Navegando por Autor "Belete, A. Bewketu"

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Cosmological evolution of quasar radio emission in the view of multifractality
(American Astronomical Society, 2019-03-11) Martins, Bruno Leonardo Canto; Belete, A. Bewketu; Femmam, Smain; Tornikosk, Merja; Lähteenmäki, Anne; Tammi, Joni; Leão, Izan de Castro; Medeiros, J. R. de
Variations in scaling behavior in the flux and emissions of distant astronomical sources with respect to their cosmic time are important phenomena that can provide valuable information about the dynamics within the sources and their cosmological evolution with time. Different studies have been applying linear analysis to understand and model quasars’ light curves. Here, we study the multifractal behavior of selected quasars’ radio emissions in their observed frame (at 22 and 37 GHz bands) and their rest frame. To this end, we apply the wavelet transform-based multifractal analysis formalism called wavelet transform modulus maxima. In addition, we verify whether the autoregressive integrated moving average (ARIMA) models fit our data. In our work, we observe strong multifractal behavior for all the sources. Additionally, we find that the degree of multifractality is strongly similar for each source and significantly different between sources at 22 and 37 GHz. This similarity implies that the two frequencies have the same radiation region and mechanism, whereas the difference indicates that the sources have intrinsically different dynamics. Furthermore, we show that the degree of multifractality is the same in the observed and rest frames of the quasars, i.e., multifractality is an intrinsic property of radio quasars. Finally, we show that the ARIMA models fit the 3C 345 quasar at 22 GHz and partially fit most of the time series, with the exception of the 3C 273 and 3C 279 quasars at 37 GHz, for which the models are found to be inadequate
Multifractality signatures in quasars time series – I. 3C 273
(Oxford University Press (OUP), 2018-05-18) Martins, Bruno Leonardo Canto; Belete, A. Bewketu; Bravo, J. P.; Leão, I. C.; Araujo, J. M. de; Medeiros, J.R. de
The presence of multifractality in a time series shows different correlations for different timescales as well as intermittent behaviour that cannot be captured by a single scaling exponent. The identification of a multifractal nature allows for a characterization of the dynamics and of the intermittency of the fluctuations in non-linear and complex systems. In this study, we search for a possible multifractal structure (multifractality signature) of the flux variability in the quasar 3C 273 time series for all electromagnetic wavebands at different observation points, and the origins for the observed multifractality. This study is intended to highlight how the scaling behaves across the different bands of the selected candidate, which can be used as an additional new technique to group quasars based on the fractal signature observed in their time series and determine whether quasars are non-linear physical systems or not. The multifractal detrended moving average algorithm (MFDMA) has been used to study the scaling in non-linear, complex, and dynamic systems. To achieve this goal, we applied the backward (θ = 0) MFDMA method for one-dimensional signals. We observe weak multifractal (close to monofractal) behaviour in some of the time series of our candidate except in the mm, UV and X-ray bands. The non-linear temporal correlation is the main source of the observed multifractality in the time series whereas the heaviness of the distribution contributes less.
A Novel Approach to Study the Variability of NGC 5548
(American Astronomical Society, 2019-07-12) Martins, Bruno Leonardo Canto; Belete, A. Bewketu; Goicoechea, L. J.; Leão, I.C.; Medeiros, J.R. de
Understanding the properties of the continuum radiation and broad emission lines of active galactic nuclei provides significant information not only to model the radiation mechanism and constrain the geometry and kinematics ofthe broad-line region (BLR) but also to probe the central engine of the sources. Here we investigate the multifractal behavior of the Hβ emission line and the 5100 Å continuum flux light curves of NGC 5548. The aim is to search for multiscaling signatures in the light curves and check if there is a possible nonlinear relationship between them. To this end, we use a multifractality analysis technique called the Multifractal Detrended Moving Average analysis. We detect multifractal (nonlinear) signatures in the full monitoring and densely sampled period of the Hβ line and 5100 Å continuum light curves of NGC 5548, possibly indicating the presence of complex and nonlinear interaction in the 5100 Å continuum and Hβ emission line regions. Moreover, the degree of multifractality of the Hβ line is found to be about twice that of the 5100 Å continuum. The nonlinearity of both emissions could be generated when the BLR reprocesses the radiation from the central compact source. Finally, we found that antipersistent long-range temporal correlation is the main source of the multifractality detected in both light curves.
Revealing the non-linear behaviour of the lensed quasar Q0957+561
(Oxford University Press (OUP), 2019-01-18) Martins, Bruno Leonardo Canto; Belete, A. Bewketu; Leão, I. C.; Medeiros, J. R. de
Knowledge about how the non-linear behaviour of the intrinsic signal from lensed background sources changes on its path to the observer provides much information, particularly about the matter distribution in lensing galaxies and the physical properties of the current universe, in general. Here, we analyse the multifractal behaviour of the optical observations of A and B images of Q0957+561 in the r and g bands. Aims: To verify the presence, or absence, of extrinsic variations in the observed signals of the quasar images and whether extrinsic variations affect the multifractal behaviour of their intrinsic signals. Method: We apply a wavelet transform modulus maxima-based multifractality analysis approach. Results: We detect strong multifractal signatures in the light curves of the quasar images. The degree of multifractality for both images in the r band changes over time in a non-monotonic way, indicating the presence of extrinsic variabilities in the light curves of the images. Additionally, in the r band, in periods of quiescent microlensing activity, we find that the degree of multifractality of image A is stronger than that of B, while B has a larger multifractal strength in recent epochs when it appears to be affected by microlensing. Finally, comparing the optical bands in a period of quiescent microlensing activity, we find that the degree of multifractality is stronger in the r band for both quasar images. In the absence of microlensing, the observed excesses of non-linearity are most likely generated when the broad-line region reprocesses the radiation from the compact sources.

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Navegando por Autor "Belete, A. Bewketu"