Navegando por Autor "Medeiros, J. R. de"

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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
Incidence of planet candidates in open clusters and a planet confirmation
(EDPSCIENCE, 2018-12-10) Martins, Bruno Leonardo Canto; Leão, I. C.; Alves, S.; Oliveira, G. Pereira de; Cortés, C.; Brucalassi, A.; Melo, C. H. F.; Freitas, D. B. de; Pasquini, L.; Medeiros, J. R. de
Context. Detecting exoplanets in clusters of different ages is a powerful tool for understanding a number of open questions, such as how the occurrence rate of planets depends on stellar metallicity, on mass, or on stellar environment. Aims. We present the first results of our HARPS long-term radial velocity (RV) survey which aims to discover exoplanets around intermediate-mass (between ∼2 and 6 M ) evolved stars in open clusters. Methods. We selected 826 bona fide HARPS observations of 114 giants from an initial list of 29 open clusters and computed the halfpeak to peak variability of the HARPS RV measurements, namely ∆RV=2, for each target, to search for the best planet-host candidates. We also performed time series analyses for a few targets for which we have enough observations to search for orbital solutions. Results. Although we attempted to rule out the presence of binaries on the basis of previous surveys, we detected 14 new binary candidates in our sample, most of them identified from a comparison between HARPS and CORAVEL data. We also suggest 11 new planet-host candidates based on a relation between the stellar surface gravity and ∆RV=2. Ten of the candidates are less than 3 M , showing evidence of a low planet occurrence rate for massive stars. One of the planet-host candidates and one of the binary candidates show very clear RV periodic variations, allowing us to confirm the discovery of a new planet and to compute the orbital solution for the binary. The planet is IC 4651 9122b, with a minimum mass of m sin i = 6:3 MJ and a semimajor axis a = 2:0 AU. The binary companion is NGC 5822 201B, with a very low minimum mass of m sin i = 0:11 M and a semimajor axis a = 6:5 AU, which is comparable to the Jupiter distance to the Sun.
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.
Statistical tracing of magnetic fields: comparing and improving the techniques
(IOP Publishing, 2018-09-20) Martins, Bruno Leonardo Canto; Yuen, Ka Ho; Chen, Junda; Hu, Yue; Ho, Kai Wa; Lazarian, A.; Lazarian, Victor; Yang, Bo; Burkhart, Blaskesley; Correia, Caio; Cho, Jungyeon; Medeiros, J. R. de
Magnetohydrodynamic turbulence displays velocity anisotropies that reflect the direction of the magnetic field. This anisotropy has led to the development of a number of statistical techniques for studying magnetic fields in the interstellar medium. In this paper, we review and compare three techniques that use radio position–position–velocity data to determine magnetic field strength and morphology: the correlation function anisotropy (CFA), principal component analysis of anisotropies (PCAA), and the more recent velocity gradient technique (VGT). We compare these three techniques and suggest improvements to the CFA and PCAA techniques to increase their accuracy and versatility. In particular, we suggest and successfully implement a much faster way to calculate nonperiodic correlation functions for the CFA. We discuss possible improvements to the current implementation of the PCAA. We show the advantages of the VGT in terms of magnetic field tracing and stress the complementary nature with the other two techniques.
The variability behaviour of CoRoT M-giant stars
(EDP Sciences, 2015-11-04) Martins, Bruno Leonardo Canto; Lopes, C. E. Ferreira; Neves, V.; Leão, I. C.; Freitas, D. B de; Costa, A. D. da; Paz-Chinchón, F.; Chagas, M. L. das; Baglin, A.; Janot-Pacheco, E.; Medeiros, J. R. de
Context. For six years the Convection, Rotation, and planetary Transits (CoRoT) space mission has been acquiring photometric data from more than 100 000 point sources towards and directly opposite the inner and outer regions of the Galaxy. The high temporal resolution of the CoRoT data, combined with the wide time span of the observations, enabled the study of short- and long-time variations in unprecedented detail. Aims. The aim of this work is to study the variability and evolutionary behaviour of M-giant stars using CoRot data. Methods. From the initial sample of 2534 stars classified as M giants in the CoRoT databases, we selected 1428 targets that exhibit well defined variability, by visual inspection. Then, we defined three catalogues: C1 – stars with Teff < 4200 K and LCs displaying semi-sinusoidal signatures; C2 – rotating variable candidates with Teff > 4200 K; C3 – long-period variable candidates (with LCs showing a variability period up to the total time span of the observations). The variability period and amplitude of C1 stars were computed using Lomb-Scargle and harmonic fit methods. Finally, we used C1 and C3 stars to study the variability behaviour of M-giant stars. Results. The trends found in the V − I vs. J − K colour–colour diagram are in agreement with standard empirical calibrations for M giants. The sources located towards the inner regions of the Galaxy are distributed throughout the diagram, while the majority of the stars towards the outer regions of the Galaxy are spread between the calibrations of M giants and the predicted position for carbon stars. The stars classified as supergiants follow a different sequence from the one found for giant stars. We also performed a Kolmogorov-Smirnov (KS) test of the period and amplitude of stars towards the inner and outer regions of the Galaxy. We obtained a low probability that the two samples came from the same parent distribution. The observed behaviour of the period-amplitude and period-effective temperature (Teff) diagrams are, in general, in agreement with those found for Kepler sources and ground based photometry, with pulsation being the dominant cause responsible for the observed modulation. We also conclude that short-time variations on M-giant stars do not exist or are very rare, and the few cases we found are possibly related to biases or background stars.
A wavelet analysis of photometric variability in Kepler white dwarf stars
(Oxford University Press (OUP), 2019-01-12) Martins, Bruno Leonardo Canto; Lira, S. R. de; Bravo, J. P.; Leão, I. C.; Costa, A. D. da; Freitas, D. B. de; Medeiros, J. R. de
This work describes a wavelet analysis of 14 Kepler white dwarf stars, in order to confirm their photometric variability behaviour and to search for periodicities in these targets. From the observed Kepler light curves we obtained the wavelet local and global power spectra. Through this procedure, one can perform an analysis in the time–frequency domain rich in detail, and so obtain a new perspective on the time evolution of the periodicities present in these stars. We identified a photometric variability behaviour in 10 white dwarfs, corresponding to period variations of ∼2 h to 18 d: among these stars, three are new candidates and seven, earlier identified from other studies, are confirmed.

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Navegando por Autor "Medeiros, J. R. de"