Constraining the PG 1553+113 binary hypothesis: interpreting a new, 22-year period

PG 1553+113 is a well-known blazar exhibiting evidence of a ∼2.2 yr quasiperiodic oscillation (QPO) in the radio, optical, X-ray, and γ-ray bands. Since QPO mechanisms often predict multiple QPOs, we search for a second QPO in its historical optical light curve covering a century of observations. Despite challenging data quality issues, we find hints of a 21.8 ± 4.7 yr oscillation. On its own, this ∼22 yr period has a modest statistical significance of 1.6σ when accounting for the look-elsewhere effect. However, the joint significance of both the 2.2 and 22 yr periods arising from colored noise alone is ∼3.6σ. The next peak of the 22 yr oscillation is predicted to occur around July 2025. We find that such a ∼10:1 relation between two periods can arise in the gas dynamics of a plausible supermassive black hole binary model of PG 1553+113. While the 22 yr QPO is preliminary, an interpretation of PG 1553+113's two QPOs in this binary model suggests that the binary engine has a mass ratio ≳0.2, an eccentricity ≲0.1, and accretes from a disk with characteristic aspect ratio ∼0.03. The putative binary radiates nHz gravitational waves, but the amplitude is ∼10–100 times too low for detection by foreseeable pulsar timing arrays.

Distortions in periodicity analysis of blazars: the impact of flares

Blazars, a unique class of active galactic nuclei, exhibit highly variable emission across the electromagnetic spectrum. This variability frequently manifests as intense flaring events, sparking an ongoing debate in recent literature about whether these flares exhibit periodic behaviour in certain sources. However, many blazars also show clear signs of stochastic, uncorrelated flares that do not follow a regular pattern. This paper explores how the presence of one such of these stochastic flares can distort an intrinsically periodic pattern of emission in blazars. Our results demonstrate that, depending on the specific circumstances, the deviations in significance and periods can exceed 100 per cent. Sometimes, these deviations can be so severe that they eliminate any evidence of a periodic pattern. These findings highlight the dramatic impact that flares can have on periodicity searches. To confront this challenge, we propose an innovative approach, the Singular Spectrum Analysis method, which appears more robust against the effects of flares. As an alternative solution, we also propose the sigma clipping technique to mitigate the impact of flares. This framework offers a valuable foundation for analysing periodicity in similar astrophysical sources that are also subject to stochastic flaring events.

Singular spectrum analysis of Fermi-LAT blazar light curves: A systematic search for periodicity and trends in the time domain

Context. A majority of blazars exhibit variable emission across the entire electromagnetic spectrum, observed over various timescales. In particular, discernible periodic patterns are detected in the γ-ray light curves (LCs) of a few blazars, such as PG 1553+113, S5 1044+71, and PKS 0426–380. The presence of trends, flares, and noise complicates the detection of periodicity, requiring careful analysis to determine whether these patterns are related to emission mechanisms within the source or occur by chance.Aims. 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 in the time domain, using 28-day binned LCs. Our aim is to isolate any potential periodic nature of the emission from trends and noise, thereby reducing uncertainties in revealing periodicity. Additionally, we aim to characterize long-term trends and develop a forecasting algorithm based on SSA, enabling accurate predictions of future emission behavior.Methods. We apply SSA to analyze 494 sources detected by Fermi-LAT, focusing on identifying and isolating oscillatory components from trends and noise in their γ-ray LCs. We calculate the Lomb-Scargle periodogram (LSP) for the oscillatory components extracted by SSA to determine the most significant periods. The local and global significance of these periods is then assessed to validate their authenticity.Results. Our analysis identifies 46 blazars as potential candidates for quasi-periodic γ-ray emissions, each with a local significance level ≥2σ. Notably, 33 of these candidates exhibit a local significance of ≥4σ (corresponding to a global significance of ≥2.2σ). Our findings introduce 25 new γ-ray candidates, effectively doubling the number of potentially periodic sources. This study provides a foundation for future investigations by identifying promising candidates and highlighting their potential significance within the context of blazar variability.

PG 1553+113: the case for a super-massive black hole binary

PG 1553+113 is a blazar exhibiting significant evidence of a 2.2-year periodic emission detected from radio to gamma rays. Here we present the first evidence of longer, 21-year periodic variability in the >100-year optical lightcurve provided by the DASH database. The 10-to-1 relationship between these two periodicities is tantalizingly consistent with the two dominant periodicities reported in recent simulations of supermassive binaries embedded in a thin disk. In that setting, the 2.2-year period comes from the binary orbit, and the 21-year period comes from the orbit of an overdensity in the circumbinary disk called a "lump." This finding provides strong evidence that a binary of super-massive black holes resides at the core of PG 1553+113. We will also discuss physical constraints that can be placed on the putative binary.

Search for Kink Events in Variable Fermi-LAT Blazars

This study explores the detection of quasiperiodic 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 nine 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 data sets. 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.

Systematic Search for Long-term Trends in Fermi-LAT Jetted Active Galactic Nuclei

Jetted active galactic nuclei (AGN) exhibit variability across a wide range of timescales. Traditionally, this variability can often be modeled well as a stochastic process. However, in certain cases, jetted AGN variability displays regular patterns, enabling us to conduct investigations aimed at understanding its origins. Additionally, a novel type of variability has emerged in jetted AGN light curves, specifically, the observation of a long-term trend characterized by a linear increase of the flux with time in blazars such as PG 1553+113, which is among the objects most likely to display periodic behavior. In this paper, we present the results of a systematic search for long-term trends, spanning ≈10 yr, utilizing 12 yr of Fermi-LAT observations. The study is focused on detecting the presence of linear or quadratic long-term trends in a sample of 3308 jetted AGN. Our analysis has identified 40 jetted AGN that exhibit long-term trends, each with distinct properties, which we also characterize in this study. These long-term trends may originate from the dynamics of a supermassive black hole binary system, or they could be the result of intrinsic phenomena within the jet itself. Our findings can help in addressing questions pertaining to the astrophysical origins of variability and periodicity within jetted AGN.

Decade-long periodicity study of 2FHL blazars with historical optical data

In our recent investigation, we utilized a century’s worth of archival optical data to search for a decade-long periodicity from the blazar PG 1553+113, finding a hint of a 22-yr period. Building on this foundation, the current study extends our analysis to include 10 blazars from the Fermi-Large Area Telescope Second Catalog of Hard Sources (2FHL) catalogue to uncover similar long-term periodic behaviour. To ensure the reliability of our findings, we consider the impact of observational limitations, such as temporal gaps and uneven sampling, which could potentially introduce artefacts or false periodic signals. Our analysis initially identifies decade-scale periodicity in four of these blazars (AP Librae, MKN 421, MKN 501, PG 1246+586). However, further investigation reveals that three of these are likely influenced by noise and poor sampling. The most promising candidate, approximately 51 9 yr signal in MKN 421, corresponds to fewer than three full cycles and cannot be considered significant. Furthermore, global significance suggests none of the candidate periodicities meet the threshold for statistical significance. These results underscore the importance of accounting for sampling artefacts and highlight the need for robust methodologies in long-term periodicity searches.

Analysis of fermi-lat blazar lightcurves in the time domain using singular spectrum analysis,” in International Conference on Time Series and Forecasting (ITISE-2023), Gran Canaria, Spain, 2023.

Enhancing Periodicity Analysis Accuracy Through Phase Fold Amplitude Minimization (PFAM) Technique

In time-series astronomy, periodicity study is one of the most useful tools to understand an astrophysical system. Research papers on periodicity study often show phase-folded plots to emphasize the presence of periodicity. Depending on the quality of the data points, we can use the phase-folded light curves (LCs) to further enhance the accuracy of the observed period. I discuss an amplitude minimization of the phase-folded LC, which can enhance the accuracy of observed periods in astrophysical LCs.

Can Blazar flares in gamma-ray LCs be explained by jet angle and geometry?

In Preparation.