Abstract
We report the detection of several quasiperiodicities around 0.6–2.5 days in the optical emission of the blazar S4 0954+658. The source was observed by the Transiting Exoplanet Survey ...Satellite in six sectors and it showed these features in all but one of them, with a quasiperiodic oscillation of 1.52 days apparently present in portions of four of them. We used the generalized Lomb–Scargle periodogram method to search for significant signals and we confirmed them using a weighted wavelet transform for time–frequency domain analyses. We discuss several possible explanations for these rapid quasiperiodic variations and suggest that an origin in the innermost part of the accretion disk is most likely. Within this framework, we provide estimates for the mass of the black hole at the core of this blazar.
ABSTRACT We use the Athena hydrodynamics code to model propagating two-dimensional relativistic jets as approximations to the growth of radio-loud active galactic nuclei for various input jet ...velocities and jet-to-ambient matter density ratios. Using results from these simulations we estimate the changing synchrotron emission by summing the fluxes from a vertical strip of zones behind the reconfinement shock, which is nearly stationary, and from which a substantial portion of the flux variability should arise. We explore a wide range of timescales by considering two light curves from each simulation; one uses a relativistic turbulence code with bulk velocities taken from our simulations as input, while the other uses the bulk velocity data to compute fluctuations caused by variations in the Doppler boosting due to changes in the direction and the speed of the flow through all zones in the strip. We then calculate power spectral densities (PSDs) from the light curves for both turbulent and bulk velocity origins for variability. The range of the power-law slopes of the PSDs for the turbulence induced variations is −1.8 to −2.3, while for the bulk velocity produced variations this range is −2.1 to −2.9; these are in agreement with most observations. When superimposed, these power spectra span a very large range in frequency (about five decades), with the turbulent fluctuations yielding most of the shorter timescale variations and the bulk flow changes dominating the longer periods.
Since 2016 October, the active galaxy PKS 2247-131 has undergone a γ-ray outburst, which we studied using data obtained with the Fermi Gamma-ray Space Telescope. The emission arises from a ...relativistic jet in PKS 2247-131, as an optical spectrum only shows a few weak absorption lines, typical of the BL Lacertae sub-class of the blazar class of active galactic nuclei. Here we report a ≃34.5 day quasi-periodic oscillation (QPO) in the emission after the initial flux peak of the outburst. Compared to one-year time-scale QPOs, previously identified in blazars in Fermi energies, PKS 2247-131 exhibits the first clear case of a relatively short, month-like oscillation. We show that this QPO can be explained in terms of a helical structure in the jet, where the viewing angle to the dominant emission region in the jet undergoes periodic changes. The time scale of the QPO suggests the presence of binary supermassive black holes in PKS 2247-131.
We have examined 40 Nuclear Spectroscopic Telescope Array (NuSTAR) light curves (LCs) of five TeV emitting high synchrotron peaked blazars: 1ES 0229+200, Mrk 421, Mrk 501, 1ES 1959+650, and PKS ...2155−304. Four of the blazars showed intraday variability in the NuSTAR energy range of 3-79 keV. Using an autocorrelation function analysis we searched for intraday variability timescales in these LCs and found indications of several between 2.5 and 32.8 ks in eight LCs of Mrk 421, a timescale around 8.0 ks for one LC of Mrk 501, and timescales of 29.6 and 57.4 ks in two LCs of PKS 2155-304. The other two blazars' LCs do not show any evidence for intraday variability timescales shorter than the lengths of those observations; however, the data were both sparser and noisier for them. We found positive correlations with zero lag between soft (3-10 keV) and hard (10-79 keV) bands for most of the LCs, indicating that their emissions originate from the same electron population. We examined spectral variability using a hardness ratio analysis and noticed a general "harder-when-brighter" behavior. The 22 LCs of Mrk 421 observed between 2012 July and 2013 April show that this source was in a quiescent state for an extended period of time and then underwent an unprecedented double-peaked outburst while monitored on a daily basis during 2013 April 10-16. We briefly discuss models capable of explaining these blazar emissions.
Abstract
We have analyzed the optical light curves of the blazar OJ 287 obtained with the Transiting Exoplanet Survey Satellite over about 80 days from 2021 October 13 to December 31, with an ...unprecedented sampling of 2 minutes. Although significant variability has been found during the entire period, we have detected two exceptional flares with flux nearly doubling and then nearly tripling over 2 days in the middle of 2021 November. We went through the light-curve analysis using the excess variance, generalized Lomb–Scargle periodogram, and continuous autoregressive moving average methods and estimated the flux halving/doubling timescales. The most probable shortest variability timescale was found to be 0.38 days in the rising phase of the first flare. We briefly discuss some emission models for the variability in radio-loud active galactic nuclei that could be capable of producing such fast flares.
ABSTRACT
We detected a possible quasi-periodic oscillation (QPO) of ∼71 d in the 0.1–300 GeV γ-ray Fermi-Large Area Telescope light curve of the high-redshift flat spectrum radio quasar B2 1520+31. ...We identify and confirm that quasi-period by Lomb–Scargle periodogram and weighted wavelet Z-transform analyses. Using this QPO period, and assuming it originates from accretion disc fluctuations at the innermost stable circular orbit, we estimate the central supermassive black hole mass to range between ∼5.4 × 109 M⊙ for a non-rotating black hole and ∼3.4 × 1010 M⊙ for a maximally rotating black hole. We briefly discuss other possible radio-loud active galactic nuclei emission models capable of producing a γ-ray QPO of such a period in a blazar.
ABSTRACT
In a previous paper, we reported evidence for quasi-periodicities in the Transiting Exoplanet Survey Satellite (TESS) light curves of BL Lacerate and two other blazars found serendipitously ...in the Sloan Digital Sky Survey (SDSS) active galactic nuclei catalogue. In this work, we find tentative evidence for quasi-periodic features in the TESS observations of five sources in the fourth catalogue of the Fermi–Large Area Telescope sources: J090453.4−573503, J2345−1555, B0422+004, J002159.2−514028, and B0537−441. We analysed the TESS light curves of these blazars that we extracted using a customized approach. The quasi-periodic oscillations (QPOs) are searched for using two timing analysis techniques: generalized Lomb–Scargle periodogram and weighted wavelet Z-transform. Their apparent periods lie in the range of 2.8–6.5 d and have at least 3σ significance in both of these methods. QPOs at such time-scales can originate from the kink instability model which relates the quasi-periodic feature with the growth of kinks in the magnetized relativistic jets. We performed Markov Chain Monte Carlo simulations to obtain the posterior distribution of parameters associated with this model and found the kink period consistent with previous studies.
ABSTRACT
We report the detection of a probable γ-ray quasi-periodic oscillation (QPO) of around 314 d in the monthly binned 0.1–300 GeV γ-ray Fermi-Large Area Telescope light curve of the well-known ...BL Lacertae blazar OJ 287. To identify and quantify the QPO nature of the γ-ray light curve of OJ 287, we used the Lomb–Scargle periodogram (LSP), REDFIT, and weighted wavelet Z-transform (WWZ) analyses. We briefly discuss possible emission models for radio-loud active galactic nuclei that can explain a γ-ray QPO of such a period in a blazar.
ABSTRACT
We report the time series analysis of Transiting Exoplanet Survey Satellite light curves of three blazars, BL Lacertae (BL Lac), 1RXS J111741.0+254858, and 1RXS J004519.6+212735, obtained ...using a customized approach for extracting active galactic nucleus light curves. We find tentative evidence for quasi-periodic oscillations (QPOs) in these light curves that range from 2 to 6 d. Two methods of analysis are used for assessing their significance: generalized Lomb–Scargle periodograms and weighted wavelet Z-transforms. The different approaches of these methods together ensure a robust measurement of the significance of the claimed periodicities. We can attribute the apparent QPOs to the kink instability model, which postulates that the observed QPOs are related to the temporal growth of kinks in the magnetized relativistic jet. We confirm the application of this model to BL Lac and extend the kink instability model to the other two BL Lac objects.
Abstract Blazars are characterized by largely aperiodic variability on timescales ranging from minutes to decades across the electromagnetic spectrum. The Transiting Exoplanet Survey Satellite (TESS) ...mission provides continuous sampling of blazar variability on timescales ranging from tens of minutes to 27 days for a single sector observation. Proper removal of the background, thermal ramping, and onboard systematic effects are crucial to the extraction of a reliable blazar light curve. Multiple publicly available procedures have been created to correct for these effects. Using ground-based observations from the Zwicky Transit Facility (ZTF) and the Asteroid Terrestrial-impact Last Alert System (ATLAS) as “ground truth” observations, we compare six different methods (regression, cotrending basis vectors, pixel-level decorrelation, eleanor , quaver , and simple differential photometry) to each other, and to our “ground truth” observations, to identify which methods properly correct light curves of a sample of 11 bright blazars, including the suspected neutrino source TXS 0506+056. In addition to comparing the resulting light curves, we compare the slopes of the power spectral densities, perform least-square fitting to simultaneous ZTF data, and quantify other statistical qualities generated from the light curves of each method. We find that only three of the six methods compared (simple differential photometry, eleanor , and quaver ) produce TESS light curves consistent with the ground-based ZTF and ATLAS observations.
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