scholarly journals Modelling X-ray RMS spectra I: intrinsically variable AGNs

2019 ◽  
Vol 492 (1) ◽  
pp. 1363-1369 ◽  
Author(s):  
M L Parker ◽  
W N Alston ◽  
Z Igo ◽  
A C Fabian
Keyword(s):  
Power Law ◽  
Low Frequency ◽  
Galactic Nuclei ◽  
Physical Parameters ◽  
Intrinsic Variability ◽  
High Frequencies ◽  
Low Frequency Variability ◽  
Variable Power ◽  
Variability Model ◽  
Variance Spectrum

ABSTRACT We present simple xspec models for fitting excess variance spectra of active galactic nuclei. Using a simple Monte Carlo approach, we simulate a range of spectra corresponding to physical parameters varying, then calculate the resulting variance spectra. Starting from a variable power law, we build up a set of models corresponding to the different physical processes that can affect the final excess variance spectrum. We show that the complex excess variance spectrum of IRAS 13224−3809 can be well described by such an intrinsic variability model, where the power-law variability is damped by relativistic reflection and enhanced by an ultra-fast outflow. The reflection flux is correlated with that of the power law, but not perfectly. We argue that this correlation is stronger at high frequencies, where reverberation lags are detected, while excess variance spectra are typically dominated by low-frequency variability.

scholarly journals The nature of the extreme X-ray variability in the NLS1 1H 0707-495

2021 ◽  
Author(s):  
M L Parker ◽  
W N Alston ◽  
L Härer ◽  
Z Igo ◽  
A Joyce ◽  
...  
Keyword(s):  
High Frequency ◽  
Low Frequency ◽  
Emission Lines ◽  
Intrinsic Variability ◽  
Low Velocity ◽  
Strong Suppression ◽  
X Ray ◽  
Low Frequency Variability ◽  
High Frequency Variability ◽  
Long Timescales

Abstract We examine archival XMM-Newton data on the extremely variable narrow-line Seyfert 1 (NLS1) active galactic nucleus (AGN) 1H 0707-495. We construct fractional excess variance (Fvar) spectra for each epoch, including the recent 2019 observation taken simultaneously with eROSITA. We explore both intrinsic and environmental absorption origins for the variability in different epochs, and examine the effect of the photoionised emission lines from outflowing gas. In particular, we show that the unusual soft variability first detected by eROSITA in 2019 is due to a combination of an obscuration event and strong suppression of the variance at 1 keV by photoionised emission, which makes the variance below 1 keV appear more extreme. We also examine the variability on long timescales, between observations, and find that it is well described by a combination of intrinsic variability and absorption variability. We suggest that the typical extreme high frequency variability which 1H 0707-495 is known for is intrinsic to the source, but the large amplitude, low frequency variability that causes prolonged low-flux intervals is likely dominated by variable low-ionisation, low velocity absorption.

Interdecadal Variability of the Southern Ocean

2006 ◽  
Vol 36 (8) ◽  
pp. 1626-1645 ◽  
Author(s):  
Andrew Mc C. Hogg ◽  
Jeffrey R. Blundell
Keyword(s):  
Baroclinic Instability ◽  
Low Frequency ◽  
Driving Mechanism ◽  
Intrinsic Variability ◽  
Low Frequency Variability ◽  
Energetic Balance ◽  
Explicit Simulation ◽  
Circumpolar Current ◽  
Mean Circulation ◽  
The Antarctic

Abstract The intrinsic variability of the Antarctic Circumpolar Current is investigated using an idealized wind-driven model. The model uses three quasigeostrophic layers, with steady wind stress forcing, and no diabatic effects. Despite the idealized nature of the model, the simulations display a robust mode of low-frequency variability in the flow. It is demonstrated that this variability is dependent upon the explicit simulation of the dynamics of mesoscale eddies. As such, the variability is sensitive to stratification, horizontal viscosity, bottom stress, and topography. The energetic balance of the variability is diagnosed, and a driving mechanism is proposed that involves positive feedback between the generation of eddies through baroclinic instability and the dynamics of the mean circulation.

Variable Sources at 408 MHz

1980 ◽  
Vol 4 (1) ◽  
pp. 70-72 ◽  
Author(s):  
W. B. McAdam
Keyword(s):  
Flux Density ◽  
High Frequency ◽  
Monitoring Program ◽  
Low Frequency ◽  
Stability Limit ◽  
High Frequencies ◽  
Low Frequency Variability ◽  
The Stability ◽  
Many Sources

Flux density changes at low frequency in extragalactic sources were discovered in 1969 by Hunstead (1972a) and this low frequency variability has since been confirmed in the range 318 to 962 MHz at five other observatories. The changes are usually small and near the stability limit of most telescopes, but occasionally large changes ~ 30% are found. Many sources show changes at both low and high frequencies, but these appear uncorrelated even with delay lags of several years, and the likelihood of low frequency variability in a source cannot be predicted from high frequency behaviour. This paper reports statistics on the proportion of sources that showed changes of different amplitudes during a monitoring program with the Molonglo Cross in 1975/76.

scholarly journals The Representation of Atmospheric Blocking and the Associated Low-Frequency Variability in Two Seasonal Prediction Systems

2014 ◽  
Vol 27 (24) ◽  
pp. 9082-9100 ◽  
Author(s):  
Panos J. Athanasiadis ◽  
Alessio Bellucci ◽  
Leon Hermanson ◽  
Adam A. Scaife ◽  
Craig MacLachlan ◽  
...  
Keyword(s):  
North Atlantic ◽  
Large Scale ◽  
Intraseasonal Variability ◽  
Low Frequency ◽  
Seasonal Prediction ◽  
Intrinsic Variability ◽  
The North ◽  
Low Frequency Variability ◽  
The North Atlantic Oscillation ◽  
Office System

Abstract Primarily as a response to boundary forcings, certain components of the atmospheric intraseasonal variability are potentially predictable. Particularly referring to the extratropics, the current generation of seasonal forecasting systems is making advancements in predicting these components by realistically initializing many components of the climate system, using higher resolution and utilizing large ensemble sizes. The operational seasonal prediction system of the Met Office (UKMO) and the corresponding system of the Centro Euro-Mediterraneo sui Cambiamenti Climatici (CMCC) are analyzed in terms of their representation of different aspects of extratropical low-frequency variability. The UKMO system achieves unprecedented high scores in predicting the winter mean phase of the North Atlantic Oscillation (NAO; correlation 0.62) and the Pacific–North American pattern (PNA; correlation 0.82). The CMCC system, despite its smaller ensemble size and coarser resolution, also exhibits significant skill (0.42 for NAO, 0.51 for PNA). Low-frequency variability is underrepresented in both models, particularly in the eastern North Atlantic. Consequently, their intrinsic variability patterns (sectoral EOFs) are somewhat different from the observed patterns. Regarding the representation of wintertime Northern Hemisphere blocking, after bias correction both systems exhibit a realistic climatology of blocking frequency. In this assessment, instantaneous blocking and large-scale persistent blocking events are identified using daily geopotential height fields at 500 hPa. The blocking signature on the circulation and the dependence of blocking frequency on the NAO are also quite realistic for both systems. Finally, the Met Office system exhibits significant skill in predicting the winter mean frequency of blocking that relates to the NAO.

scholarly journals Nonlinear stratospheric variability: multifractal de-trended fluctuation analysis and singularity spectra

2016 ◽  
Vol 472 (2191) ◽  
pp. 20150864 ◽  
Author(s):  
Gualtiero Badin ◽  
Daniela I. V. Domeisen
Keyword(s):  
Time Scales ◽  
Power Law ◽  
Scaling Laws ◽  
Low Frequency ◽  
Small Scale ◽  
Fluctuation Analysis ◽  
Scaling Exponent ◽  
Low Frequency Variability ◽  
Generalized Hurst Exponent ◽  
Stratospheric Variability

Characterizing the stratosphere as a turbulent system, temporal fluctuations often show different correlations for different time scales as well as intermittent behaviour that cannot be captured by a single scaling exponent. In this study, the different scaling laws in the long-term stratospheric variability are studied using multifractal de-trended fluctuation analysis (MF-DFA). The analysis is performed comparing four re-analysis products and different realizations of an idealized numerical model, isolating the role of topographic forcing and seasonal variability, as well as the absence of climate teleconnections and small-scale forcing. The Northern Hemisphere (NH) shows a transition of scaling exponents for time scales shorter than about 1 year, for which the variability is multifractal and scales in time with a power law corresponding to a red spectrum, to longer time scales, for which the variability is monofractal and scales in time with a power law corresponding to white noise. Southern Hemisphere (SH) variability also shows a transition at annual scales. The SH also shows a narrower dynamical range in multifractality than the NH, as seen in the generalized Hurst exponent and in the singularity spectra. The numerical integrations show that the models are able to reproduce the low-frequency variability but are not able to fully capture the shorter term variability of the stratosphere.

scholarly journals Jet Jumping: Low-Frequency Variability in the Southern Ocean

2013 ◽  
Vol 43 (5) ◽  
pp. 990-1003 ◽  
Author(s):  
Christopher C. Chapman ◽  
Andrew McC. Hogg
Keyword(s):  
Numerical Model ◽  
Plane Channel ◽  
Low Frequency ◽  
Mesoscale Eddies ◽  
Time Varying ◽  
Intrinsic Variability ◽  
Simple Mode ◽  
Low Frequency Variability ◽  
Eddy Fluxes ◽  
Model Variability

Abstract The authors study intrinsic variability in the position of jets in a β-plane channel ocean with simple topography using a quasigeostrophic numerical model. This study links the variability in jet position with abyssal anticyclones that form as a result of interaction of mesoscale eddies and subsurface topography, reminiscent of such flows as the Zapiola anticyclone. A simple dynamical framework explaining this behavior is developed. In this framework, this study shows that the topographic anticyclones form closed regions of homogenized yet time-varying potential vorticity. Neighboring topographic anticyclones are coupled by eddy fluxes. Interaction of a baroclinic jet with these two (or more) anticyclones can drive variability in local jet strength. Predictions of the dynamical framework are then compared with the results of the numerical model, and it is demonstrated that this model has merit in explaining the observed model variability. This study argues that this simple mode of variability has relevance for the ocean.

scholarly journals Rheology of rounded mammalian cells over continuous high-frequencies

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Gotthold Fläschner ◽  
Cosmin I. Roman ◽  
Nico Strohmeyer ◽  
David Martinez-Martin ◽  
Daniel J. Müller
Keyword(s):  
High Frequency ◽  
Mammalian Cells ◽  
Mechanical Response ◽  
Cell Mass ◽  
Low Frequency ◽  
Mass Measurements ◽  
High Frequencies ◽  
Polymer Relaxation ◽  
Continuous Frequency ◽  
Rheological Experiments

AbstractUnderstanding the viscoelastic properties of living cells and their relation to cell state and morphology remains challenging. Low-frequency mechanical perturbations have contributed considerably to the understanding, yet higher frequencies promise to elucidate the link between cellular and molecular properties, such as polymer relaxation and monomer reaction kinetics. Here, we introduce an assay, that uses an actuated microcantilever to confine a single, rounded cell on a second microcantilever, which measures the cell mechanical response across a continuous frequency range ≈ 1–40 kHz. Cell mass measurements and optical microscopy are co-implemented. The fast, high-frequency measurements are applied to rheologically monitor cellular stiffening. We find that the rheology of rounded HeLa cells obeys a cytoskeleton-dependent power-law, similar to spread cells. Cell size and viscoelasticity are uncorrelated, which contrasts an assumption based on the Laplace law. Together with the presented theory of mechanical de-embedding, our assay is generally applicable to other rheological experiments.

The GLEAM 4-Jy Sample: The brightest radio-sources in the southern sky

2019 ◽  
Vol 15 (S356) ◽  
pp. 375-375
Author(s):  
Sarah White
Keyword(s):  
Radio Emission ◽  
Active Galactic Nuclei ◽  
Visual Inspection ◽  
Low Frequency ◽  
Galactic Nuclei ◽  
Radio Sources ◽  
Relativistic Jets ◽  
The Galaxy ◽  
Murchison Widefield Array ◽  
Past Activity

AbstractLow-frequency radio emission allows powerful active galactic nuclei (AGN) to be selected in a way that is unaffected by dust obscuration and orientation of the jet axis. It also reveals past activity (e.g. radio lobes) that may not be evident at higher frequencies. Currently, there are too few “radio-loud” galaxies for robust studies in terms of redshift-evolution and/or environment. Hence our use of new observations from the Murchison Widefield Array (the SKA-Low precursor), over the southern sky, to construct the GLEAM 4-Jy Sample (1,860 sources at S151MHz > 4 Jy). This sample is dominated by AGN and is 10 times larger than the heavily relied-upon 3CRR sample (173 sources at S178MHz > 10 Jy) of the northern hemisphere. In order to understand how AGN influence their surroundings and the way galaxies evolve, we first need to correctly identify the galaxy hosting the radio emission. This has now been completed for the GLEAM 4-Jy Sample – through repeated visual inspection and extensive checks against the literature – forming a valuable, legacy dataset for investigating relativistic jets and their interplay with the environment.

scholarly journals Shift Detection in Hydrological Regimes and Pluriannual Low-Frequency Streamflow Forecasting Using the Hidden Markov Model

Water ◽  
2020 ◽  
Vol 12 (7) ◽  
pp. 2058 ◽  
Author(s):  
Larissa Rolim ◽  
Francisco de Souza Filho
Keyword(s):  
Time Series ◽  
Markov Model ◽  
Hidden Markov Model ◽  
Extreme Events ◽  
Hidden Markov ◽  
Low Frequency ◽  
Streamflow Forecasting ◽  
Hydrologic Time Series ◽  
Low Frequency Variability ◽  
The Impact

Improved water resource management relies on accurate analyses of the past dynamics of hydrological variables. The presence of low-frequency structures in hydrologic time series is an important feature. It can modify the probability of extreme events occurring in different time scales, which makes the risk associated with extreme events dynamic, changing from one decade to another. This article proposes a methodology capable of dynamically detecting and predicting low-frequency streamflow (16–32 years), which presented significance in the wavelet power spectrum. The Standardized Runoff Index (SRI), the Pruned Exact Linear Time (PELT) algorithm, the breaks for additive seasonal and trend (BFAST) method, and the hidden Markov model (HMM) were used to identify the shifts in low frequency. The HMM was also used to forecast the low frequency. As part of the results, the regime shifts detected by the BFAST approach are not entirely consistent with results from the other methods. A common shift occurs in the mid-1980s and can be attributed to the construction of the reservoir. Climate variability modulates the streamflow low-frequency variability, and anthropogenic activities and climate change can modify this modulation. The identification of shifts reveals the impact of low frequency in the streamflow time series, showing that the low-frequency variability conditions the flows of a given year.

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