Features In Light Curves Research Articles

New candidates for AM Canum Venaticorum stars among ASAS-SN transients

Abstract We studied Zwicky Transient Facility (ZTF) light curves of 34 dwarf nova candidates discovered by the All-Sky Automated Survey for Supernovae (ASAS-SN) between 2020 May 12 and September 9 and found six AM CVn-type candidates. All objects showed short outbursts (post-superoutburst rebrightenings) on the fading tail. Two objects (ASASSN-20eq, ASASSN-20la) showed double superoutbursts. Three objects (ASASSN-20jt, ASASSN-20ke, and ASASSN-20lr) showed short superoutbursts (5–6 d). These features in the light curve can be used in discriminating AM CVn-type candidates from hydrogen-rich systems. In contrast to hydrogen-rich systems, some object did not show red color excess during the rebrightening or fading tail phase. We interpret that this is due to the higher ionization temperature in helium disks. Two objects had long (likely) supercycles: ASASSN-20gx (8.5 yr) and ASASSN-20lr (7 yr). We provide a scheme for identifying AM CVn-type candidates based on the light curve characteristics.

A Low-mass Binary Neutron Star: Long-term Ejecta Evolution and Kilonovae with Weak Blue Emission

Abstract We study the long-term evolution of ejecta formed in a binary neutron star (NS) merger that results in a long-lived remnant NS by performing a hydrodynamics simulation with the outflow data of a numerical relativity simulation as the initial condition. At the homologously expanding phase, the total ejecta mass reaches ≈ 0.1 M ⊙ with an average velocity of ≈ 0.1 c and lanthanide fraction of ≈ 0.005. We further perform the radiative transfer simulation employing the obtained ejecta profile. We find that, contrary to a naive expectation from the large ejecta mass and low lanthanide fraction, the optical emission is not as bright as that in GW170817/AT2017gfo, while the infrared emission can be brighter. This light-curve property is attributed to preferential diffusion of photons toward the equatorial direction due to the prolate ejecta morphology; large opacity contribution of Zr, Y, and lanthanides; and low specific heating rate of the ejecta. Our results suggest that these light-curve features could be used as an indicator for the presence of a long-lived remnant NS. We also found that the bright optical emission broadly consistent with GW170817/AT2017gfo is realized for the case in which the high-velocity ejecta components in the polar region are suppressed. These results suggest that the remnant in GW170817/AT2017gfo is unlikely to be a long-lived NS but might have collapsed to a black hole within s.

Modelling the projected separation of microlensing events using systematic time-series feature engineering

Gravitational microlensing events have led to the discovery of more than 80 planets. In anticipation of the launch of the NASA Nancy Grace Roman Space Telescope, observations of gravitational microlensing events are expected to become much more numerous, and current manual techniques for their analysis will become insufficient. As a first step towards the automated estimated of microlensing event parameters, we present a workflow for identifying characteristics of light curves to estimate the projected separation from microlensing observations. We based this method on systematic time-series feature analysis of simulated light curves and validated it for a parameter space reduced to the planet–star separation. We determined a set of seven time-series features for making accurate predictions of the separation parameter. This reduced feature space of light curves serves as reliable input to both parametric and nonparametric regression models. Specifically, we validated a Random Forest regressor with respect to noise and data outages, which are common to current microlensing data, and found that the model is very robust. For this purpose, we created an empirical noise model from known microlensing events and introduced a model for simulating missing data due to data outages. Furthermore, we present an implementation of Bayesian Linear Regression on polynomial combinations of these seven light curve features, which computes probability distributions for recovered planet–star separations. The Random Forest and Bayesian Linear Regression regressors have an out-of-sample mean absolute error of 0.00057 R E and 0.00110 R E , respectively. The presented feature extraction workflow is expected to open new opportunities for mapping observed light curves to the large parameter space of microlensing events, which will be very useful for analysing the data from the Roman telescope mission.

Classifying High-cadence Microlensing Light Curves. I. Defining Features

Abstract Microlensing is a powerful tool for discovering cold exoplanets, and the Roman Space Telescope microlensing survey will discover over 1000 such planets. Rapid, automated classification of Roman’s microlensing events can be used to prioritize follow-up observations of the most interesting events. Machine learning is now often used for classification problems in astronomy, but the success of such algorithms can rely on the definition of appropriate features that capture essential elements of the observations that can map to parameters of interest. In this paper, we introduce tools that we have developed to capture features in simulated Roman light curves of different types of microlensing events, and we evaluate their effectiveness in classifying microlensing light curves. These features are quantified as parameters that can be used to decide the likelihood that a given light curve is due to a specific type of microlensing event. This method leaves us with a list of parameters that describe features like the smoothness of the peak, symmetry, the number of peaks, and the width and height of small deviations from the main peak. This will allow us to quickly analyze a set of microlensing light curves and later use the resulting parameters as input to machine learning algorithms to classify the events.

Years-delayed X-Ray Afterglows of TDEs Originated from Wind–Torus Interactions

Abstract Tidal disruption events (TDEs) occurring in active galactic nuclei (AGNs) are a special class of sources with outstanding scientific significance. TDEs can generate ultrafast winds, which should almost inevitably collide with the preexisting AGN dusty tori. We perform analytical calculations and simulations on the wind–torus interactions and find such a process can generate considerable X-ray afterglow radiation several years or decades later after the TDE outburst. This provides a new origin for the years delayed X-rays in TDEs. The X-ray luminosity can reach 1041−42 erg s−1, and the light curve characteristics depend on the parameters of winds and tori. We apply the model to two TDE candidates, and provide lower limits on the masses of the disrupted stars, as well as rigorous constraints on the gas densities of tori. Our results suggest that the observations of the time delay, spectral shape, luminosity, and the light curve of the X-ray afterglow can be used to constrain the physical parameters of both TDE winds and tori, including the wind velocity, wind density, cloud density, and cloud size.

Cloudy Atmospheres on Directly Imaged Exoplanets: The Need for Accurate Particulate Representation in Photopolarimetric Simulations

Abstract Missions like the upcoming Roman Space Telescope and its follow-on missions, Habitable Exoplanet Observatory (HabEx) and the Large UV/Optical/IR Surveyor (LUVOIR), will provide direct imaging observations of stellar light reflected by exoplanets with successively closer orbits. The synergistic use of ground-based polarimeters like Gemini Planet Imager and Very Large Telescope/Spectro-Polarimetric High-contrast Exoplanet Research instrument (SPHERE) would allow us to characterize cloudy exoplanet atmospheres using spectropolarimetric direct imaging. We present an extension of our semianalytic 3D radiative transfer modeling framework for brown dwarfs to include stellar light reflected by exoplanets with cloudy atmospheres. Using Mie theory to compute scattering by cloud and haze consisting of spherical particles, we show that the currently widespread use of approximations like the scalar Two-Term Henyey–Greenstein or the vector Henyey–Greenstein Rayleigh (HGR) composite result in a blurring of the phase-dependent features of exoplanet lightcurves, causing a 10%–39% loss of sensitivity to atmospheric parameters in an average measurement for signal-to-noise ratios (S/Ns) between 5 and 500. The HGR approximation creates the misleading impression that clouds are as polarizing as Rayleigh scatterers, regardless of their droplet size. This not only causes significant errors in the scientific interpretation of polarimetric measurements, but also results in a negligible sensitivity of HGR simulations to polarization measurements at the S/Ns considered, whereas Mie simulations show a 10%–30% gain in parametric sensitivity through the addition of polarimetry.

What does FRB light-curve variability tell us about the emission mechanism?

ABSTRACT A few fast radio bursts’ (FRBs) light curves have exhibited large intrinsic modulations of their flux on extremely short ($t_{\rm r}\sim 10\, \mu$s) time-scales, compared to pulse durations (tFRB ∼ 1 ms). Light-curve variability time-scales, the small ratio of rise time of the flux to pulse duration, and the spectro-temporal correlations in the data constrain the compactness of the source and the mechanism responsible for the powerful radio emission. The constraints are strongest when radiation is produced far (≳1010 cm) from the compact object. We describe different physical set-ups that can account for the observed tr/tFRB ≪ 1 despite having large emission radii. The result is either a significant reduction in the radio production efficiency or distinct light-curve features that could be searched for in observed data. For the same class of models, we also show that due to high-latitude emission, if a flux f1(ν1) is observed at t1 then at a lower frequency ν2 < ν1 the flux should be at least (ν2/ν1)2f1 at a slightly later time (t2 = t1ν1/ν2) independent of the duration and spectrum of the emission in the comoving frame. These features can be tested, once light-curve modulations due to scintillation are accounted for. We provide the time-scales and coherence bandwidths of the latter for a range of possibilities regarding the physical screens and the scintillation regime. Finally, if future highly resolved FRB light curves are shown to have intrinsic variability extending down to ${\sim}\mu$s time-scales, this will provide strong evidence in favour of magnetospheric models.

Non-radially pulsating stars as microlensing sources

ABSTRACT We study the microlensing of non-radially pulsating (NRP) stars. Pulsations are formulated for stellar radius and temperature using spherical harmonic functions with different values of l, m. The characteristics of the microlensing light curves from NRP stars are investigated in relation to different pulsation modes. For the microlensing of NRP stars, the light curve is not a simple multiplication of the magnification curve and the intrinsic luminosity curve of the source star, unless the effect of finite source size can be ignored. Three main conclusions can be drawn from the simulated light curves. First, for modes with m ≠ 0 and when the viewing inclination is more nearly pole-on, the stellar luminosity towards the observer changes little with pulsation phase. In this case, high-magnification microlensing events are chromatic and can reveal the variability of these source stars. Secondly, some combinations of pulsation modes produce nearly degenerate luminosity curves (e.g. (l, m) = (3, 0), (5, 0)). The resulting microlensing light curves are also degenerate, unless the lens crosses the projected source. Finally, for modes involving m = 1, the stellar brightness centre does not coincide with the coordinate centre, and the projected source brightness centre moves in the sky with pulsation phase. As a result of this time-dependent displacement in the brightness centroid, the time of the magnification peak coincides with the closest approach of the lens to the brightness centre as opposed to the source coordinate centre. Binary microlensing of NRP stars and in caustic-crossing features are chromatic.

Constraining the oblateness of transiting planets with photometry and spectroscopy

ABSTRACT Rapid planetary rotation can cause the equilibrium shape of a planet to be oblate. While planetary oblateness has mostly been probed by examining the subtle ingress and egress features in photometric transit light curves, we investigate the effect of oblateness on the spectroscopic Rossiter–McLaughlin (RM) signals. We found that a giant planet, with planet-to-star radius ratio of 0.15 and Saturn-like oblateness of 0.098, can cause spectroscopic signatures with amplitudes up to 1.1 m s−1 which is detectable by high-precision spectrographs such as ESPRESSO. We also found that the spectroscopic oblateness signals are particularly amplified for transits across rapidly rotating stars and for planets with spin-orbit misalignment thereby making them more prominent than the photometric signals at some transit orientations. We compared the detectability of oblateness in photometry and spectroscopy and found that photometric light curves are more sensitive to detecting oblateness than the spectroscopic RM signals mostly because they can be sampled with higher cadence to better probe the oblateness ingress and egress anomaly. However, joint analyses of the light curve and RM signal of a transiting planet provides more accurate and precise estimate of the planet’s oblateness. Therefore, ESPRESSO alongside ongoing and upcoming photometric instruments such as TESS, CHEOPS, PLATO, and JWST will be extremely useful in measuring planet oblateness.

Transiting Exoplanet Discovery Using Machine Learning Techniques: A Survey

Spatial missions such as the Kepler mission, and the Transiting Exoplanet Survey Satellite (TESS) mission, have encouraged data scientists to analyze light curve datasets. The purpose of analyzing these data is to look for planet transits, with the aim of discovering and validating exoplanets, which are planets found outside our Solar System. Furthermore, transiting exoplanets can be better characterized when light curves and radial velocity curves are available. The manual examination of these datasets is a task that requires big quantities of time and effort, and therefore is prone to errors. As a result, the application of machine learning methods has become more common on exoplanet discovery and categorization research. This survey presents an analysis on different exoplanet transit discovery algorithms based on machine learning, some of which even found new exoplanets. The analysis of these algorithms is divided into four steps, namely light curve preprocessing, possible exoplanet signal detection, and identification of the detected signal to decide whether it belongs to an exoplanet or not. We propose a model to create synthetic datasets of light curves, and we compare the performance of several machine learning models used to identify transit exoplanets, with inputs preprocessed with and without using the Discrete Wavelet Transform (DWT). Our experimental results allow us to conclude that multiresolution analysis in the time-frequency domain can improve exoplanet signal identification, because of the characteristics of light curves and transiting exoplanet signals.

X-Ray Afterglows from the Gamma-Ray Burst “Large-angle” Emission

Abstract We derive basic analytical results for the timing and decay of the gamma-ray burst (GRB) counterpart and delayed afterglow light curves for a brief emission episode from a relativistic surface endowed with angular structure, consisting of a uniform core of size (Lorentz factor and surface emissivity are angle independent) and an axially symmetric power-law envelope ( ). In this “large-angle emission” model, radiation produced during the prompt emission phase (GRB) at angles arrives at the observer well after the burst (delayed emission). The dynamical time range of the very fast decaying GRB “tail” and of the flat afterglow “plateau” and the morphology of the GRB counterpart/afterglow are all determined by two parameters: the core's parameter and the envelope's Lorentz factor index g, leading to three types of light curves that display three post-GRB phases (type 1: tail, plateau/slow decay, post-plateau/normal decay), two post-GRB phases (type 2: tail and fast decay), or just one (type 3: normal decay). We show how X-ray light-curve features can be used to determine core and envelope dynamical and spectral parameters. Testing of the large-angle emission model is done using the Swift/XRT X-ray emission of two afterglows of type 1 (GRB 060607A, GRB 061121), one of type 2 (GRB 061110A), and one of type 3 (GRB 061007). We find that the X-ray afterglows with plateaus require an envelope Lorentz factor and a comoving-frame emissivity ; thus, for a typical afterglow spectrum , the lab-frame energy release is uniform over the emitting surface.

Classification of Continuous Sky Brightness Data Using Random Forest

Sky brightness measuring and monitoring are required to mitigate the negative effect of light pollution as a byproduct of modern civilization. Good handling of a pile of sky brightness data includes evaluation and classification of the data according to its quality and characteristics such that further analysis and inference can be conducted properly. This study aims to develop a classification model based on Random Forest algorithm and to evaluate its performance. Using sky brightness data from 1250 nights with minute temporal resolution acquired at eight different stations in Indonesia, datasets consisting of 15 features were created to train and test the model. Those features were extracted from the observation time, the global statistics of nightly sky brightness, or the light curve characteristics. Among those features, 10 are considered to be the most important for the classification task. The model was trained to classify the data into six classes (1: peculiar data, 2: overcast, 3: cloudy, 4: clear, 5: moonlit-cloudy, and 6: moonlit-clear) and then tested to achieve high accuracy (92%) and scores (F-score = 84% and G-mean = 84%). Some misclassifications exist, but the classification results are considerably good as indicated by posterior distributions of the sky brightness as a function of classes. Data classified as class-4 have sharp distribution with typical full width at half maximum of 1.5 mag/arcsec2, while distributions of class-2 and -3 are left skewed with the latter having lighter tail. Due to the moonlight, distributions of class-5 and -6 data are more smeared or have larger spread. These results demonstrate that the established classification model is reasonably good and consistent.

On some strange features in Kepler EB light curves.

On some strange features in Kepler EB light curves.

Structured Jets and X-Ray Plateaus in Gamma-Ray Burst Phenomena

Abstract The first multi-messenger detection of a binary neutron star merger, GW170817, brought to the forefront the structured jet model as a way to explain multiwavelength observations taken more than a year after the event. Here, we show that the high-latitude emission from a structured jet can naturally produce an X-ray plateau in gamma-ray burst (GRB) light curves, independent of the radiation from an external shock. We calculate the radiation from a switched-off shell featuring an angular structure in both its relativistic bulk motion and intrinsic brightness. Our model is able to explain the shallow decay phase (plateau) often observed in GRB X-ray light curves. We discuss the possible contribution of the structured jet high-latitude emission to other distinctive features of GRB X-ray light curves, and its capability to explain the chromatic optical/X-ray light-curve properties.

Humps and bumps: the effects of shocks on the optical light curves of fundamental-mode RR Lyrae stars

We present the most extended and homogeneous study carried out so far of the main and early shocks in 1485 RR Lyrae stars in the Galactic bulge observed by the Optical Gravitational Lensing Experiment. We selected nonmodulated fundamental-mode RR Lyrae stars with good-quality photometry. Using a self-developed method, we determined the centers and strengths of main and early shock features in the phased light curves. We found that the positions of both humps and bumps are highly correlated with the pulsation properties of the studied variables. Pulsators with a pronounced main shock are concentrated in the low-amplitude regime of the period–amplitude diagram, while stars with a strong early shock have average and above-average pulsation amplitudes. A connection between the main and early shocks and the Fourier coefficients is also observed. In the color–magnitude diagram, we see a separation between stars with strong and weak shocks. Variables with a pronounced main shock cluster close to the fundamental red edge of the instability strip (IS), while stars with a strong early shock tend to clump in the center and near the fundamental blue edge of the IS. The appearance of shocks and their properties appear to be independent of the direction of evolution estimated from the period change rate of the studied stars. In addition, the differences in the period change rate between the two main Oosterhoff groups found in the Galactic bulge suggest that stars of Oosterhoff type I are located close to the zero-age horizontal branch while Oosterhoff type II variables are on their way toward the fundamental red edge of the instability strip, and have therefore already left the zero-age horizontal branch.

Photometric, kinematic, and variability study in the young open cluster NGC 1960

ABSTRACT We present a comprehensive photometric analysis of a young open cluster NGC 1960 (= M36) along with the long-term variability study of this cluster. Based on the kinematic data of Gaia DR2, the membership probabilities of 3871 stars are ascertained in the cluster field among which 262 stars are found to be cluster members. Considering the kinematic and trigonometric measurements of the cluster members, we estimate a mean cluster parallax of 0.86 ± 0.05 mas and mean proper motions of μRA = −0.143 ± 0.008 mas yr−1 and μDec. = −3.395 ± 0.008 mas yr−1. We obtain basic parameters of the cluster such as E(B − V) = 0.24 ± 0.02 mag, log(Age/yr) = 7.44 ± 0.02, and d = 1.17 ± 0.06 kpc. The mass function slope in the cluster for the stars in the mass range of 0.72–7.32 M⊙ is found to be γ = −1.26 ± 0.19. We find that mass segregation is still taking place in the cluster which is yet to be dynamically relaxed. This work also presents first high-precision variability survey in the central 13 arcmin × 13 arcmin region of the cluster. The V-band photometric data accumulated on 43 nights over a period of more than 3 yr reveals 76 variable stars among which 72 are periodic variables. Among them, 59 are short period (P < 1 d) and 13 are long period (P > 1 d). The variable stars have V magnitudes ranging between 9.1 to 19.4 mag and periods between 41 min and 10.74 d. On the basis of their locations in the H–R diagram, periods, and characteristic light curves, 20 periodic variables belong to the cluster are classified as 2 δ-Scuti, 3 γ-Dor, 2 slowly pulsating B stars, 5 rotational variables, 2 non-pulsating B stars, and 6 as miscellaneous variables.

Luminous Red Novae: population models and future prospects

ABSTRACT A class of optical transients known as Luminous Red Novae (LRNe) have recently been associated with mass ejections from binary stars undergoing common-envelope evolution. We use the population synthesis code COMPAS to explore the impact of a range of assumptions about the physics of common-envelope evolution on the properties of LRNe. In particular, we investigate the influence of various models for the energetics of LRNe on the expected event rate and light curve characteristics, and compare with the existing sample. We find that the Galactic rate of LRNe is ∼0.2 yr−1, in agreement with the observed rate. In our models, the luminosity function of Galactic LRNe covers multiple decades in luminosity and is dominated by signals from stellar mergers, consistent with observational constraints from iPTF and the Galactic sample of LRNe. We discuss how observations of the brightest LRNe may provide indirect evidence for the existence of massive (>40 M⊙) red supergiants. Such LRNe could be markers along the evolutionary pathway leading to the formation of double compact objects. We make predictions for the population of LRNe observable in future transient surveys with the Large Synoptic Survey Telescope and the Zwicky Transient Facility. In all plausible circumstances, we predict a selection-limited observable population dominated by bright, long-duration events caused by common envelope ejections. We show that the Large Synoptic Survey Telescope will observe 20–750 LRNe per year, quickly constraining the luminosity function of LRNe and probing the physics of common-envelope events.

Neutrino Emissions from Tidal Disruption Remnants

Abstract We study high-energy neutrino emissions from tidal disruption remnants (TDRs) around supermassive black holes. The neutrinos are produced by the decay of charged pions originating in ultrarelativistic protons that are accelerated there. In the standard theory of tidal disruption events (TDEs), there are four distinct phases from the debris circularization of stellar debris to super- and sub-Eddington to radiatively inefficient accretion flows (RIAFs). In addition, we consider the magnetically arrested disk (MAD) state in both the super-Eddington accretion and RIAF phases. We find that there are three promising cases to produce neutrino emissions: the super-Eddington accretion phase of the MAD state and the RIAF phases of both the non-MAD and MAD states. In the super-Eddington MAD state, the enhanced magnetic field makes it possible to accelerate the protons to with the other given appropriate parameters. The neutrino energy is then at the peak of the energy spectrum. For M bh ≳ 107.7 M ⊙, the neutrino light curve is proportional to , while it follows the standard decay rate for . In both cases, the large luminosity and characteristic light curves diagnose the super-Eddington MAD state in TDEs. In the RIAF phase of the non-MAD state, we find and , and its light curve is proportional to . This indicates that one can identify whether the existing RIAFs are the TDE origin or not. TDRs are potentially a population of hidden neutrino sources invisible in gamma-rays.

Characterization of material around the centaur (2060) Chiron from a visible and near-infrared stellar occultation in 2011

ABSTRACT The centaur (2060) Chiron exhibits outgassing behaviour and possibly hosts a ring system. On 2011 November 29, Chiron occulted a fairly bright star (R ∼ 15 mag) as observed from the 3-m NASA Infrared Telescope Facility (IRTF) on Mauna Kea and the 2-m Faulkes Telescope North (FTN) at Haleakala. Data were taken as visible wavelength images and simultaneous, low-resolution, near-infrared (NIR) spectra. Here, we present a detailed examination of the light-curve features in the optical data and an analysis of the NIR spectra. We place a lower limit on the spherical diameter of Chiron's nucleus of 160.2 ± 1.3 km. Sharp, narrow dips were observed between 280 and 360 km from the centre (depending on event geometry). For a central chord and assumed ring plane, the separated features are 298.5–302 and 308–310.5 km from the nucleus, with normal optical depth ∼0.5–0.9, and a gap of 9.1 ± 1.3 km. These features are similar in equivalent depth to Chariklo's inner ring. The absence of absorbing/scattering material near the nucleus suggests that these sharp dips are more likely to be planar rings than a shell of material. The region of relatively increased transmission is within the 1:2 spin-orbit resonance, consistent with the proposed clearing pattern for a non-axisymmetric nucleus. Characteristics of possible azimuthally incomplete features are presented, which could be transient, as well as a possible shell from ∼900–1500 km: future observations are needed for confirmation. There are no significant features in the NIR light curves, nor any correlation between optical features and NIR spectral slope.

On the Role of a Cavity in the Hypernova Ejecta of GRB 190114C

Abstract Within the binary-driven hypernova I (BdHN I) scenario, the gamma-ray burst GRB190114C originates in a binary system composed of a massive carbon–oxygen core (COcore), and a binary neutron star (NS) companion. As the COcore undergoes a supernova explosion with the creation of a new neutron star (νNS), hypercritical accretion occurs on the companion binary neutron star until it exceeds the critical mass for gravitational collapse. The formation of a black hole (BH) captures 1057 baryons by enclosing them within its horizon, and thus a cavity of approximately 1011 cm is formed around it with initial density 10−7 g cm−3. A further depletion of baryons in the cavity originates from the expansion of the electron-positron-photon (e + e − γ) plasma formed at the collapse, reaching a density of 10−14 g cm−3 by the end of the interaction. It is demonstrated here using an analytical model complemented by a hydrodynamical numerical simulation that part of the e + e − γ plasma is reflected off the walls of the cavity. The consequent outflow and its observed properties are shown to coincide with the featureless emission occurring in a time interval of duration t rf, measured in the rest frame of the source, between 11 and 20 s of the GBM observation. Moreover, similar features of the GRB light curve were previously observed in GRB 090926A and GRB 130427A, all belonging to the BdHN I class. This interpretation supports the general conceptual framework presented in R. Ruffini et al. and guarantees that a low baryon density is reached in the cavity, a necessary condition for the operation of the “inner engine” of the GRB presented in an accompanying article.