scholarly journals Complex variability of Kepler AGN revealed by recurrence analysis

2020 ◽  
Vol 497 (3) ◽  
pp. 3418-3439 ◽  
Author(s):  
R A Phillipson ◽  
P T Boyd ◽  
A P Smale ◽  
M S Vogeley
Keyword(s):  
Time Series ◽  
Light Curve ◽  
Time Scales ◽  
Power Spectra ◽  
Periodic Oscillation ◽  
Light Curves ◽  
Recurrence Analysis ◽  
Spectra Analysis ◽  
Modes Of Variability ◽  
Underlying Processes

ABSTRACT The advent of new time domain surveys and the imminent increase in astronomical data expose the shortcomings of traditional time series analysis (such as power spectra analysis) in characterizing the abundantly varied, complex, and stochastic light curves of Active Galactic Nuclei (AGNs). Recent applications of novel methods from non-linear dynamics have shown promise in characterizing higher modes of variability and time-scales in AGN. Recurrence analysis in particular can provide complementary information about characteristic time-scales revealed by other methods, as well as probe the nature of the underlying physics in these objects. Recurrence analysis was developed to study dynamical trajectories in phase space, which can be constructed from 1D time series such as light curves. We apply the methods of recurrence analysis to two optical light curves of Kepler-monitored AGN. We confirm the detection and period of an optical quasi-periodic oscillation in one AGN, and confirm multiple other time-scales recovered from other methods ranging from 5 to 60 d in both objects. We detect regions in the light curves that deviate from regularity, provide evidence of determinism and non-linearity in the mechanisms underlying one light curve (KIC 9650712), and determine realizations of a linear stochastic process describe the dominant variability in the other light curve (Zwicky 229–015). We discuss possible underlying processes driving the dynamics of the light curves and their diverse classes of variability.

scholarly journals Periodicities in the K2 light curve of HP Librae

2020 ◽  
Vol 500 (1) ◽  
pp. 1222-1230
Author(s):  
Siddhant Solanki ◽  
Thomas Kupfer ◽  
Omer Blaes ◽  
Elmé Breedt ◽  
Simone Scaringi
Keyword(s):  
Light Curve ◽  
Power Spectra ◽  
Periodic Oscillation ◽  
Light Curves ◽  
Precession Frequency ◽  
Frequency Range ◽  
Quasi Periodic Oscillation ◽  
Rates Of Change ◽  
Orbital Frequency ◽  
The Continuum

ABSTRACT We analyse Kepler/K2 light-curve data of the AM CVn system HP Librae (HP Lib). We detect with confidence four photometric periodicities in the system: the orbital frequency, both positive and negative superhumps, and the positive apsidal precession frequency of the accretion disc. This is only the second time that the apsidal precession frequency has ever been directly detected in the photometry of a helium accreting system, after SDSS J135154.46-064309.0. We present phase-folded light curves and sliding power spectra of each of the four periodicities. We measure rates of change of the positive superhump period of ∼10−7 d. We also redetect a quasi-periodic oscillation (QPO) at ∼300 cyc d–1, a feature that has been stable over decades, and show that it is harmonically related to two other QPOs, the lowest of which is centred on the superhump/orbital frequency. The continuum power spectrum is consistent with a single power law with no evidence of any breaks within our observed frequency range.

scholarly journals A fractal analysis application of the pre-whitening technique to δ Scuti stars time series

2019 ◽  
Vol 487 (3) ◽  
pp. 4457-4463 ◽  
Author(s):  
S de Franciscis ◽  
J Pascual-Granado ◽  
J C Suárez ◽  
A García Hernández ◽  
R Garrido ◽  
...  
Keyword(s):  
Time Series ◽  
Power Spectra ◽  
Coarse Graining ◽  
Light Curves ◽  
Pulsating Stars ◽  
Oscillation Modes ◽  
Analysis Application ◽  
New Criterion ◽  
Scuti Stars ◽  
Δ Scuti Stars

ABSTRACT Fractal fingerprints have been found recently in the light curves of several δ Scuti stars observed by Convection Rotation and planetary Transits(CoRoT) satellite. This sole fact might pose a problem for the detection of pulsation frequencies using classical pre-whitening techniques, but it is also a potentially rich source for information about physical mechanisms associated with stellar variability. Assuming that a light curve is composed of a superposition of oscillation modes with a fractal background noise, in this work we applied the Coarse Graining Spectral Analysis (CGSA), a fast Fourier transform (FFT)-based algorithm, which can discriminate in a time series the stochastic fractal power spectra from the harmonic one. We have found that the fractal background component is determining the frequency content extracted using classical pre-whitening techniques in the light curves of δ Scuti stars. This might be crucial to understand the amount of frequencies excited in these kinds of pulsating stars. Additionally, CGSA resulted to be relevant in order to extract the oscillation modes, this points to a new criterion to stop the pre-whitening cascade based on the percentage of fractal component in the residuals.

scholarly journals Optical, X-ray, and γ-ray observations of the candidate transitional millisecond pulsar 4FGL J0427.8-6704

2020 ◽  
Vol 494 (3) ◽  
pp. 3912-3926
Author(s):  
M R Kennedy ◽  
R P Breton ◽  
C J Clark ◽  
V S Dhillon ◽  
M Kerr ◽  
...  
Keyword(s):  
Light Curve ◽  
Time Scales ◽  
Periodic Oscillation ◽  
Optical Data ◽  
Data Sets ◽  
Millisecond Pulsar ◽  
X Ray ◽  
State Classification ◽  
Optical Light Curve ◽  
Γ Ray

ABSTRACT We present an optical, X-ray, and γ-ray study of 1SXPS J042749.2-670434, an eclipsing X-ray binary that has an associated γ-ray counterpart, 4FGL J0427.8-6704. This association has led to the source being classified as a transitional millisecond pulsar (tMSP) in an accreting state. We analyse 10.5 yr of Fermi LAT data and detect a γ-ray eclipse at the same phase as optical and X-ray eclipses at the >5 σ level, a significant improvement on the 2.8 σ level of the previous detection. The confirmation of this eclipse solidifies the association between the X-ray source and the γ-ray source, strengthening the tMSP classification. However, analysis of several optical data sets and an X-ray observation do not reveal a change in the source’s median brightness over long time-scales or a bi-modality on short time-scales. Instead, the light curve is dominated by flickering, which has a correlation time of 2.6 min alongside a potential quasi-periodic oscillation at ∼21 min. The mass of the primary and secondary stars is constrained to be $M_1=1.43^{+0.33}_{-0.19}$ M⊙ and $M_2=0.3^{+0.17}_{-0.12}$ M⊙ through modelling of the optical light curve. While this is still consistent with a white dwarf primary, we favour the tMSP in a low accretion state classification due to the significance of the γ-ray eclipse detection.

scholarly journals TESS photometry of extreme helium stars PV Tel and V821 Cen

2020 ◽  
Vol 495 (1) ◽  
pp. L135-L138 ◽  
Author(s):  
C Simon Jeffery ◽  
Geert Barentsen ◽  
Gerald Handler
Keyword(s):  
Light Curve ◽  
Time Scales ◽  
Characteristic Time ◽  
Satellite Observations ◽  
Light Curves ◽  
Single Site ◽  
Helium Stars ◽  
First Time

ABSTRACT PV Tel variables are extreme helium (EHe) stars known to be intrinsic light and velocity variable on characteristic time-scales of 0.1–25 d. With two exceptions, they are best described as irregular. Light curves have invariably been obtained from single-site terrestrial observatories. We present Transiting Exoplanet Survey Satellite observations of two bright EHe stars, Popper’s star (V821 Cen) and Thackeray’s star (PV Tel). PV Tel is variable on time-scales previously reported. V821 Cen is proven to be variable for the first time. Neither light curve shows any evidence of underlying regularity. Implications are considered.

scholarly journals Possible ~1 hour quasi-periodic oscillation in narrow-line Seyfert 1 galaxy MCG–06–30–15

2018 ◽  
Vol 616 ◽  
pp. L6 ◽  
Author(s):  
Alok C. Gupta ◽  
Ashutosh Tripathi ◽  
Paul J. Wiita ◽  
Minfeng Gu ◽  
Cosimo Bambi ◽  
...  
Keyword(s):  
Light Curve ◽  
Energy Range ◽  
Periodic Oscillation ◽  
Light Curves ◽  
Narrow Line ◽  
Quasi Periodic Oscillation ◽  
X Ray ◽  
S Period

We found a possible ~1 h quasi-periodic oscillation (QPO) in a ~55 ks X-ray observation of the narrow-line Seyfert 1 galaxy MCG–06–30–15 made with the XMM-Newton EPIC/pn detector in the energy range 0.3–10 keV. We identify a total modulation of ~16% in the light curve and find a ≃3670 s quasi-period using Lomb-Scargle periodogram (LSP) and weighted wavelet Z-transform (WWZ) techniques. Our analyses of eight light curves of MCG–06–30–15, indicated the possible presence of an oscillation during one of them. The LSP indicates a statistically significant (≃3σ) QPO detection. A WWZ analysis shows that the signal at this possible roughly 3670 s period is present, and rather persistent, throughout the observation; however, a signal around 8735 s is more persistent. We briefly discuss models that can produce X-ray QPOs with such periods in narrow line Seyfert 1 galaxies, as both other claimed QPO detections in this class of AGN had very similar periods.

scholarly journals The dual nature of blazar fast variability: Space and ground observations of S5 0716+714

2020 ◽  
Vol 501 (1) ◽  
pp. 1100-1115
Author(s):  
C M Raiteri ◽  
M Villata ◽  
D Carosati ◽  
E Benítez ◽  
S O Kurtanidze ◽  
...  
Keyword(s):  
Light Curve ◽  
Time Scales ◽  
Light Curves ◽  
Spectral Variability ◽  
Short Term ◽  
Dual Nature ◽  
X Ray ◽  
Geometric Origin ◽  
Optical Light Curve ◽  
Doppler Factor

ABSTRACT Blazar S5 0716+714 is well-known for its short-term variability, down to intraday time-scales. We here present the 2-min cadence optical light curve obtained by the TESS space telescope in 2019 December–2020 January and analyse the object fast variability with unprecedented sampling. Supporting observations by the Whole Earth Blazar Telescope Collaboration in B, V, R, and I bands allow us to investigate the spectral variability during the TESS pointing. The spectral analysis is further extended in frequency to the UV and X-ray bands with data from the Neil Gehrels Swift Observatory. We develop a new method to unveil the shortest optical variability time-scales. This is based on progressive de-trending of the TESS light curve by means of cubic spline interpolations through the binned fluxes, with decreasing time bins. The de-trended light curves are then analysed with classical tools for time-series analysis (periodogram, autocorrelation, and structure functions). The results show that below 3 d there are significant characteristic variability time-scales of about 1.7, 0.5, and 0.2 d. Variability on time-scales $\lesssim 0.2$ d is strongly chromatic and must be ascribed to intrinsic energetic processes involving emitting regions, likely jet substructures, with dimension less than about 10−3 pc. In contrast, flux changes on time-scales $\gtrsim 0.5$ d are quasi-achromatic and are probably due to Doppler factor changes of geometric origin.

scholarly journals Variability characteristics of Blazar 0J 287

1996 ◽  
Vol 175 ◽  
pp. 45-46
Author(s):  
L.O. Takalo ◽  
A. Sillanpää ◽  
T. Pursimo ◽  
H.J. Lehto ◽  
K. Nilsson ◽  
...  
Keyword(s):  
Light Curve ◽  
Time Scales ◽  
Light Curves ◽  
Data Archive ◽  
Extragalactic Object ◽  
Optical Light Curve ◽  
Blazar Oj 287

Blazar OJ 287 is one of the best observed extragalactic objects. It's historical light curve goes back to 1890′s. Based on the historical behaviour Sillanpää et al. (1988) showed that OJ 287 displays large periodic outbursts, with a period of 11.7 years. We have monitored OJ 287 intensively for two years, during the OJ-94 project. This project was created for monitoring OJ 287 during its predicted new outburst in 1994. In the data archive we have over 7000 observations on OJ 287, in the radio, infrared and optical bands. This data archive contains the best ever obtained light curves for any extragalactic object. The optical light curve shows continuous variability down to time scales of tens of minutes. The variability observed in OJ 287 can be broken down to (at least) four different categories:

PNN NGC 246: A Complex Photometric Behaviour That Requires Wet

2003 ◽  
Vol 12 (1) ◽  
Author(s):  
J. M. González Pérez ◽  
J.-E. Solheim ◽  
T. N. Dorokhova ◽  
N. I. Dorokhov
Keyword(s):  
Wavelet Analysis ◽  
Light Curve ◽  
Time Scales ◽  
Light Curves ◽  
Single Site ◽  
Harmonic Structure ◽  
Peculiar Characteristic

AbstractWe present a study over three single-site campaigns to investigate the photometric behaviour of the PNN NGC 246. We observed this object in 2000 and 2001. The analysis of the light curves indicates complex and variable temporal spectra. Using wavelet analysis we have found evidences for changes on time scales of hours in the 2000 dataset. The temporal spectra obtained during 2001 are quite different from the results of the previous year. The modulations in the light curve are more noticeable and the temporal spectra present a higher number of modulation frequencies. One peculiar characteristic is the presence of a variable harmonic structure related to one of these modulation frequencies. This complex photometric behaviour may be explained by a more complicated unresolved combination of modulation frequencies, but more likely due to a combination of pulsations of the star plus modulations related to interaction with a close companion, maybe indicating a disc. However, these characteristics cannot be confirmed from single site observations. The complex and variable behaviour of NGC 246 needs the WET co-operation in order to completely resolve its light curve.

scholarly journals What does FRB light-curve variability tell us about the emission mechanism?

2020 ◽  
Vol 498 (1) ◽  
pp. 651-664 ◽  
Author(s):  
Paz Beniamini ◽  
Pawan Kumar
Keyword(s):  
Light Curve ◽  
Time Scales ◽  
Production Efficiency ◽  
Compact Object ◽  
Light Curves ◽  
Radio Bursts ◽  
Intrinsic Variability ◽  
Temporal Correlations ◽  
Small Ratio ◽  
Powerful Radio Emission

ABSTRACT A few fast radio bursts’ (FRBs) light curves have exhibited large intrinsic modulations of their flux on extremely short ($t_{\rm r}\sim 10\, \mu$s) time-scales, compared to pulse durations (tFRB ∼ 1 ms). Light-curve variability time-scales, the small ratio of rise time of the flux to pulse duration, and the spectro-temporal correlations in the data constrain the compactness of the source and the mechanism responsible for the powerful radio emission. The constraints are strongest when radiation is produced far (≳1010 cm) from the compact object. We describe different physical set-ups that can account for the observed tr/tFRB ≪ 1 despite having large emission radii. The result is either a significant reduction in the radio production efficiency or distinct light-curve features that could be searched for in observed data. For the same class of models, we also show that due to high-latitude emission, if a flux f1(ν1) is observed at t1 then at a lower frequency ν2 < ν1 the flux should be at least (ν2/ν1)2f1 at a slightly later time (t2 = t1ν1/ν2) independent of the duration and spectrum of the emission in the comoving frame. These features can be tested, once light-curve modulations due to scintillation are accounted for. We provide the time-scales and coherence bandwidths of the latter for a range of possibilities regarding the physical screens and the scintillation regime. Finally, if future highly resolved FRB light curves are shown to have intrinsic variability extending down to ${\sim}\mu$s time-scales, this will provide strong evidence in favour of magnetospheric models.

scholarly journals Burrell-Optical-Kepler Survey (BOKS): A Variability Search in the Kepler Field

2008 ◽  
Vol 4 (S253) ◽  
pp. 416-419
Author(s):  
Amanda L. Proctor ◽  
Steve B. Howell ◽  
William H. Sherry ◽  
Kaspar von Braun ◽  
Mark E. Everett ◽  
...  
Keyword(s):  
Time Series ◽  
Light Curve ◽  
Light Curves ◽  
Field Of View ◽  
Schmidt Telescope ◽  
Photometric Observations ◽  
Mission Field

AbstractWe present the results of eighteen non-continuous nights of time series photometric observations of a 1.25 deg2 field in Cygnus centered on the NASA Kepler Mission field of view. Using the Case Western Burrell Schmidt telescope we gathered a dataset containing light curves of roughly 30,000 stars with 14 < r < 19. We have statistically examined each light curve to test for variability, periodicity, and unusual light curve trends, including exoplanet transits. We present a summary of our photometric project including a characterization of the level and content of stellar variability in this field. We will also discuss our potential exoplanet candidates.

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