arXiv subjects

D. B. de Freitas

Publications and source records attributed to D. B. de Freitas.

At least 19 recordsLinked to original sources

arXiv2025

Nonextensive entropic behavior observed in Quasar 3C 273

We investigate the flux intensities spanning from radio waves to X-rays across 39 light curves of Quasar 3C 273, utilizing publicly available data collected by the Integral Science Data Centre (ISDC) database. Our results suggest that Quasar 3C 273 exhibits nonextensive behavior. Furthermore, we calculate the $q$ entropic indices for these light curves using the $q$-Gaussian distribution with a predominant observation of cases where $q>1$. Based on this index, we estimate the non-extensive entropy ($S_{q}$) and explore its correlation with the energy (in eV). In this context, we identify two jump-like increases in entropy, particularly evident in the infrared (IR) and X-ray wavebands. The peak in the far-IR band, around 0.34 eV, results from synchrotron flares evolving from higher to lower energies and thermal radiation emitted by hot dust near the sublimation radius. However, the second entropic peak in the hard X-ray range lacks statistical robustness due to limited data or large measurement uncertainties.

astro-ph.HE↗

arXiv2024

Anomalous Entropic Behavior Observed in Quasar 3C 273

We investigate the flux intensities spanning from radio waves to $γ$-rays across 36 light curves of Quasar 3C 273, utilizing publicly available data collected by the Integral Science Data Centre (ISDC) database. Our analysis reveals a consistent adherence of all light curves from this quasar to $q$-Gaussian distribution. This compelling finding strongly suggests a nonextensive behavior exhibited by Quasar 3C 273. Moreover, we derive the $q$ entropic indices for these light curves, providing insights into the degree of nonextensivity, where cases with $q>1$ were primarily observed. Utilizing this index, we estimate the nonextensive entropy ($S_{q}$) and explore its correlation with the energy (in eV) and the $q$ index. Notably, we observe a tendency for the $q$ value to increase as the Tsallis entropy decreases. Remarkably, our most significant observation pertains to the relationship between the entropy $S_{q}$ and the energy of the source. We identify an anomalous behavior in entropy, particularly evident in the infrared and $γ$-ray wavebands.

astro-ph.HE↗

arXiv2024

Exploring Detached Eclipsing Binary Systems from TESS Observations: Insights into OBA-type Systems and Orbital Circularization

This study presents photometric solutions for a carefully selected sample of 181 detached eclipsing binary systems recently observed by the Transiting Exoplanet Survey Satellite (TESS) mission. Our findings contribute to discussions about OBA-type systems and the orbital circularization influenced by tidal interactions. The results encompass crucial parameters, including the sum of radii fraction ($r_{\rm a}\,+\,r_{\rm b}$), radii ratio ($k$), orbital inclination ($i$), surface brightness ratio ($J$), and combinations of the longitude of the periastron and orbital eccentricity ($e\,\sin\,ω$ and $e\,\cos\,ω$). Geometric light curve fitting is performed using version 43 of the {\sc jktebop} code, serving as the primary tool for determining orbital elements and physical ratios. Careful uncertainty estimates for these findings are derived through bootstrap simulations. This work introduces unprecedented photometric solutions for 172 well-detached eclipsing systems, while the results for the remaining nine systems exhibit excellent agreement with those previously reported in the literature. Additionally, a critical primary radius fraction ($R_{\rm a}/a$) of 0.266 is identified for our sample of radiative envelope stars, consistent with observational and theoretical findings in the literature. Finally, we determine a circularization period of 5.762 days based on a function from the literature, revealing a discrepancy with Zahn's theory for hot stars.

astro-ph.SR↗

arXiv2024

Multifractal nature of seismic sequences distributed along the Pacific Ring of Fire

A multifractal methodology was utilized to analyze a set of seismic sequences distributed along the Pacific Ring of Fire, sourced from the National Earthquake Information Center (NEIC) catalog. The analysis employed the Multifractal Detrended Moving Average Analysis (MFDMA) method to characterize the multi-scale behavior using various geometrical parameters of the multifractal spectrum. The findings of this study can be summarized as follows: firstly, our research suggests that seismic sequences along the Ring of Fire exhibit distinct dynamics; and secondly, it indicates that long-range correlations may influence larger magnitude earthquakes, as demonstrated by the correlation with the $b_{\rm GR}$ index. These results contribute to an enhanced understanding of the multifractal characteristics of seismic activity and their implications for earthquake dynamics.

physics.geo-ph↗

arXiv2023

Multiscale structure of the gravitational wave signal from GW150914 based on the nonextensivity $q$-triplet

We study the first gravitational wave, GW150914, detected by advanced LIGO and constructed from the data of measurement of strain relative deformation of the fabric of spacetime. We show that the time series from the gravitational wave obeys Tsallis's $q$-Gaussian distribution as a probability density and its dynamics evolve of the three associated Tsallis' indices named $q$-triplet. This fact strongly suggests that these black hole merger systems behave in a non-extensive manner. Furthermore, our results point out that the entropic indexes obtained as a function of frequency are useful statistical parameters to determine the dominant frequency when black hole coalescence is achieved.

gr-qc↗

arXiv2022

Multifractal charactarization as a function of timescale in the light curves with planetary signal observed by the kepler mission

Astrophysical data, in the domains of time, involve a wide range of stellar variability phenomena, among them the magnetic activity of the order of a few hours until the signature of an extra-solar planet which can cover a scale of time of a few days until tens of years. Numerous instruments are being developed to detect Earth-sized exoplanets. Exoplanets with this dimension challenge scientific instrumentation and the field of research in the data processing. In this context, our study offers a powerful framework to explain dynamical properties as a function of timescale in light curves with the planetary signal. For that, we selected the stellar target Kepler-30 to test our methods and procedures. In this sense, we investigate the multifractal behavior of the Kepler-30 system composed of a sun-like star with a rotation period of ~16 days and three planets with masses between 2 Earth and 2.5 Jupiter masses. Furthermore, this system has an orbital period varying from 29 to 143 days and orbits almost coplanar. This system is highly interesting because starspots dynamics are strongly affected by the passing of a planet in front of the star. We used about 1600 days of high-precision photometry collected by the Kepler mission to investigate the quasi-periodic variation caused by the rotation of the star and the effect of spot evolution as a function of timescale. We applied indexes extract from multifractal analysis to model the flux rotational modulation induced by active regions. Our results that stellar flux variations in Kepler-30 star caused by rotational modulation can be replicated in detail with just four recent-known multifractal indexes. These indexes will greatly simplify spot modelling of current TESS and future PLATO data.

astro-ph.EP↗

arXiv2022

Measuring deviation from Skumanich braking index in active stars observed by Kepler mission

The aim of this work is to determine the deviation of the value of magnetic braking index $q$ from Skumanich $q=3$ canonical value for giant and main-sequence stars. In this context, the present work attempts to analytically calculate the braking index based on the balance of gravitational and centrifugal forces, a determining factor for understanding the delicate mechanisms that control the spin-down of stars in these evolutionary phases. In the present study, we used a wide sample of stellar targets from the \textit{Kepler} mission with well-defined mass, radius, and rotation period. As a result, \textit{Kepler} stellar parameters provide rather precise values of $q$ index limited in the range $1\leq q\leq 3$, which is consistent with the predictions of the model of magnetic stellar wind. Our results show conclusively that, within the model used in this work, any significant deviation of the braking index away from the value $q=3$ occurs at masses higher than the Kraft break.

astro-ph.SR↗

arXiv2021

On the multiscale behaviour of stellar activity and rotation of the planet host Kepler-30

Kepler-30 is a unique target to study stellar activity and rotation in a young solar-like star accompanied by a compact planetary system. We use about 4 years of high-precision photometry collected by the Kepler mission to investigate the fluctuations caused by photospheric convection, stellar rotation, and starspot evolution as a function of the timescale. Our main goal is to apply methods for the analysis of timeseries to find the timescales of the phenomena that affect the light variations. We correlate those timescales with periodicities in the star as well as in the planetary system. We model the flux rotational modulation induced by active regions using spot modelling and apply the MFDMA in standard and multiscale versions for analysing the behaviour of variability and light fluctuations that can be associated with stellar convection and the evolution of magnetic fields on timescales ranging from less than 1 day up to about 35 days. The light fluctuations produced by stellar activity can be described by the multifractal Hurst index that provides a measure of their persistence. The spot modeling indicates a lower limit to the relative surface differential rotation of $ΔΩ/Ω\sim 0.02\pm 0.01$ and suggests a short-term cyclic variation in the starspot area with a period of $\sim 34$ days, virtually close to the synodic period of 35.2 days of the planet Kepler-30b. By subtracting the two timeseries of the SAP and PDC Kepler pipelines, we reduce the rotational modulation and find a 23.1-day period close to the synodic period of Kepler-30c. This period also appears in the multifractal analysis as a crossover of the fluctuation functions associated with the characteristic evolutionary timescales of the active regions in Kepler-30 as confirmed by spot modelling. These procedures and methods may be greatly useful for analysing current TESS and future PLATO data.

astro-ph.SR↗

arXiv2020

Non-extensive processes associated with heating of the Galactic disc

We analyse the mechanisms ruling galactic disc heating through the dynamics of space velocities $U$, $V$ and $W$, extracted from the Geneva-Copenhagen catalogue. To do this, we use a model based on non-extensive statistical mechanics, where we derive the probability distribution functions that quantify the non-Gaussian effects. Furthermore, we find that the deviation $q-1$ at a given stellar age follows non-random behaviour. As a result, the $q$-index behaviour indicates that the vertical component $W$, perpendicular to the Galactic plane, does not ``heat up'' at random, which is in disagreement with previous works that attributed the evolution of $W$ to randomness. Finally, our results bring a new perspective to this matter and open the way for studying Galactic kinematic components through the eyes of more robust statistical models that consider non-Gaussian effects.

astro-ph.SR↗

arXiv2020

A new insight on stellar age I. Theoretical trends with gyro-age and rotation

In this first work attempts to analytically explain the effects on the magnetic braking index, $q$, caused by the evolution of stellar velocity in main-sequence stars, and estimated by de Freitas \& De Medeiros (2013). We have found that the effect of $q$ is here a determining factor for understanding the delicate mechanisms that control the spin-down of stars as a function of the mass of stars. We note that our models predict that the calculated ages are distinct from gyrochronology ages. Indeed, the gyro-ages are measured considering only the canonical value of the Skumanich relation ($q$=3). As a result, we find that the age of stars can be well-determined when $q$ is free parameter. We also verified that for rotation periods less than $\sim$ 5 days (i.e., fast rotators) there is a strong discrepancy among the different indexes $q$. In addition, the ages measured by gyrochronology model can be underestimated according to mass range selected. In conclusion, we suggest that the generalized gyro-ages by magnetic braking index can be an interesting way to better understand the idea of rotation as a clock.

astro-ph.SR↗

arXiv2019

Investigating the signatures of long-range persistence in seismic sequences along Circum-Pacific subduction zones

In the present paper, we analyze the signatures of long-range persistence in seismic sequences along Circum-Pacific subduction zones, from Chile to Kermadec, extracted from the National Earthquake Information Center (NEIC) catalog. This region, known as the Pacific Ring of Fire, is the world's most active fault line, containing about 90$\%$ of the world's earthquakes. We used the classical rescaled range ($R/S$) analysis to estimate the long-term persistence signals derived from a scaling parameter called the Hurst exponent, $H$. We measured the referred exponent and obtained values of $H>0.5$, indicating that a long-term memory effect exists. We found a possible fractal relationship between $H$ and the $b_{s}(q)$-index, which emerges from the non-extensive Gutenberg-Richter law as a function of the asperity. Therefore, $H$ can be associated with a mechanism that controls the level of seismic activity. Finally, we concluded that the dynamics associated with fragment-asperity interactions can be classified as a self-affine fractal phenomenon.

physics.geo-ph↗

arXiv2019

Multifractal detrended moving average analysis of Kepler stars with surface differential rotation traces

A multifractal formalism is employed to analyse high-precision time-series data of \textit{Kepler} stars with surface differential rotation traces. The multifractal detrended moving average analysis (MFDMA) algorithm has been explored to characterize the multi-scale behaviour of the observed time series from a sample of 662 stars selected with parameters close to those of the Sun, e.g., effective temperature, mass, effective gravity and rotation period. Among these stars, 141 have surface differential rotation traces, whereas 521 have no detected differential rotation signatures. In our sample, we also include the Sun in its active phase. Our results can be summarized in two points: first, our work suggests that starspots for time series with and without differential rotation have distinct dynamics, and second, the magnetic fields of active stars are apparently governed by two mechanisms with different levels of complexity for fluctuations. Throughout the course of the study, we identified an overall trend whereby the differential rotation is distributed in two $H$ regimes segregated by the degree of asymmetry $A$, where $H$-index denotes the global Hurst exponent which is used as a measure of long-term memory of time series. As a result, we show that the degree of asymmetry can be considered a segregation factor that distinguishes the differential rotation behaviour when related to the effect of the rotational modulation on the time series. In summary, the multifractality signals in our sample are the result of magnetic activity control mechanisms leading to activity-related long-term persistent signatures.

astro-ph.SR↗

arXiv2019

New Suns in the Cosmos V: Stellar rotation and multifractality in active \textit{Kepler} stars

In the present study, high-precision time series photometry for the active \emph{Kepler} stars is described in the language of multifractals. We explore the potential of using the rescaled range analysis ($R/S$) and multifractal detrended moving average analysis (MFDMA) methods to characterize the multiscale structure of the observed time series from a sample of $\sim$40 000 active stars. Among these stars, 6486 have surface differential rotation measurement, whereas 1846 have no signature of differential rotation. As a result, the Hurst exponent ($H$) derived from both methods shows a strong correlation with the period derived from rotational modulation. In addition, the variability range $R_{var}$ reveals how this correlation follows a high activity ``line''. We also verify that the $H$-index is an able parameter for distinguishing the different signs of stellar rotation that can exist between the stars with and without differential rotation. In summary, the results indicate that the Hurst exponent is a promising index for estimating photometric magnetic activity.

astro-ph.SR↗

arXiv2019

A nonextensive insight to the stellar initial mass function

the present paper, we propose that the stellar initial mass distributions as known as IMF are best fitted by $q$-Weibulls that emerge within nonextensive statistical mechanics. As a result, we show that the Salpeter's slope of $\sim$2.35 is replaced when a $q$-Weibull distribution is used. Our results point out that the nonextensive entropic index $q$ represents a new approach for understanding the process of the star-forming and evolution of massive stars.

astro-ph.SR↗

arXiv2019

A wavelet analysis of photometric variability in Kepler white dwarf stars

This work brings a wavelet analysis for 14 Kepler white dwarf stars, in order to confirm their photometric variability behavior 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 time-frequency domain rich in details, and so to obtain a new perspective on the time evolution of the periodicities present in these stars. We identified a photometric variability behavior in ten white dwarfs, corresponding to period variations of ~ 2 h to 18 days: among these stars, three are new candidates and seven, earlier identified from other studies, are confirmed.

astro-ph.SR↗

arXiv2018

Debris disks among Kepler solar rotational analog stars

Observations of circumstellar disks provide a powerful tool for our understanding of planetary systems dynamics. Analogs to the Solar System asteroid belts, debris disks result from the collision of the remaining solid material of the planet formation process. Even if the presence of disk is now reported for hundreds of stars, its detection around stars similar to the Sun is still very sparse. We report the results of a search for debris disks around Kepler stars with surface physical parameters close to solar values, including rotation period, using observations by the Wide-field infrared Survey Explorer (WISE). From the entire sample of Kepler stars, 881 targets were identified with these parameters and only six of them (KIC 1868785, 7267949, 7435796, 10533222, 11352643, and KIC 11666436) show unambiguous infrared excess, for which we determined debris disk physical parameters. Interestingly, the present study reveals traces of debris disks much more massive and brighter than the Solar System zodiacal dust, probably resulting from recent violent collisional events, orbiting stars with ages around the solar values.

astro-ph.SR↗

arXiv2018

On the incidence of planet candidates in open clusters and a planet confirmation

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. 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 Msun) evolved stars in open clusters. We selected 826 bona fide HARPS observations of 114 giants from an initial list of 29 open clusters and computed the half peak-to-peak variability of the HARPS RV measurements, namely DeltaRV/2, for each target, to search for the best planet-host candidates. We also performed time series analysis for a few targets with enough observations to search for orbital solutions. 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 DeltaRV/2. Ten of the candidates have less than 3 Msun, 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 msini = 6.3 MJ and a semi-major axis a = 2.0 AU. The binary companion is NGC 5822 201B, with a very low minimum mass of msini = 0.11 Msun and a semi-major axis a = 6.5 AU, which is comparable to the Jupiter distance to the Sun.

astro-ph.SR↗

arXiv2017

On a possible fractal relationship between the Hurst exponent and the nonextensive Gutenberg-Richter index

In the present paper, we analyze the fractal structures in magnitude time series for a set of unprecedented sample extracted from the National Earthquake Information Center (NEIC) catalog corresponding to 12 Circum-Pacific subduction zones from Chile to Kermadec. For this end, we used the classical Rescaled Range ($R/S$) analysis for estimating the long-term persistence signature derived from scaling parameter so-called Hurst exponent, $H$. As a result, we measured the referred exponent and obtained all values of $H>0.5$, indicating that a long-term memory effect exists. The main contribution of our paper, we found a possible fractal relationship between $H$ and the $b_{s}(q)$-index which emerges from nonextensive Gutenberg-Richter law as a function of the asperity, i.e., we show that the values of $H$ can be associated with the mechanism which controls the abundance of magnitude and, therefore, the level of activity of earthquakes. Finally, we concluded that dynamics associated with fragment-asperity interactions can be emphasized as a self-affine fractal phenomenon.

physics.geo-ph↗