Light Curves Research Articles

Detecting and sizing the Earth with PLATO: A feasibility study based on solar data

The PLAnetary Transits and Oscillations of stars (PLATO) mission will observe the same area of the sky continuously for at least two years in an effort to detect transit signals of an Earth-like planet orbiting a solar-like star. We aim to study how short-term solar-like variability caused by oscillations and granulation would affect PLATO's ability to detect and size Earth if PLATO were to observe the Solar System itself. We also compare different approaches to mitigate noise caused by short-term solar-like variability and perform realistic transit fitting of transit signals in PLATO-like light curves. We injected Earth-like transit signals onto real solar data taken by the Helioseismic and Magnetic Imager (HMI) instrument on board the Solar Dynamics Observatory (SDO). We isolated short-term stellar variability in the HMI observations by removing any variability with characteristic timescales longer than five hours using a smooth Savitzky-Golay filter. We then added a noise model for a variety of different stellar magnitudes computed by assuming an observation by all 24 normal cameras. We first compared four different commonly used treatments of correlated noise in the time domain by employing them in a transit fitting scheme. We then tried to recover pairs of transit signals using an algorithm similar to the transit least squares algorithm. Finally, we performed transit fits using realistic priors on planetary and stellar parameters and assessed how accurately the pair of two injected transits was recovered. We find that short-term solar-like variability affects the correct retrieval of Earth-like transit signals in PLATO data. Variability models accounting for variations with typical timescales at the order of one hour are sufficient to mitigate these effects. We find that when the limb-darkening coefficients of the host star are properly constrained, the impact parameter does not negatively affect the detectability of a transit signal or the retrieved transit parameters, except for high values ($b > 0.8$). For bright targets (8.5 - 10.5 mag), the transit signal of an Earth analogue can reliably be detected in PLATO data. For faint targets a detection is still likely, though the results of transit search algorithms have to be verified by transit-fitting algorithms to avoid false positive detections being flagged. For bright targets (V-mag leq 9.5), the radius of an Earth-like planet orbiting a solar-like star can be correctly determined at a precision of 3<!PCT!> or less, assuming that at least two transit events are observed and the characteristics of the host star are well understood.

Studying the X-ray absorption characteristics of Centaurus X-3 using nearly 14 years of MAXI/GSC data

Centaurus X-3 is a persistent high-mass X-ray binary with the long-term light curve from the source exhibiting orbit-to-orbit intensity variations with no apparent superorbital periodicity. We used sim 13.5 years of MAXI /GSC data to study the long-term behaviour of X-ray absorption caused by the stellar wind from the companion star and any absorbing structures present in the binary. We used orbital-phase-resolved spectroscopy to study the variation in the photoelectric absorption along the line of sight of the source for both the intensity-averaged data and intensity-resolved data after dividing all the data binned with orbital period into three intensity levels. We find an asymmetric variation in the photoelectric absorption along the line of sight across an orbit of the source. The orbital-phase-resolved spectra show a clear increase in photoelectric absorption after $ orb 0.5, which deviates from a spherically symmetric stellar wind model. The flux of Cen X-3 shows significant variation between consecutive orbits. An intensity-resolved spectral analysis of the source was performed, followed by an intensity-resolved and orbital-phase-resolved spectral analysis, which showed that at the medium and high intensity levels, the orbital-phase-resolved photoelectric absorption is slightly asymmetric with respect to mid-phase ($ orb =$ 0.5). The asymmetry is very pronounced at the lowest intensity level and cannot be explained by a spherically symmetric wind from the companion star. The differences in the orbital phase-dependence of absorption for different intensity levels suggest that the presence of an accretion wake, photoionization wake, or tidal stream is more prominent at a lower intensity level for Centaurus X-3 than at a higher intensity level.

Variability of young stellar objects in the Orion nebula

The Orion Nebula is one of the best-studied star-forming regions in our Galaxy and provides an ideal laboratory to study the early stages of star formation. Understanding the variability of young stellar objects (YSOs) can provide valuable insights into the underlying physical processes that occur during this crucial phase of stellar evolution. In this context, we have conducted a photometric survey with high temporal resolution of a six-square-degree region centered on NGC\,1980. We used two filters ($R$ and $I$) between 2012 and 2015 with the 15\,cm twin telescope RoBoTT at the Universit\"atssternwarte Bochum in Chile. Our goal is to identify and characterize the variable objects in this sample using a unique observational setup that allows us to obtain high-quality light curves with time sampling on the order of minutes. This is the first time such an intensive ground-based photometric campaign has been conducted in the Orion Nebula. After careful quality control, more than 13000 light curves were produced in either $R$ or $I$; the total number of matched light curves is 11889. The corresponding stars were subsequently matched with the 2MASS point source catalog, resulting in 8197 stars with $I < 16.5$ mag and AAA quality flags. We employed three methods -- amplitude, standard deviation, and the Stetson J index -- to identify variable objects in our sample, successfully classifying 561 stars as variables (irregular, periodic, and quasi-periodic). These stars serve as the foundation for further analysis. We confirmed 456 of these as known pre-main-sequence stars through cross-referencing with existing YSO catalogs, supported by spectroscopic and photometric data. The youth of the remaining stars was established based on their optical and infrared color signatures. The period, $P$, could be determined for 147 sources and ranges from 0.2 days to several weeks. The frequency of very short periods ($ < 1$ day) is two to four times higher than reported in previous studies. A significant population (10<!PCT!>) comes primarily from weak-line T Tauri stars, which are characterized by rotation periods of less than 1 day. These results emphasize the advantages of observations with an exceptional time sampling strategy that allows the identification of variability on scales from hours to years.

The fast rise of the unusual type IIL/IIb SN 2018ivc

We present an analysis of the photometric and spectroscopic dataset of the type II supernova (SN) 2018ivc in the nearby (10 Mpc) galaxy Messier 77. Thanks to our high-cadence data, we observed the SN rising very rapidly by nearly three magnitudes in five hours (or 18 mag d$^ $). The $r$-band light curve presents four distinct phases: the maximum light, which was reached in just one day, followed by a first, rapid linear decline and a short-duration plateau. Finally, the long, slower linear decline lasted for one year. Thanks to the ensuing radio re-brightening, we were able to detect SN 2018ivc four years after the explosion. The early spectra show a blue, nearly featureless continuum, but the spectra go on to evolve rapidly; after about ten days, a prominent Halpha line starts to emerge, characterised by a peculiar profile. However, the spectra are heavily contaminated by emission lines from the host galaxy. The He I lines, namely $ are also strong. In addition, strong absorption from the Na I doublet is evident and indicative of a non-negligible internal reddening. From its equivalent width, we derived a lower limit on the host reddening of $A_V mag. From the Balmer decrement and a match of the $B-V$ colour curve of SN 2018ivc to that of the comparison objects, we obtained a host reddening of $A_V mag. The spectra are similar to those of SNe II, but with strong He lines. Given the peculiar light curve and spectral features, we suggest SN 2018ivc could be a transitional object between the type IIL and type IIb SNe classes. In addition, we found signs of an interaction with the circum-stellar medium (CSM) in the light curve, also making SN 2018ivc an interacting event. Finally, we modelled the early multi-band light curves and photospheric velocity of SN 2018ivc to estimate the physical parameters of the explosion and CSM.

New orbital periods of high-inclination dwarf novae based on Gaia Alerts photometry

The orbital period of a cataclysmic variable stands as a crucial parameter for investigating the structure and physics of these binary systems, as well as understanding their evolution. We use photometric Gaia data for dwarf novae (DNe) in the quiescent state ---which are available for a number of years--- to determine new orbital periods and improve or modify previously suggested period values. Two approaches are implemented for selecting high-inclination targets, either eclipsing or with ellipsoidal variations. We determine new orbital periods for 75 DNe and improve ephemerides for 27 more (three of which change significantly), contributing 9.4<!PCT!> of the known DNe periods of between 0.05 and 2.0 days, and doubling the number of known periods exceeding 0.44 days. Their phase-folded light curves are presented and arranged by orbital period, illustrating the transition from short-period systems, dominated by radiation from the accretion disc and the hot spot, to longer-period DNe, where the Roche-lobe-filling secondary star is the primary visual flux source. This transition ---which occurs around the well-known period gap (between sim 2 and 3 hours)--- is expected, as DNe with larger orbital periods typically harbour more massive donors, which contribute to the visible flux. However, this transition is not abrupt. Within the same range of periods, we observe systems dominated by ellipsoidal variations, where the companion star is clearly visible, as well as others dominated by the disc and the hot spot. The presence of some DNe with ellipsoidal variations near the lower edge of the period gap is striking, as the companions in these systems are expected to be cool low-mass M-dwarfs not visible in the light curve. This could indicate that we are observing systems where the donor star was originally much more massive and underwent significant nuclear evolution before mass-transfer began, as has been suggested previously for QZ Ser.

The Dark Energy Survey Supernova Program: Light Curves and 5 Yr Data Release

We present griz photometric light curves for the full 5 yr of the Dark Energy Survey Supernova (DES-SN) program, obtained with both forced point-spread function photometry on difference images (DiffImg) performed during survey operations, and scene modelling photometry (SMP) on search images processed after the survey. This release contains 31,636 DiffImg and 19,706 high-quality SMP light curves, the latter of which contain 1635 photometrically classified SNe that pass cosmology quality cuts. This sample spans the largest redshift (z) range ever covered by a single SN survey (0.1 < z < 1.13) and is the largest single sample from a single instrument of SNe ever used for cosmological constraints. We describe in detail the improvements made to obtain the final DES-SN photometry and provide a comparison to what was used in the 3 yr DES-SN spectroscopically confirmed Type Ia SN sample. We also include a comparative analysis of the performance of the SMP photometry with respect to the real-time DiffImg forced photometry and find that SMP photometry is more precise, more accurate, and less sensitive to the host-galaxy surface brightness anomaly. The public release of the light curves and ancillary data can be found at github.com/des-science/DES-SN5YR and doi:10.5281/zenodo.12720777.

Fundamental Parameters of a Binary System Consisting of a Red Dwarf and a Compact Star

TIC 157365951 has been classified as a δ Scuti type by the International Variable Star Index. Through the spectra from Large Sky Area Multi-Object Fiber Spectroscopic Telescope and its light curve, we further discovered that it is a binary system. This binary system comprises a red-dwarf star and a compact star. Through the spectral energy distribution fitting, we determined the mass of the red dwarf star as M 1 = 0.31 ± 0.01M ⊙ and its radius as R 1 = 0.414 ± 0.004R ⊙. By fitting the double-peaked Hα emission, we derived the mass ratio of q = 1.76 ± 0.04, indicating a compact star mass of M 2 = 0.54 ± 0.01M ⊙. Using Phoebe to model the light curve and radial velocity curve for the detached binary system, we obtained a red dwarf star mass of M 1 = 0.29 ± 0.02M ⊙, a radius of R 1 = 0.39 ± 0.04R ⊙, and a Roche–lobe filling factor of f = 0.995 ± 0.129, which is close to the f = 1 expected for a semidetached system. The Phoebe model gives a compact star mass M 2 = 0.53 ± 0.05M ⊙. Constraining the system to be semidetached gives M 1 = 0.34 ± 0.02M ⊙, R 1 = 0.41 ± 0.01R ⊙, and M 2 = 0.62 ± 0.03M ⊙. The consistency of the models is encouraging. The value of the Roche–lobe filling factor suggests that there might be ongoing mass transfer. The compact star mass is as massive as a typical white dwarf.

Corrections of the Distribution of Nuclei Due to Neutron Degeneracy and Their Effects in the R-Process in Neutron Stars—Black Hole Mergers

The r-process is one of the processes that produces heavy elements in the Universe. One of its possible astrophysical sites is the neutron star–black hole (NS-BH) merger. We first show that the neutrons can degenerate before and during the r-process in these mergers. Previous studies assumed neutrons were non-degenerate and the related rates were calculated under Maxwell–Boltzmann approximations. Hence, we corrected the related rates with neutron degeneracy put in the network code and calculated with the trajectories of NS-BH mergers. We show that there are differences in the nuclei distributions. The heating rates and the temperature at most can be two times larger. The change in heating rates and temperature can affect the light curves of the kilonovae. However, this has little effect on the final abundances.

Separating the Spectral Counterparts in NGC 1275/Perseus Cluster in X-Rays

We present a model-independent method for separating the spectral counterparts of the active galactic nucleus (AGN) NGC 1275 from the surrounding emission of the Perseus cluster, as observed by Suzaku/XIS cameras. The Perseus cluster emission extends to higher energies than typically observed in AGN environments, reaching up to 9–10 keV. This necessitates precise separation of AGN and cluster spectra. To circumvent the degeneracy arising from numerous spectral fitting parameters, including elemental abundances, thermal and Compton emissions from the nucleus, and spectral parameters of the jet synchrotron self-Compton/inverse Compton emissions, we avoid traditional spectral fitting methods. Instead, we leverage spatial resolution and employ a double background subtraction approach. We apply this procedure to the complete set of Suzaku/XIS observational data for NGC 1275, resulting in cleaned spectra and a light curve of the AGN emission in this system. To demonstrate the applicability of our method, we also utilize the available XMM-Newton/EPIC data.

Pre-Outburst Signal in the Light Curves of the Recurrent Novae RS Oph and T CrB

Pre-Outburst Signal in the Light Curves of the Recurrent Novae RS Oph and T CrB

On the nature of the eccentric eclipsing binary star SY Phe with a pulsating γ Dor component

ABSTRACT Spectroscopic observations of the eccentric binary system SY Phe were made at the South African Astronomical Observatory in 2019, 2020, and 2021, and its mid-resolution spectra were obtained. The radial velocities of the component stars were measured using the cross-correlation method and Fourier disentangling of the spectra. The spectral type (hence the effective temperature) of the primary star was determined from a model-atmosphere analysis. The radial velocity and Transiting Exoplanet Survey Satellite (TESS) light curves of the system were analysed, and its absolute parameters were derived. A strong (5.2 mmag) periodic signal with a frequency typical of $\gamma$ Dor stars (1.169 cycles per day) dominates the Fourier spectrum of the light curve between the eclipses. Apsidal motion parameters of SY Phe were calculated by studying eclipse timing variations. The Geneva evolution models indicate an evolutionary age of 2 Gyr and solar metallicity for the primary component; however, although the position of the secondary component in the H–R diagram matches the isochrone of 2 Gyr, it appears to have a larger radius and higher effective temperature than expected for its determined mass. Here, the secondary component has too large a radius, which is in accordance with the radius discrepancy problem that has been encountered in other studies, especially in late-type dwarfs, and has not been solved for half a century.

Generalized linear model study of the T90–T50 relation of gamma-ray bursts

ABSTRACT Gamma-ray bursts (GRBs) can be studied with their linearly dependent parameters alongside the standard $T_{90}$ distribution. The generalized linear model (GLM) identifies the number of linear dependences in a two-parameter space. Classically, GRBs are classified into two classes by the presence of bimodality in the histogram of $T_{90}$. However, additional classes and different features of GRBs are fascinating topics to explore. In this work, we investigate GRB features in the $T_{90} {-}T_{50}$ plane using the GLM for three major catalogues: Swift, the Fermi Gamma-ray Burst Monitor (GBM), and the Burst And Transient Source Experiment (BATSE). This study shows five linear features for the Fermi GBM catalogue and four linear features for the BATSE catalogue, directing us towards the possibility of non-Gaussianity in the light curves of GRBs.

MeerKAT observations of pair-plasma induced birefringence in the double pulsar eclipses

ABSTRACT PSR J0737−3039A/B is unique among double neutron star systems. Its near-perfect edge-on orbit causes the fast spinning pulsar A to be eclipsed by the magnetic field of the slow spinning pulsar B. Using high-sensitivity MeerKAT radio observations combined with updated constraints on the system geometry, we studied the impact of these eclipses on the incident polarization properties of pulsar A. Averaging light curves together after correcting for the rotation of pulsar B revealed enormous amounts of circular polarization and rapid changes in the linear polarization position angle, which occur at phases where emission from pulsar A is partially transmitted through the magnetosphere of pulsar B. These behaviours confirm that the eclipse mechanism is the result of synchrotron absorption in a relativistic pair-plasma confined to the closed-field region of pulsar B’s truncated dipolar magnetic field. We demonstrate that changes in circular polarization handedness throughout the eclipses are directly tied to the average line of sight magnetic field direction of pulsar B, from which we unambiguously determine the complete magnetic and viewing geometry of the pulsar.

Different manifestations of a loop-like transient brightening in solar atmospheres

Small-scale transient brightenings that are the consequence of magnetic reconnection play pivotal roles in the heating process of solar atmospheres. These phenomena contain key information about the dynamic evolution of the solar magnetic field. The fine-scale structures triggered by instabilities in these brightenings are intimately connected with the release of magnetic energy. To better understand the conversion and release of magnetic energy in small-scale heating events, we investigated the thermal-dynamical behaviors of a loop-like transient brightening (LTB) with plasma blobs. We used the spectroscopic and slit-jaw imaging observations taken from the Interface Region Imaging Spectrograph and the extreme-ultraviolet images taken from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory to analyze the plasma properties of an LTB that occurred on February 28, 2014. The space-time maps were created to present the spatial evolution of the LTB, and the light curves were calculated to illustrate the heating process. Additionally, we employed the differential emission measure (DEM) method to compute the temperature and emission measure of the LTB. In order to investigate the plasma motion along the line-of-sight direction, a double-Gaussian function was used to fit the Si IV spectral profiles. The spectrum and DEM analysis indicate that the LTB was constituted by multithermal plasma with temperatures reaching up to $5.4 $ K. The space-time maps of the emission and the Gaussian-fitting results of the Si IV line demonstrate that the LTB not only exhibited bidirectional flows, but was also twisted. Several plasma blobs were identified in the spine of the LTB, suggesting the potential presence of a tearing-mode instability. The low-temperature bands peaked approximately one minute prior to the high-temperature bands, suggesting the occurrence of a heating process driven by magnetic reconnection. The appearance of plasma blobs closely coincided with the sudden increase in the velocity and the quick rise of light curves, providing evidence that plasma blobs facilitate the release of magnetic energy during solar activity. Based on these findings, we speculate that the LTB was a complex structure that occurred in the upper chromosphere-transition region. These results clearly demonstrate that plasma blobs are important for the conversion and release processes of magnetic energy.

X-Ray Emission of Nearby Low-mass and Sunlike Stars with Directly Imageable Habitable Zones

Stellar X-ray and UV radiation can significantly affect the survival, composition, and long-term evolution of the atmospheres of planets in or near their host star’s habitable zone (HZ). Especially interesting are planetary systems in the solar neighborhood that may host temperate and potentially habitable surface conditions, which may be analyzed by future ground- and space-based direct-imaging surveys for signatures of habitability and life. To advance our understanding of the radiation environment in these systems, we leverage ∼3 Ms of XMM-Newton and Chandra observations in order to measure three fundamental stellar properties at X-ray energies for 57 nearby FGKM stellar systems: the shape of the stellar X-ray spectrum, the luminosity, and the timescales over which the stars vary (e.g., due to flares). These systems possess HZs that will be directly imageable to next-generation telescopes such as the Habitable Worlds Observatory and ground-based Extremely Large Telescopes. We identify 29 stellar systems with L X/L bol ratios similar to (or less than) that of the Sun; any potential planets in the HZs of these stars therefore reside in present-day X-ray radiation environments similar to (or less hostile than) modern Earth, though a broader set of these targets could host habitable planets. An additional 19 stellar systems have been observed with the Swift X-ray Telescope; in total, only ∼30% of potential direct imaging target stars has been observed with XMM-Newton, Chandra, or Swift. The data products from this work (X-ray light curves and spectra) are available via a public Zenodo repository (doi:10.5281/zenodo.11490574).

A Study of Stochastic Low-frequency Variability for Galactic O-type Stars

In order to explore how the ubiquitous stochastic low-frequency (SLF) variability of O-type stars is related to various stellar characteristics, we compiled a sample of 150 O-type stars observed via ground-based spectroscopic surveys, alongside photometric data obtained from the Transiting Exoplanet Survey Satellite (TESS). We analyzed 298 light curves obtained from TESS Sectors 1–65 for the stars in our sample. Leveraging the spectroscopic parameters, we used Bonnsai to determine masses, radii, fractional main-sequence ages, and mass-loss rates for stars of our sample. Subsequently, we identified possible correlations between the fitted parameters of SLF variability and stellar properties. Our analysis unveiled four significant correlations between the amplitude and stellar parameters, including mass, radius, fractional main-sequence ages, and mass-loss rate. For stars with ≳30 M ⊙, we observed a decrease in characteristic frequency and steepness with increasing radius. Finally, we compared various physical processes that may account for the SLF variability with our results. The observed SLF variability may arise from the combined effects of the iron convection zone (FeCZ) and internal gravity waves (IGWs), with IGWs potentially more dominant in the early stages of stellar evolution, and the contribution of FeCZ becoming more significant as stars evolve. Meanwhile, our results indicate that the SLF variability of O-type stars bears certain signatures of the line-driven wind instability and granulation.

Photometry and Models of Seven Main-Belt Asteroids

The China Near-Earth Object Survey Telescope (CNEOST) conducted four photometric surveys from 2015 to 2018 using image processing and aperture photometry techniques to obtain extensive light curve data on asteroids. The second-order Fourier series method was selected for its efficiency in determining the rotation periods of the observed asteroids. Our study successfully derived rotation periods for 892 asteroids, with 648 of those matching values recorded in the LCDB (for asteroids with U &gt; 2). To enhance the reliability of the derived spin parameters and shape models, we also amassed a comprehensive collection of published light curve data supplemented by additional photometric observations on a targeted subset of asteroids conducted using multiple telescopes between 2021 and 2022. Through the application of convex inversion techniques, we successfully derived spin parameters and shape models for seven main-belt asteroids (MBAs): (2233) Kuznetsov, (2294) Andronikov, (2253) Espinette, (4796) Lewis, (1563) Noel, (2912) Lapalma, and (5150) Fellini. Our thorough analysis identified two credible orientations for the rotational poles of these MBAs, shedding light on the prevalent issue of “ambiguity in pole direction” that often accompanies photometric inversion processes. CNEOST continues its observational endeavors, and future collected data combined with other independent photometric measurements will facilitate further inversion to better constrain the spin parameters and yield more refined shape models.

Rapid Analysis for α-Tocopherol and Its Oxidative Products in the Pisum sativum L. Leaf Using Supercritical Fluid Chromatography-Medium Vacuum Chemical Ionization Tandem Mass Spectrometry.

A method for the rapid determination of α-tocopherol (α-T) and its oxidative products in plant tissue has been developed using supercritical fluid extraction (SFE) coupled with supercritical fluid chromatography (SFC) and medium vacuum chemical ionization (MVCI) with tandem mass spectrometry. The method is designed to study changes in levels for α-T and its oxidative products in plant cells during photosynthesis, aiming to observe the light response curves. α-T oxidation is a non-enzymatic self-defense mechanism in plant cells. Unlike enzyme-involved reactions, it cannot be stopped, so the oxidation continues in crude extracts even after extraction. Therefore, a real-time in-situ method is essential for tracking the light response curves. To optimize the selective reaction monitoring method, the reaction mixture of α-T and singlet oxygen (1O2), generated by rose Bengal under light illumination, was used as the source of oxidative products. The relative abundance changes in α-tocopherylquinone and 8a-hydroperoxy tocopherone in Pisum sativum L. (Pea) leaves under excessive light illumination have been preliminarily analyzed as part of the light response curve study. The method archives a throughput of 10-15 minutes for analyzing duplicate leaf samples. This process includes cutting off the leaf, sectioning it, placing the sample in a frozen SFE vessel, and conducting SFE/SFC analysis. Consequently, the average throughput is approximately 5-7 minutes per sample.

A regularisation technique to precisely infer limb darkening using transit measurements: Can we estimate stellar surface magnetic fields?

Abstract The high-precision measurements of exoplanet transit light curves that are now available contain information about the planet properties, their orbital parameters, and stellar limb darkening (LD). Recent 3D magneto-hydrodynamical (MHD) simulations of stellar atmospheres have shown that LD depends on the photospheric magnetic field, and hence its precise determination can be used to estimate the field strength. Among existing LD laws, the uses of the simplest ones may lead to biased inferences, whereas the uses of complex laws typically lead to a large degeneracy among the LD parameters. We have developed a novel approach in which we use a complex LD model but with second derivative regularisation during the fitting process. Regularisation controls the complexity of the model appropriately and reduces the degeneracy among LD parameters, thus resulting in precise inferences. The tests on simulated data suggest that our inferences are not only precise but also accurate. This technique is used to re-analyse 43 transit light curves measured by the NASA Kepler and TESS missions. Comparisons of our LD inferences with the corresponding literature values show good agreement, while the precisions of our measurements are better by up to a factor of 2. We find that 1D non-magnetic model atmospheres fail to reproduce the observations while 3D MHD simulations are qualitatively consistent. The LD measurements, together with MHD simulations, confirm that Kepler-17, WASP-18, and KELT-24 have relatively high magnetic fields (&amp;gt;200 G). This study paves the way for estimating the stellar surface magnetic field using the LD measurements.

An acceleration-radiation model for nonthermal flares from Sgr A*

is the electromagnetic counterpart of the accreting supermassive black hole in the Galactic center. Its emission is variable in the near-infrared (NIR) and X-ray wavelengths on short timescales (several minutes to a few hours). The NIR light curve displays red-noise variability, while the X-ray light curve exhibits bright flares that rise by many orders of magnitude upon the stable X-ray quiescent emission. Every X-ray flare is associated with a bright NIR flux change, but the opposite is not always true. The physical origin of NIR and X-ray flares is still under debate. We introduce a model for the production of NIR and X-ray flares flares from an active region in where particle acceleration takes place intermittently. A fraction of electrons from their thermal pool is accelerated to higher energies while they radiate via synchrotron and synchrotron self-Compton (SSC) processes. in contrast to other radiation models for flares, the particle acceleration is not assumed to be instantaneous. We studied the evolution of the particle distribution and the emitted electromagnetic radiation from the flaring region by numerically solving the kinetic equations for electrons and photons. Our calculations took the finite duration of particle acceleration, radiative energy losses, and physical escape from the flaring region into account. To gain better insight into the relation of the model parameters, we complemented our numerical study with analytical calculations. Flares are produced when the acceleration episode has a finite duration. The rising part in the light curve of a flare is related to the particle acceleration timescale, while the decay is controlled by the cooling or escape timescale of particles. The emitted synchrotron spectra are power laws whose photon index is determined by the ratio of the acceleration and escape timescales, followed by an exponential cutoff. This occurs at the characteristic synchrotron photon energy emitted by particles with the maximum Lorentz factor (where energy loss and gain rates become equal). The NIR flux increases before the onset of the X-ray flare, and the time lag is linked to the particle acceleration timescale. Bright X-ray flares, such as the one observed in 2014, have gamma -ray counterparts that might be detected by the Cherenkov Telescope Array Observatory. Our generic model for NIR and X-ray flares favors an interpretation of diffusive nonresonant particle acceleration in magnetized turbulence. If direct acceleration by the reconnection electric field in macroscopic current sheets causes the energization of particles during flares in then models considering the injection of preaccelerated particles into a blob where particles cool and/or escape would be appropriate to describe the flare.