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Register a new userMulticomponent Power Density Spectra of Kepler AGNs, an instrumental artifact or a physical origin?
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We analysed the light curves of four active galactic nuclei (AGN) from the Kepler field, and find multicomponent power density spectra with characteristic frequencies that are surprisingly similar to other Kepler AGNs (including ZW229-15). An identical time series analysis of randomly selected planet candidate stars revealed the same features, suggesting an instrumental origin for the variability. This result is enigmatic, as these signals have been confirmed for ZW229-15 using independent observations from Swift. Based on our re-analysis of these Swift data and test simulations, we now distinguish the instrumental artifact in Kepler data from the real pattern in Swift observations. It appears that some other AGNs observed with instruments such as XMM-Newton show similar frequency components. This supports the conclusion that the similarity between the variability timescales of the Kepler artifact and real Swift features is coincidental.
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Many nearby AGNs display a significant short-term variability. In this work we re-analyze photometric data of four active galactic nuclei observed by Kepler in order to study the flickering activity, having as main goal that of searching for multiple components in the power density spectra. We find that all four objects have similar characteristics, with two break frequencies at approximately log(f/Hz)=-5.2 and -4.7. We consider some physical phenomena whose characteristic time-scales are consistent with those observed, in particular mass accretion fluctuations in the inner geometrically thick disc (hot X-ray corona) and unstable relativistic Rayleigh-Taylor modes. The former is supported by detection of the same break frequencies in the Swift X-ray data of ZW229-15. We also discuss rms-flux relations, and we detect a possible typical linear trend at lower flux levels. Our findings support the hypothesis of a multiplicative character of variability, in agreement with the propagating accretion fluctuation model.
The high quality light curves of Kepler space telescope make it possible to analyze the optical variability of AGNs with an unprecedented time resolution. Studying the asymmetry in variations could give independent constraints on the physical models for AGN variability. In this paper, we use Kepler observations of 19 sources to perform analyses on the variability asymmetry of AGNs. We apply smoothing-correction to light curves to deduct the bias to high frequency variability asymmetry, caused by long term variations which are poorly sampled due to the limited length of light curves. A parameter $beta$ based on structure functions is introduced to quantitively describe the asymmetry and its uncertainty is measured using extensive Monte-Carlo simulations. Individual sources show no evidence of asymmetry at timescales of $1sim20$ days and there is not a general trend toward positive or negative asymmetry over the whole sample. Stacking data of all 19 AGNs, we derive averaged $bar{beta}$ of 0.00$pm$0.03 and -0.02$pm$0.04 over timescales of 1$sim$5 days and 5$sim$20 days, respectively, statistically consistent with zero. Quasars and Seyfert galaxies show similar asymmetry parameters. Our results indicate that short term optical variations in AGNs are highly symmetric.
Supernova remnants (SNRs) have a variety of overall morphology as well as rich structures over a wide range of scales. Quantitative study of these structures can potentially reveal fluctuations of density and magnetic field originating from the interaction with ambient medium and turbulence in the expanding ejecta. We have used $1.5$GHz (L band) and $5$GHz (C band) VLA data to estimate the angular power spectrum $C_{ell}$ of the synchrotron emission fluctuations of the Kepler SNR. This is done using the novel, visibility based, Tapered Gridded Estimator of $C_{ell}$. We have found that, for $ell = (1.9 - 6.9) times 10^{4}$, the power spectrum is a broken power law with a break at $ell = 3.3 times 10^{4}$, and power law index of $-2.84pm 0.07$ and $-4.39pm 0.04$ before and after the break respectively. The slope $-2.84$ is consistent with 2D Kolmogorov turbulence and earlier measurements for the Tycho SNR. We interpret the break to be related to the shell thickness of the SNR ($0.35 $ pc) which approximately matches $ell = 3.3 times 10^{4}$ (i.e., $0.48$ pc). However, for $ell > 6.9 times 10^{4}$, the estimated $C_{ell}$ of L band is likely to have dominant contribution from the foregrounds while for C band the power law slope $-3.07pm 0.02$ is roughly consistent with $3$D Kolmogorov turbulence like that observed at large $ell$ for Cas A and Crab SNRs.
The Event Horizon Telescope (EHT) has mapped the central compact radio source of the elliptical galaxy M87 at 1.3 mm with unprecedented angular resolution. Here we consider the physical implications of the asymmetric ring seen in the 2017 EHT data. To this end, we construct a large library of models based on general relativistic magnetohydrodynamic (GRMHD) simulations and synthetic images produced by general relativistic ray tracing. We compare the observed visibilities with this library and confirm that the asymmetric ring is consistent with earlier predictions of strong gravitational lensing of synchrotron emission from a hot plasma orbiting near the black hole event horizon. The ring radius and ring asymmetry depend on black hole mass and spin, respectively, and both are therefore expected to be stable when observed in future EHT campaigns. Overall, the observed image is consistent with expectations for the shadow of a spinning Kerr black hole as predicted by general relativity. If the black hole spin and M87s large scale jet are aligned, then the black hole spin vector is pointed away from Earth. Models in our library of non-spinning black holes are inconsistent with the observations as they do not produce sufficiently powerful jets. At the same time, in those models that produce a sufficiently powerful jet, the latter is powered by extraction of black hole spin energy through mechanisms akin to the Blandford-Znajek process. We briefly consider alternatives to a black hole for the central compact object. Analysis of existing EHT polarization data and data taken simultaneously at other wavelengths will soon enable new tests of the GRMHD models, as will future EHT campaigns at 230 and 345 GHz.
The $textit{Kepler}$ satellite potentially provides the highest precision photometry of active galactic nuclei (AGN) available to investigate short-timescale optical variability. We targeted quasars from the Sloan Digital Sky Survey that lie in the fields of view of the $textit{Kepler/K2}$ campaigns. Based on those observations, we report the discovery and properties of a previously unidentified instrumental signature in K2. Systematic errors in K2, beyond those due to the motion of the detector, plague our AGN and other faint-target, guest-observer science proposals. Weakly illuminated pixels are dominated by low frequency trends that are both non-astrophysical and correlated from object to object. A critical clue to understanding this instrumental noise is that different targets observed in the same channels of Campaign 8 (rear facing) and Campaign 16 (forward facing) had nearly identical light curves after time reversal of one of the campaigns. This observation strongly suggests that the underlying problem relates to the relative Sun-spacecraft-field orientation, which was approximately the same on day 1 of Campaign 8 as the last day of Campaign 16. Furthermore, we measure that the instrumental signature lags in time as a function of radius from the center of the detector, crossing channel boundaries. Systematics documented in this investigation are unlikely to be due to Moir{e} noise, rolling band, or pointing jitter. Instead this work strongly suggests temperature-dependent focus changes that are further subject to channel variations. Further characterization of this signature is crucial for rehabilitating K2 data for use in investigations of AGN light curves.
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