristics and the indicator of the accretion rate from the disk/Eddington luminosity ratio $λ=L_{AD}/L_{Edd} = 0.23 \pm 0.10$, along with a high Compton dominance, favour a FSRQ-like nature, contrary to the argued changing-look nature.

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arXiv2025

Transient QPOs of Fermi-LAT blazars with Linearly Multiplicative Oscillations

We present a study on the detection and characterization of transient quasi-periodic oscillations (QPOs) in the $γ$-ray emission of blazars 4C +31.03, MG1 J123931+0443, and PKS 1622$-$253. Using light curves derived from \textit{Fermi} Large Area Telescope data, we investigate oscillatory patterns characterized by periodic multiplicative amplitudes that vary linearly over time. By segmenting the light curves into increasing and decreasing trends, we analyze each segment independently, allowing for precise measurements of both the periodicity and long-term variations. To interpret these QPOs, we explore various theoretical scenarios that could explain their origin and underlying physical mechanisms. The variability observed in 4C~+31.03 is more consistent with a stochastic process, whereas the periods estimated for MG1~J123931+0443 and PKS~1622$-$253 align with the precessional dynamics expected from binary supermassive black hole systems. However, the current results remain tentative and do not allow for a definitive conclusion.

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arXiv2025

Polarimetric Diversity in Tidal Disruption Events: Comparative Study of Low-Polarised sources with AT2020mot

Tidal disruption events (TDEs) occur when a star is disrupted by the tidal forces of a supermassive black hole (SMBH), producing bright multi-wavelength flares. Among these events, AT2020mot has so far exhibited the highest recorded optical polarisation, with tidal shocks proposed as the primary source of its polarised emission. We present a comprehensive analysis of 13 TDEs with available polarimetric observations, aiming to determine whether the unusually high polarisation of AT2020mot stems from unique physical processes or arises from mechanisms shared by other TDEs. We present new optical polarisation measurements of TDEs obtained from multiple ground-based telescopes, combining them with optical, UV, and X-ray light curves from the Zwicky Transient Facility and the Swift observatory, respectively. We derive intrinsic TDE properties, such as SMBH and stellar masses, using MOSFiT and TDEMass, and compare them with the ones of the sample population. Our population study reveals that AT2020mot agrees with the broader TDE sample in most physical properties, including blackbody temperature, luminosity, and rise timescales. However, its optical polarisation degree is exceptionally high compared to the low or undetected polarisation observed in other events. Additionally, AT2020mot appears to have an elevated column density from our MOSFiT fits, suggesting a more complex environment than is typically assumed. We conclude that although AT2020mot fits well within the general TDE population in terms of global characteristics, its extraordinarily high polarisation and higher column density challenge current models based purely on shock or reprocessing mechanisms. More extensive, time-resolved polarimetric monitoring of newly discovered TDEs will be critical to determine whether AT2020mot represents an outlier or the extreme end of a continuum of TDE properties.

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arXiv2025

Transient QPOs of Fermi-LAT blazars under the Curved Jet Model

This study explores transient quasi-periodic oscillations (QPOs) in the $γ$-ray emission of two blazars, PMN J0531$-$4827 and PKS 1502+106, using over a decade of Fermi Large Area Telescope observations. The analysis focuses on identifying QPO signatures in their long-term light curves and interpreting the variability through a curved jet model, which predicts multiplicative oscillations with exponentially decaying amplitudes. We develop an analysis methodology to characterize the QPO and the specific properties of the amplitude of such QPOs. The findings offer insights into the dynamic processes driving relativistic jet evolution and their potential connections to underlying mechanisms, such as binary systems or other phenomena influencing the observed characteristics of these blazars.

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arXiv2025

Search for Kink Events in Variable Fermi-LAT Blazars

This study explores the detection of Quasi-Periodic Oscillations (QPOs) in blazars as a method to identify kink events within their jets, utilizing both $γ$-ray and polarized light observations. Focusing on a sample of 9 blazars, we analyze $γ$-ray light curves to identify significant QPOs. In addition to $γ$-ray data, we incorporated polarized light data corresponding to the same temporal segments to cross-validate the presence of QPOs. However, the limited availability of comprehensive polarized data restricted our ability to perform a thorough analysis across all datasets. Despite these limitations, our analysis reveals a segment where QPOs in polarized light coincided with those observed in $γ$-rays, providing preliminary evidence supporting the kink origin of these oscillations.

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arXiv2025

Exploring the capability of the HH 80-81 protostellar jet to accelerate relativistic particles

Context. Protostellar jets driven by massive protostars are collimated outflows producing high-speed shocks through dense interstellar medium. Fast shocks can accelerate particles up to relativistic energies via diffusive shock acceleration, producing non-thermal emission that can originate $γ$-ray photons. HH 80-81 is one of the most powerful collimated protostellar jets in our galaxy, with non-thermal emission detected in radio, X-ray, and $γ$-ray bands. Characterize the $γ$-ray emission originated by the accelerated particles of the region is crucial for demonstrating the capability of protostars to accelerate cosmic rays. Aims. Our goal is to determine the particle distribution that is producing the $γ$-ray spectrum of HH 80-81 in order to ascertain the leptonic/hadronic origin of the $γ$-ray emission. We aim at associating the high-energy emission in the region with the HH 80-81 system, characterize its spectrum, and elaborate emission models based on what we expect from the diffusive shock acceleration. Methods. We use the 15 yr database provided by the Fermi-LAT satellite to study the high-energy emission of the jet, spanning from 300 MeV to 100 GeV. In addition, we perform a source association based on positional arguments. Then, we employ the naima and Gamera softwares to analyze the possible mechanisms that are producing $γ$-rays considering the ambient conditions. We perform a radiative fitting and study the nature of the particles behind the $γ$-ray emission. Results. By analyzing all the candidates to produce the $γ$-ray emission that we detect, we conclude that HH 80-81 is the most probable candidate to explain the $γ$-ray emission in the region. The detected spectrum can be explained by both hadronic and leptonic particle components.

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arXiv2024

Multi-wavelength picture of the misaligned BL Lac object 3C 371

The BL Lac object 3C 371 is one of the targets that are regularly monitored by the Whole Earth Blazar Telescope (WEBT) Collaboration to study blazar variability on both short and long timescales. We aim to evaluate the long-term multiwavelength (MWL) behaviour of 3C 371, comparing it with the results derived for its optical emission in our previous study. For this, we make use of the multi-band campaigns organized by the WEBT Collaboration in optical and radio between January 2018 and December 2020, and of public data from Swift and Fermi satellites and the MOJAVE Very Large Interferometry programme. We evaluate the variability shown by the source in each band with the amplitude variability quantification, as well as possible interband correlation using the z-Discrete Correlation Function. We also present a deep analysis of the optical-UV, X-ray and $γ$-ray spectral variability. With the MOJAVE data we perform a kinematics analysis, looking for components propagating along the jet, calculating its kinematics parameters. This set of parameters is later used for the interpretation of the source MWL behaviour, modelling the broadband spectral energy distribution (SED) of the source with theoretical blazar emission scenarios.

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arXiv2024

A Unified Multi-Wavelength Data Analysis Workflow with gammapy. Constraining the Broadband Emission of FSRQ OP 313

The Flat Spectrum Radio Quasar OP~313 entered an enhanced activity phase in November 2023 and has undergone multiple flares since then which have motivated the organisation of several large multi-wavelength campaigns, including two deep observations from the hard X-ray telescope NuSTAR. The broadband emission from OP~313 during these two observations is investigated under a new unified analysis framework, with data spanning from optical to gamma rays. Traditional methods for analyzing blazar emissions often rely on proprietary software tailored to specific instruments, making it challenging to integrate and interpret data from multi-wavelength campaigns comprehensively. This study demonstrates the feasibility of utilizing gammapy, an open-source Python package, and common data formats originally developed for gamma-ray instrumentation, to perform a consistent multi-instrument analysis. This enables a forward folding approach that fully incorporates source observations, detector responses, and various instrumental and astrophysical backgrounds. The methodology is applied as an example to recent data collected from the distant quasar OP~313. We present a comprehensive data reconstruction and analysis for instruments including the Liverpool Telescope's IO:O detector, Swift-UVOT, Swift-XRT, NuSTAR, and Fermi-LAT. The resulting spectral analysis is validated against the native tools for each instrument. Additionally, a multi-wavelength phenomenological model of the source emission, encompassing optical to gamma-ray bands, is developed, incorporating absorption components across different energy regimes. We introduce and validate a new unified framework for multi-wavelength forward folding data analysis based on gammapy and open data formats, demonstrating its application to spectral data from the quasar OP~313.

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arXiv2024

Distance estimation of gamma-ray emitting BL Lac objects from imaging observations

Direct redshift determination of BL Lac objects is highly challenging as the emission in the optical and near-infrared (NIR) bands is largely dominated by the non-thermal emission from the relativistic jet that points very close to our line of sight. Therefore, their optical spectra often show no emission lines from the host galaxy. In this work, we aim to overcome this difficulty by attempting to detect the host galaxy and derive redshift constraints based on assumptions on the galaxy magnitude ("imaging redshifts"). Imaging redshifts are derived by obtaining deep optical images under good seeing conditions, so that it is possible to detect the host galaxy as weak extension of the point-like source. We then derive the imaging redshift by using the host galaxy as a standard candle using two different methods. We determine imaging redshift for 9 out of 17 blazars that we observed as part of this program. The redshift range of these targets is 0.28-0.60 and the two methods used to derive the redshift give very consistent results within the uncertainties. We also performed a detailed comparison of the imaging redshifts with those obtained by other methods, like direct spectroscopic constraints or looking for groups of galaxies close to the blazar. We show that the constraints from different methods are consistent and that for example in the case of J2156.0+1818, which is the most distant source for which we detect the host galaxy, combining the three constraints narrows down the redshift to $0.63<z<0.71$. This makes the source interesting for future studies of extragalactic background light in the Cherenkov Telescope Array Observatory era.

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arXiv2024

Optical variability of the blazar 3C 371: from minute to year timescales

The BL Lac object 3C 371 has been observed by the Transiting Exoplanet Survey Satellite (\textit{TESS}) for approximately a year, between July 2019 and July 2020, with an unmatched 2-minute observing cadence. In parallel, the Whole Earth Blazar Telescope (WEBT) Collaboration organized an extensive observing campaign, providing three years of continuous optical monitoring between 2018 and 2020. These datasets allow for a thorough investigation of the variability of the source. The goal of this study is evaluating the optical variability of 3C 371. Taking advantage of the remarkable cadence of \textit{TESS} data, we aim to characterize the intra-day variability (IDV) displayed by the source and identify its shortest variability timescale. With this estimate, constraints on the size of the emitting region and black hole mass can be calculated. Moreover, WEBT data is used to investigate long-term variability (LTV), including understanding spectral behaviour of the source and the polarization variability. Based on the derived characteristics, information on the origin of the variability on different timescales is extracted. We evaluated the variability applying the variability amplitude tool that quantifies how variable the emission is. Moreover, we employed common tools like ANOVA (ANalysis Of VAariance) tests, wavelet and power spectral density (PSD) analyses to characterize the shortest variability timescales present in the emission and the underlying noise affecting the data. Short- and long-term colour behaviours have been evaluated to understand the spectral behaviour of the source. The polarized emission was analyzed, studying its variability and possible rotation patterns of the electric vector position angle (EVPA). Flux distributions of IDV and LTV were also studied with the aim of linking the flux variations to turbulent and/or accretion disc related processes.

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arXiv2024

Characterization of High-polarization Stars and Blazars with DIPOL-1 at Sierra Nevada Observatory

We report here the performance and first results of the new multiband optical polarimeter DIPOL-1, installed at the Sierra Nevada Observatory 90 cm T90 telescope (SNO, Granada, Spain). DIPOL-1 is equipped with a plane parallel calcite plate and $λ$/2 retarder for modulating the intensity of two perpendicularly polarized beams, and a high readout speed CMOS camera that allows for fast, time-dense coverage. We characterize the performance of this instrument through a series of tests on zero- and high-polarization standard stars. The instrumental polarization in the Nasmyth focus was well determined, with a very stable contribution of 4.0806% $\pm$ 0.0014% in the optical $R$ band. For bright high-polarization standards ($m_{R}<8$) we reach precisions $<$0.02% in polarization degree and 0.1$^{\circ}$ in polarization angle for exposures of 2$-$4 minutes. The polarization properties of these stars have been constrained, providing more recent results also about possible variability for future studies on some of the most used calibrators. Moreover, we have tested the capability of observing much fainter objects, in particular through blazar observations, where we reach a precision $<$0.5$-$0.6% and $<$0.5$^{\circ}$ for faint targets ($m_{R}\sim16.5$) with exposures of $\sim$1 hour. For brighter targets ($m_{R}\sim14.5-15$), we can aim for time-dense observations with errors $<$$0.2-0.4$% and $<$$1-1.5^{\circ}$ in 5-20 minutes. We have successfully performed a first campaign with DIPOL-1, detecting significant polarized emission of several blazars, with special attention to the highest ever polarization degree measured from blazar 3C~345 at $\sim$32%.

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arXiv2024

Multiwavelength Variability Analysis of Fermi-LAT Blazars

Blazars present highly variable $γ$-ray emission. This variability, which can range from a few minutes to several years, is also observed at other wavelengths across the entire electromagnetic spectrum. We make use of the first 12 years of data from the Fermi Large Area Telescope (LAT), complemented with multiwavelength (MWL) archival data from different observatories and facilities in radio, infrared and optical bands, to study the possible periodic emission from 19 blazars previously claimed as periodic candidates. A periodicity analysis is performed with a pipeline for periodicity searches. Moreover, we study the cross-correlations between the $γ$-ray and MWL light curves. Additionally, we use the fractional variability and the structure function to evaluate the variability timescales. We find five blazars showing hints of periodic modulation with $\geq$3.0$σ$ ($\approx$0$σ$ post-trials), with periods ranging from 1.2 to 4 years, both in their $γ$-ray and MWL emission. The results provide clues for understanding the physical mechanisms generating the observed periodicity.

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arXiv2023

Multiwavelength Analysis of Fermi-LAT Blazars with High-Significance Periodicity: Detection of a Long-Term Rising Emission in PG 1553+113

Blazars display variable emission across the entire electromagnetic spectrum, with timescales that can range from a few minutes to several years. Our recent work has shown that a sample of five blazars exhibit hints of periodicity with a global significance $\gtrsim2\,σ$ at $γ$-ray energies, in the range of 0.1~GeV$<$E$<$800~GeV. In this work, we study their multiwavelength (MWL) emission, covering the X-ray, ultraviolet, optical, and radio bands. We show that three of these blazars present similar periodic patterns in the optical and radio bands. Additionally, fluxes in the different bands of the five blazars are correlated, suggesting a co-spatial origin. Moreover, we detect a long-term ($\approx$10 year) rising trend in the light curves of PG~1553+113, and we use it to infer possible constraints on the binary black hole hypothesis.

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arXiv2023

Variability and evolution of the optical polarization of a sample of gamma-ray blazars

We present a polarization variability analysis of a sample of 26 $γ$-ray blazars monitored by the Steward Observatory between 2008 and 2018 in the optical band. We investigate the properties and long-term variability of their optical polarization, searching for differences between blazar types. We observe that BL Lac objects are typically less polarized and less variable than flat spectrum radio quasars (FSRQs). Moreover, BL Lacs display a distribution of their polarization angle typically oriented in a preferential direction, contrary to the rather random distribution of FSRQs. For the latter blazar type, as well as those sources showing a bright stellar emission, we take into account the depolarizing effect introduced by the broad line region and the host galaxy on the measured polarization degree. In this sample we also observe that BL Lacs present an uncorrelated evolution of the flux and the polarization. Contrary, FSRQs show a correlation before the depolarization correction, that is lost however after considering this effect. In addition, we study the behaviour of the polarization angle, searching for angle rotations in its long-term evolution. We derive that the FSRQs studied here show rotations more frequently than BL Lac objects by a factor $\sim$1.5. During these periods we also observe a systematic decrease of the polarization fraction, as well as a marginal flux increase, not significant however to connect rotations with optical flares. We interpret these results within the extended shock-in-jet scenario, able to explain the overall features observed here for the polarization of the blazar sample.

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arXiv2023

The relation between optical and $γ$-ray emission in BL Lac sources

The relativistic jets produced by some Active Galactic Nuclei (AGNs) are among the most efficient persistent sources of non-thermal radiation and represent an ideal laboratory for studying high-energy interactions. In particular, when the relativistic jet propagates along the observer's line of sight, the beaming effect produces dominant signatures in the observed spectral energy distribution (SED), from the radio domain up to the highest energies, with the further possibility of resulting in radiation-particle multi-messenger associations. In this work, we investigate the relationships between the emission of $γ$ rays and the optical spectra of a sample of AGN, selected from BL Lac sources detected by the $Fermi$ Large Area Telescope ($Fermi$-LAT). We find that there is a close relationship between the optical and $γ$-ray spectral indices. Despite all the limitations due to the non-simultaneity of the data, this observation strongly supports a substantial role of Synchrotron-Self Compton (SSC) radiation in a single zone leptonic scenario for most sources. This result simplifies the application of theoretical models to explore the physical parameters of the jets in this type of sources.

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arXiv2022

Multiwavelength periodicity search in a sample of $γ$-ray bright blazars

We present the results of a long-term periodicity search in a sample of $γ$-ray blazars within a multiwavelength context. These blazars have been selected from the Steward Observatory sample as part of its optical monitoring program between 2008 and 2018. We study 15 sources with a temporal coverage in their optical total and polarized emission sufficiently large ($>9$ years) to perform a reliable long-term periodicity analysis. We collect data from several observatories to extend the coverage, enabling the search of longer periods. In addition, data are also gathered in the high-energy ($E>100$ MeV) $γ$-ray band from the $\textit{Fermi}$ Large Area Telescope; and in the 15-GHz radio band from the Owens Valley Radio Observatory. We identify 5 promising candidates to host quasi-periodic emission, AO 0235+164, PKS 1222+216, Mrk 501, BL Lacertae and 1ES 2344+514 with periods in one or more bands and statistical significances $\sim$3$σ$ after trial factor correction. AO 0235+164 shows a period of $\sim$8.2 years in the R band; PKS 1222+216 has a quasi-periodic modulation in its total and polarized optical emission of $\sim$1.6 years; Mrk 501 displays a $\sim$5-year quasi-periodicity in optical and radio wavelengths; BL Lacertae presents a period of $\sim$1.8 years in its polarized emission; and 1ES 2344+514 shows a hint of a $\sim$5.5-year period in its optical R band. We interpret these results in the framework of the most common models and scenarios, namely the presence of a binary supermassive black hole system; or geometrical effects like helical or precessing jets.

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arXiv2022

A statistical study of the optical spectral variability in gamma-ray blazars

Blazars optical emission is generally dominated by relativistic jets, although the host galaxy, accretion disk and broad line region (BLR) may also contribute significantly. Disentangling their contributions has been challenging for years due to the dominance of the jet. To quantify the contributions to the spectral variability, we use the statistical technique for dimensionality reduction Non-Negative Matrix Factorization on a spectroscopic data set of 26 $γ$-ray blazars. This technique allows to model large numbers of spectra in terms of a reduced number of components.We use a priori knowledge to obtain components associated to meaningful physical processes. The sources are classified according to their optical spectrum as host-galaxy dominated BL Lac objects (BL Lacs), BL Lacs, or Flat Spectrum Radio Quasars (FSRQs). Host-galaxy sources show less variability, as expected, and bluer-when-brighter trends, as the other BL Lacs. For FSRQs, more complicated colour-flux behaviours are observed: redder-when-brighter for low states saturating above a certain level and, in some cases, turning to bluer-when-brighter. We are able to reproduce the variability observed during 10 years using only 2 to 4 components, depending on the type. The simplest scenario corresponds to host-galaxy blazars, whose spectra are reconstructed using the stellar population and a power law for the jet. BL Lac spectra are reproduced using from 2 to 4 power laws. Different components can be associated to acceleration/cooling processes taking place in the jet. The reconstruction of FSRQs also incorporates a QSO-like component to account for the BLR, plus a very steep power law, associated to the accretion disk.

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