arXiv subjects

P. Peñil

Publications and source records attributed to P. Peñil.

13 recordsLinked to original sources

arXiv2026

Profile Analysis of the Multiwavelength 2.1-year Oscillations of PG 1553+113

We investigate the morphology of the oscillation profiles of the blazar PG~1553+113 in relation to its well-known $\sim$2.1 yr periodicity. We identify individual cycles in the $γ$-ray, X-ray, UV, and optical light curves and characterize their temporal profiles using analytical models for single- and multi-peaked events. We find that the oscillations are generally described by a broad activity envelope with shorter-timescale substructure, showing that the $\sim$2.1 yr signal is not a strictly sinusoidal or self-similar modulation. The internal morphology varies across cycles and energy bands. This is particularly evident in X-rays, where all analyzed cycles show a strong formal preference for multi-component profiles, unlike the $γ$-ray band, where several cycles admit statistically comparable empirical descriptions. The contemporaneous MWL oscillations show broadly aligned activity episodes, but the timing and relative amplitudes of secondary components are not systematically repeated. This suggests that a common long-term modulation affects the broadband emission, while additional local or energy-dependent processes shape individual cycles. Such a picture is compatible with a geometric, jet-related contribution to the broad recurrent envelope, with intrinsic variability superimposed on it. We also identify new cycles with a dominant peak accompanied by weaker twin-peak-like features, similar to structures previously discussed in a supermassive black hole binary scenario for PG~1553+113. Although our results do not provide definitive evidence for this interpretation, the recurrence of comparable morphologies in newly analyzed cycles keeps this scenario viable.

astro-ph.GA↗

arXiv2026

Gamma-Ray Periodicity in Jetted AGN: Revisiting Periodicity Candidates with >17 years of Fermi-LAT Data

We reanalyze previously reported gamma-ray periodicity candidates in jetted active galactic nuclei using more than 17 years of Fermi-LAT observations. The updated data provide a robust test of whether earlier results correspond to persistent periodic behavior, to transient quasi-periodic oscillations (QPOs), or to fluctuations produced by stochastic variability. We apply complementary timing methods, including the Generalized Lomb-Scargle periodogram, Phase Dispersion Minimization, and Singular Spectrum Analysis, and estimate the test statistics using artificial light curves generated from different variability models, including simple and bending power-law power spectral densities, as well as ARIMA and ARFIMA autoregressive models. We find that most previously proposed candidates in literature are not confirmed when the longer baseline is considered, indicating that many reported periods were likely driven by limited temporal coverage and red-noise variability or transient QPO-like features rather than persistent periodic behavior. Only eight sources retain hints of periodic behavior at the >2sigma local test statistics. Among them, PG 1553+113 and S5 1044+71 remain the most significant cases, with local test statistics above 3sigma, and with a global significance consistent with ~0sigma (because of the large trial factor). In addition, we assess predictions from previous studies as an independent test of the proposed periods and find that some are consistent with the new observations.

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arXiv2026

Testing X-ray Periodicity and Long-Term Trend in PG 1553+113 via Targeted Swift-XRT Monitoring

PG~1553+113 is the blazar with the most-significantly detected periodic patter in its multiwavelength (MWL) emission, making it one of the most promising candidates for hosting a supermassive black hole binary. However, the presence of this periodic behavior in the X-ray band remains under debate, largely due to the lack of continuous monitoring. This has led to differing conclusions in previous studies. In addition, we aim to examine whether the recently identified linear long-term trends in the gamma-ray and optical bands also exist in the X-ray regime. Here, we evaluate the 2.1-year period in the X-ray light curve of PG 1553+113 using two dedicated monitoring campaigns with Swift-XRT and UVOT, guided by predictions of future oscillation phases. We also examine whether the long-term trend is present in X-rays, the potential periodic behavior of the X-ray power-law photon index, and its potential correlation to the X-ray flux. As a result, we find tentative evidence for a correlation between the predicted high-emission states in the gamma-ray band and those observed in the X-ray and UV bands. Therefore, we do not find a strong evidence of the same periodic pattern in X-ray. In addition, we find that the X-ray light curve is consistent with the presence of a long-term linear trend, in agreement with those previously reported in gamma-ray, optical, and radio. Overall, these results indicate that the X-ray emission is likely to share the same long-term behavior observed in the gamma-ray and optical bands. Nevertheless, the pronounced stochastic variability that characterizes the X-ray light curve limits our ability to draw firm conclusions regarding the presence of the periodic behavior.

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arXiv2025

Extensive Analysis of gamma-Ray Periodicity in Jetted AGN from the 4FGL Catalog Using Fermi-LAT Observations

The quest to uncover periodic patterns within the $γ$-ray emissions of jetted active galactic nuclei (AGN) has recently emerged as a focal point in astrophysics. One of the primary challenges has been the necessity for prolonged exposures in the $γ$-ray energy band. In our investigation, we leverage 12 years' worth of observations from the \textit{Fermi}-LAT to systematically explore periodicity across 1492 jetted AGN cataloged in 4FGL, representing the largest sample analyzed to date. Our analysis involves a robust pipeline employing nine distinct techniques designed to detect potential periodic emissions within their $γ$ rays. We note that 24 objects with previous hints of periodicity are deliberately excluded in the present work since they were reanalyzed in a dedicated paper using a similar methodology. Using this thorough approach, we do not find any evidence for periodic signals in the 1492 jetted AGN $γ$-ray light curves analyzed here.

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arXiv2025

Distortions in Periodicity Analysis of Blazars II: The Impact of Gaps

Time series analysis is fundamental to characterizing the variability inherent in multi-wavelength emissions from blazars. However, a major observational challenge lies in the need for well-sampled, temporally uniform data, which is often hindered by irregular sampling and data gaps. These gaps can significantly affect the reliability and accuracy of methods used to probe source variability. This paper investigates the impact of such observational gaps on time series analysis of blazar emissions. To do so, we systematically evaluate how these gaps alter observed variability patterns, mask genuine periodic signals, and introduce false periodicity detections. This evaluation is conducted using both simulated and real observational data. We assess a range of widely used time series analysis methods, including the Lomb-Scargle periodogram, Phase Dispersion Minimization, and the recently proposed Singular Spectrum Analysis (SSA). Our results demonstrate a clear and significant degradation in period detection reliability when the percentage of gaps exceeds 50\%. In such cases, the period-significance relationship becomes increasingly distorted, often leading to misleading results. Among the tested methods, SSA stands out for its ability to yield consistent and robust detections despite high data incompleteness. Additionally, the analyzed methods tend to identify artificial periodicities of around one year, likely due to seasonal sampling effects, which can result in false positives if not carefully recognized. Finally, the periods detected with $\geq$3$σ$ confidence are unlikely to result from stochastic processes or from the presence of gaps in the analyzed time series.

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arXiv2025

Search for Periodic Variability in $γ$-ray Blazars Using \textit{Fermi}-LAT

Blazars are known to exhibit variability across a broad range of timescales. This behavior can include periodicity in their $γ$-ray emission, whose clear detection remains an ongoing challenge, partly due to the inherent stochasticity of the processes involved and also the lack of adequately well-sampled light curves. In this study, we perform a systematic search for periodicity in a selected sample of 24 $γ$-ray blazars using twelve years of Fermi-LAT data. The sample comprises the most promising candidates selected from a previous study, extending the light curves by three additional years, expanding the analyzed energy range from $>$1~GeV to $>$0.1~GeV to improve photon statistics, and enhancing the time-series analysis methodology. We incorporate upper-limit flux points in the analysis rather than discarding them, thereby preserving the temporal structure in the light curves. A suite of seven complementary time-series analysis methods is employed to ensure statistical robustness, including autoregressive models, representing a methodological advancement over the prior work. A further improvement is the explicit estimation of the look-elsewhere effect, which allows us to assess the global significance of any detected signals. The study is also supported by additional statistical treatments employed to minimize false detections and strengthen the reliability of the results. Our analysis reveals a hint of periodicity in PG 1553+113 with a global significance of $\approx$1.8$σ$. For the remaining sources in the sample, the re-evaluation of previously reported periodicities indicates that they are statistically consistent with arising from stochastic variability.

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arXiv2025

Singular Spectrum Analysis of Fermi-LAT Blazar Light Curves: A Systematic Search for Periodicity and Trends in the Time Domain

A majority of blazars exhibit variable emission across the entire electromagnetic spectrum, observed over various time scales. In particular, discernible periodic patterns are detected in the $γ$-ray light curves of a few blazars, such as PG 1553+113, S5 1044+71, and PKS 0426-380. The presence of trends, flares, and noise complicates periodicity detection, requiring careful analysis to determine whether these patterns arise from emission mechanisms or occur by chance. We employ Singular Spectrum Analysis (SSA) for the first time on data from the Large Area Telescope (LAT) aboard the Fermi Gamma-ray Space Telescope to systematically search for periodicity using 28-day binned light curves. We aim to isolate any potential periodic nature of the emission from trends and noise, reducing uncertainties in revealing periodicity. Additionally, we characterize long-term trends and develop a forecasting algorithm based on SSA to predict future emission behavior. We apply SSA to analyze 494 sources detected by Fermi-LAT, focusing on isolating oscillatory components from trends and noise in their $γ$-ray light curves. We compute the Lomb-Scargle Periodogram for oscillatory components extracted by SSA to determine the most significant periods, assessing their local and global significance. Our analysis identifies 46 blazars as potential candidates for quasi-periodic $γ$-ray emissions, each with a local significance $\geq 2σ$. Notably, 33 exhibit a local significance of $\geq 4σ$ (corresponding to a global significance of $\geq 2.2σ$). Our findings introduce 25 new $γ$-ray candidates, doubling the number of potentially periodic sources. This study provides a foundation for future investigations by identifying promising candidates and their significance in blazar variability.

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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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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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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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arXiv2020

Systematic search for gamma-ray periodicity in active galactic nuclei detected by the Fermi Large Area Telescope

We use nine years of gamma-ray data provided by the Fermi Large Area Telescope (LAT) to systematically study the light curves of more than two thousand active galactic nuclei (AGN) included in recent Fermi-LAT catalogs. Ten different techniques are used, which are organized in an automatic periodicity-search pipeline, in order to search for evidence of periodic emission in gamma rays. Understanding the processes behind this puzzling phenomenon will provide a better view about the astrophysical nature of these extragalactic sources. However, the observation of temporal patterns in gamma-ray light curves of AGN is still challenging. Despite the fact that there have been efforts on characterizing the temporal emission of some individual sources, a systematic search for periodicities by means of a full likelihood analysis applied to large samples of sources was missing. Our analysis finds 11 AGN, of which 9 are identified for the first time, showing periodicity at more than 4sigma in at least four algorithms. These findings will help in solving questions related to the astrophysical origin of this periodic behavior.

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arXiv2020

Quasi-periodic behaviour in the optical and γ-ray light curves of blazars 3C 66A and B2 1633+38

We report on quasi-periodic variability found in two blazars included in the Steward Observatory Blazar Monitoring data sample: the BL Lac object 3C 66A and the Flat Spectrum Radio Quasar B2 1633+38. We collect optical photometric and polarimetric data in V and R bands of these sources from different observatories: St. Petersburg University, Crimean Astrophysical Observatory, WEBT-GASP, Catalina Real-Time Transient Survey, Steward Observatory, STELLA Robotic Observatory and Katzman Automatic Imaging Telescope. In addition, an analysis of the $γ$-ray light curves from $\textit{Fermi}$-LAT is included. Three methods are used to search for any periodic behaviour in the data: the Z-transform Discrete Correlation Function, the Lomb-Scargle periodogram and the Weighted Wavelet Z-transform. We find evidences of possible quasi-periodic variability in the optical photometric data of both sources with periods of $\sim$3 years for 3C 66A and $\sim$1.9 years for B2 1633+38, with significances between 3$σ$ and 5$σ$. Only B2 1633+38 shows evidence of this behaviour in the optical polarized data set at a confidence level of 2$σ$-4$σ$. This is the first reported evidence of quasi-periodic behaviour in the optical light curve of B2 1633+38. Also a hint of quasi-periodic behaviour is found in the $γ$-ray light curve of B2 1633+38 with a confidence level $\geqslant$2$σ$, while no periodicity is observed for 3C 66A in this energy range. We propose different jet emission models that could explain the quasi-periodic variability and the differences found between these two sources.

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