Light Curve Variations Research Articles

Magnetic activity on two contact binaries: VW Boo and V1128 Tau from TESS data

Using the continuous TESS data, we found small intrinsic variations in the light curves of VW Boo and V1128 Tau. We selected five sets of light curves and derived the photometric solutions by using the Wilson-Devinney code. The results indicate that the intrinsic light curve variations in VW Boo are attributed to the evolution of a cool spot. In the case of V1128 Tau, the disappearance or reappearance of a cool spot might account for the variation in three sets of light curves. The obtained solutions also reveal that these two systems are W-type shallow-contact binaries (f≤15%). Period analyses for these two systems were carried out based on all available times of minimum. The period analysis of VW Boo shows that its period is decreasing at a rate of dP/dt=−1.52×10−7d yr−1, together with a cyclic oscillation with a period of 27.17 yr and a semi-amplitude of 0.0063 d. Similarly, the orbital period of V1128 Tau also shows a secular decrease (dP/dt=−1.40×10−7d yr−1), accompanied by a cyclic oscillation (P3= 16.11 yr and K3=0.0011 d). The secular decrease might be caused by mass transfer, while the cyclic oscillation might result from the presence of a third body or magnetic activity.

Machine-learning Morphological Classification of TESS Light Curves of T Tauri Stars

We present a variability and morphological classification study of TESS light curves for T Tauri star candidates in the Orion, IC 348, γ Velorum, Upper Scorpius, Corona Australis, and Perseus OB2 regions. We propose 11 morphological classes linking brightness variation behaviors with possible physical or geometric phenomena present in T Tauri stars, and develop a supervised machine-learning algorithm to automate the classification among these. Our algorithm optimizes and compares the true positive rate (recall) among k-nearest neighbors, classification trees, random forests, and support vector machines. This is done characterizing light curves with features depending on time, periodicity, and magnitude distribution. Binary and multiclass classifiers are trained and interpreted in a way that allows our final algorithm to have single or mixed classes. In the testing sample, the algorithm assigns mixed classes to 27% of the stars, reaching up to five simultaneous classes. A catalog of 3672 T Tauri star candidates is presented, along with their possible period estimations, predicted morphological classes, and visually revised ones. The cross-validation estimated performance of the final classifiers is reported. Binary classifiers surpass multiclass recall values for classes with less representation in the training sample. Support vector machines and random forest classifiers obtain better recalls. For comparison, another performance estimation of the final classifiers is calculated using the revised classes of our testing sample, indicating that this performance excels in singled classed stars, which happens in about 75% of the testing sample.

Long-term hard X-ray variability properties of Swift-BAT blazars

ABSTRACT We present results from the first dedicated study in the time domain of the hard X-ray variability behaviour of blazars on long time-scales based on ∼13 yr of continuous hard X-ray data in the 14–195 keV band. We use monthly binned data from the recent 157-month Swift-BAT (Burst Alert Telescope) catalogue to characterize the hard X-ray variability of 127 blazars and search for potential differences between the variability of BL Lacertae objects (BL Lacs) and flat-spectrum radio quasars (FSRQs). A significant portion of the blazars in the sample (∼37 per cent) do not show statistically significant hard X-ray variability on monthly time-scales, which is deeply at odds with previous studies that show that blazars are highly variable in the X-rays and other energy bands on a wide range of time-scales. We also find that, on average, the FSRQs and BL Lacs for which we do detect variability exhibit similar flux variability; this suggests that the variability in these FSRQs is not necessarily driven by variations in the source function of scattered external radiation arriving from extended regions, and that it is instead possibly driven by processes that lead to variations in particle injection. In addition, only five blazars in our sample show significant spectral variability in the long-term light curves. For three blazars, we find that a power law that changes slope on monthly time-scales is sufficient to characterize the variable hard X-ray spectrum, suggesting that, at least for some bright blazars, the long-term spectra in the hard X-rays may be described in a relatively simple fashion.

Negative Lags on the Viscous Timescale in Quasar Photometry and Prospects for Detecting More with LSST

The variability of quasar light curves can be used to study the structure of quasar accretion disks. For example, continuum reverberation mapping uses delays between variability in short and long wavelength bands (short lags) to measure the radial extent and temperature profile of the disk. Recently, a potential reverse lag, where variations in shorter wavelength bands lag the longer wavelength bands at the much longer viscous timescale, was detected for Fairall 9. Inspired by this detection, we derive a timescale for these long negative lags from fluctuation propagation models and recent simulations. We use this timescale to forecast our ability to detect long lags using the Vera Rubin Legacy Survey of Space and Time (LSST). After exploring several methods, including the interpolated cross-correlation function, a Von-Neumann estimator, javelin, and a maximum-likelihood Fourier method, we find that our two main methods, javelin and the maximum-likelihood method, can together detect long lags of up to several hundred days in mock LSST light curves. Our methods work best on proposed LSST cadences with long season lengths, but can also work for the current baseline LSST cadence, especially if we add observations from other optical telescopes during seasonal gaps. We find that LSST has the potential to detect dozens to hundreds of additional long lags. Detecting these long lags can teach us about the vertical structure of quasar disks and how it scales with different quasar properties.

New rotation period measurements of 67 163 Kepler stars

Context. The Kepler space telescope leaves a legacy of tens of thousands of stellar rotation period measurements. While many of these stars show strong periodicity, there is an even bigger fraction of stars with irregular variability for which rotation periods are rarely visible or in most cases unknown. As a consequence, many studies of stellar activity might be strongly biased toward the behavior of more active stars, for which rotation periods have been determined. Aims. With the goal to at least partially lift this bias, we apply a new method capable of determining rotation periods of stars with irregular light curve variability. This effort greatly increases the number of stars with well-determined periods, especially for stars with small variabilities similar to that of the Sun. Methods. We employed a novel method based on the gradient of the power spectrum (GPS). The maximum of the gradient corresponds to the position of the inflection point (IP), namely, the point where the curvature of the high-frequency tail of the power spectrum changes its sign. Previously, it was shown that the stellar rotation period, Prot, is linked to the inflection point period, PIP, by the simple equation Prot = PIP/α, where α is a calibration factor. The GPS method is superior to classical methods (such as auto-correlation functions (ACF)) because it does not require a repeatable variability pattern in the time series, making it an ideal tool for detecting periods of stars with very short-lived spots. Results. From the initial sample of 142 168 stars with effective temperatures Teff < 6500 K and log g > 4.0 in the Kepler archive, we could measure rotation periods for 67 163 stars by combining the GPS and the ACF method. We further report the first determination of a rotation period for 20 397 stars. The GPS periods show good agreement with previous period measurements using classical methods, when available. Furthermore, we show that the scaling factor a increases for very cool stars with effective temperatures below 4000 K, which we interpret as spots located at higher latitudes. Conclusions. We conclude that new techniques, such as the GPS method, ought to be applied in detecting the rotation periods of stars with small and more irregular variabilities. Ignoring these stars will distort the overall picture of stellar activity, particular with respect to solar-stellar comparison studies.

VarIabiLity seLection of AstrophysIcal sources iN PTF (VILLAIN)

Context. Light-curve variability is well-suited to characterising objects in surveys with high cadence and a long baseline. This is especially relevant in view of the large datasets to be produced by the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST). Aims. We aim to determine variability parameters for objects in the Palomar Transient Factory (PTF) and explore differences between quasars (QSOs), stars, and galaxies. We relate variability and colour information in preparation for future surveys. Methods. We fit joint likelihoods to structure functions (SFs) of 71 million PTF light curves with a Markov chain Monte Carlo method. For each object, we assume a power-law SF and extract two parameters: the amplitude on timescales of one year, A, and a power-law index, γ. With these parameters and colours in the optical (Pan-STARRS1) and mid-infrared (WISE), we identify regions of parameter space dominated by different types of spectroscopically confirmed objects from SDSS. Candidate QSOs, stars, and galaxies are selected to show their parameter distributions. Results. QSOs show high-amplitude variations in the R band, and the highest γ values. Galaxies have a broader range of amplitudes and their variability shows relatively little dependency on timescale. With variability and colours, we achieve a photometric selection purity of 99.3% for QSOs. Even though hard cuts in monochromatic variability alone are not as effective as seven-band magnitude cuts, variability is useful in characterising object subclasses. Through variability, we also find QSOs that were erroneously classified as stars in the SDSS. We discuss perspectives and computational solutions in view of the upcoming LSST.

Subsecond periodic radio oscillations in a microquasar.

Powerful relativistic jets are one of the ubiquitous features of accreting black holes in all scales1-3. GRS 1915 + 105 is a well-known fast-spinning black-hole X-ray binary4 with a relativistic jet, termed a 'microquasar', as indicated by its superluminal motion of radio emission5,6. It has exhibited persistent X-ray activity over the last 30 years, with quasiperiodic oscillations of approximately 1-10 Hz (refs. 7-9) and 34 and 67 Hz in the X-ray band10. These oscillations probably originate in the inner accretion disk, but other origins have been considered11. Radio observations found variable light curves with quasiperiodic flares or oscillations with periods of approximately 20-50 min (refs. 12-14). Here we report two instances of approximately 5-Hz transient periodic oscillation features from the source detected in the 1.05- to 1.45-GHz radio band that occurred in January 2021 and June 2022. Circular polarization was also observed during the oscillation phase.

Variable stars in the residual light curves of OGLE-IV eclipsing binaries towards the Galactic Bulge

Context. The Optical Gravitational Lensing Experiment (OGLE) observed around 450 000 eclipsing binaries (EBs) towards the Galactic Bulge. Decade-long photometric observations such as these provide an exceptional opportunity to thoroughly examine the targets. However, observing dense stellar fields such as the Bulge may result in blends and contamination by close objects. Aims. We searched for periodic variations in the residual light curves of EBs in OGLE-IV and created a new catalogue for the EBs that contain ‘background’ signals after the investigation of the source of the signal. Methods. From the about half a million EB systems, we selected those that contain more than 4000 data points. We fitted the EB signal with a simple model and subtracted it. To identify periodical signals in the residuals, we used a GPU-based phase dispersion minimisation python algorithm called cuvarbase and validated the found periods with Lomb-Scargle periodograms. We tested the reliability of our method with artificial light curves. Results. We identified 354 systems where short-period background variation was significant. In these cases, we determined whether it is a new variable or just the result of contamination by an already catalogued nearby one. We classified 292 newly found variables into EB, δ Scuti, or RR Lyrae categories, or their sub-classes, and collected them in a catalogue. We also discovered four new doubly eclipsing systems and one eclipsing multiple system with a δ Scuti variable, and modelled the outer orbits of the components.

Impact of jets on kilonova photometric and polarimetric emission from binary neutron star mergers

ABSTRACT A merger of binary neutron stars creates heavy unstable elements whose radioactive decay produces a thermal emission known as a kilonova. In this paper, we predict the photometric and polarimetric behaviour of this emission by performing 3D Monte Carlo radiative transfer simulations. In particular, we choose three hydrodynamical models for merger ejecta, two including jets with different luminosities and one without a jet structure, to help decipher the impact of jets on the light curve and polarimetric behaviour. In terms of photometry, we find distinct colour evolutions across the three models. Models without a jet show the highest variation in light curves for different viewing angles. In contrast to previous studies, we find models with a jet to produce fainter kilonovae when viewed from orientations close to the jet axis, compared to a model without a jet. In terms of polarimetry, we predict relatively low levels (≲0.3–0.4 per cent) at all orientations that, however, remain non-negligible until a few days after the merger and longer than previously found. Despite the low levels, we find that the presence of a jet enhances the degree of polarization at wavelengths ranging from 0.25 to $2.5\rm{\mu m}$, an effect that is found to increase with the jet luminosity. Thus, future photometric and polarimetric campaigns should observe kilonovae in blue and red filters for a few days after the merger to help constrain the properties of the ejecta (e.g. composition) and jet.

Full-frame Data Reduction Method: A Data Mining Tool to Detect the Potential Variations in Optical Photometry

A Synchronous Photometry Data Extraction (SPDE) program, performing indiscriminate monitoring of all stars appearing in the same field of view of an astronomical image, is developed by integrating several Astropy affiliated packages to make full use of time series observed by traditional small/medium aperture ground-based telescopes. The complete full-frame stellar photometry data reductions implemented for the two time series of cataclysmic variables: RX J2102.0+3359 and Paloma J0524+4244 produce 363 and 641 optimal light curves, respectively. A cross-identification with SIMBAD finds 23 known stars, of which 16 are red giant-/horizontal-branch stars, 2 W UMa-type eclipsing variables, 2 program stars, an X-ray source and 2 Asteroid Terrestrial-impact Last Alert System variables. Based on the data products from the SPDE program, a follow-up light curve analysis program identifies 32 potential variable light curves, of which 18 are from the time series of RX J2102.0+3359, and 14 are from that of Paloma J0524+4244. They are preliminarily separated into periodic, transient, and peculiar types. By querying for the 58 VizieR online data catalogs, their physical parameters and multi-band brightness spanning X-ray to radio are compiled for future analysis.

WISE J141530.7+592234: A Very Low Mass Ratio Contact Binary with Significant Light Curve Variations

Multi-band photometric observations and period investigations of the total eclipsing binary WISE J141530.7+592234 (hereinafter J141530), whose mass ratio close to the low mass ratio limit, were carried out for the first time. The stellar atmospheric parameters of the primary component were calculated by the spectral fitting, yielding the following results: T eff = 5890 ± 22 K, [Fe/H] = 0.01 ± 0.03 dex, log g = 4.53 ± 0.04 cm s−2. Our observations show that the light curve of V-band on May 2 is about 0.1 mag brighter than that observed in February and March at phase 0.6. Comparing with the small amplitude (about 0.2 mag), the variations are remarkable. Meanwhile, obvious light curve variations and quick reversal of O’Connell effects are found in Transiting Exoplanet Survey Satellite data, which is likely caused by magnetic activity. Photometric solutions derived from multi-band light curves indicate that J141530 has a high contact degree (f ∼ 70%) and a very low mass ratio (q ∼ 0.0546). Based on the O − C diagram, a long-term period increase was found. The period increase rate is calculated to be dP/dt = 3.90 × 10−7 day yr−1, which can be explained by mass transfer from the low mass component to the large mass component. By analyzing the merging criterion, such as the ratio of spin angular momentum (J spin) to the orbital angular momentum (J orb), the instability separation and the instability mass ratio, J141530 can be regarded as a potential progenitor of luminous red novae.

Investigating the variability of Swift-BAT blazars with NICER

ABSTRACT We present results of X-ray spectral and time-domain variability analyses of four faint, ‘quiescent’ blazars from the Swift-BAT 105-month catalogue. We use observations from a recent, 5-month long NICER campaign, as well as archival BAT data. Variations in the 0.3–2 keV flux are detected on minute, ∼weekly, and monthly time-scales, but we find that the fractional variability Fvar on these time-scales is <25 per cent and decreases on longer time-scales, implying generally low-amplitude variability across all sources and showing very low variability on monthly time-scales ($F_{\rm var}\lesssim 13{{\ \rm per\ cent}}$), which is at odds with previous studies that show that blazars are highly variable in the X-rays on a wide range of time-scales. Moreover, we find that the flux variability on very short time-scales appears to be characterized by long periods of relative quiescence accompanied by occasional short bursts, against the relatively time-stationary nature of the variability of most other AGN light curves. Our analysis also shows that the broadband X-ray spectra (0.3–195 keV) of our sources can be described with different power-law models. As is the case with most blazars, we find that two sources (2MASS J09343014-1721215 and PKS 0312-770) are well-modelled with a simple power law, while the remaining two (1RXS J225146.9-320614 and PKS 2126-15) exhibit curvature in the form of a log-parabolic power law. We also find that, in addition to the continuum, PKS 2126-15 requires significant absorption at the soft X-rays (≲1 keV) to fully describe the observed curvature, possibly due to absorption from the intergalactic medium.

Rarefied Broad-line Regions in Active Galactic Nuclei: Anomalous Responses in Reverberation Mapping and Implications for Weak Emission-line Quasars

Reverberation mapping (RM) is a widely used method for probing the physics of broad-line regions (BLRs) in active galactic nuclei (AGNs). There is increasing preliminary evidence that the RM behaviors of broad emission lines are influenced by BLR densities; however, the influences have not been investigated systematically from a theoretical perspective. In this paper, we adopt a locally optimally emitting cloud model and use CLOUDY to obtain the one-dimensional transfer functions of the prominent UV and optical emission lines for different BLR densities. We find that the influences of BLR densities to RM behaviors mainly have three aspects. First, rarefied BLRs (with low gas densities) may show anomalous responses in RM observations. Their emission-line light curves inversely respond to the variations in continuum light curves, which may have been observed in some UV RM campaigns. Second, the different BLR densities in AGNs may result in correlations between the time lags and equivalent widths of emission lines, and may contribute to the scatters of the radius–luminosity relationships. Third, the variations in BLR densities may explain the changes in time lags in individual objects for different years. Some weak emission-line quasars are probably extreme cases of rarefied BLRs. We predict that their RM observations may show anomalous responses.

Tidally locked rotation of the dwarf planet (136199) Eris discovered via long-term ground-based and space photometry

The rotational states of the members in the dwarf planet-satellite systems in the trans-Neptunian region are determined by formation conditions and the tidal interaction between the components. These rotational characteristics serve as prime tracers of their evolution. A number of authors have claimed a very broad range of values for the rotation period for the dwarf planet Eris, ranging from a few hours to a rotation that is (nearly) synchronous with the orbital period (15.8 d) of its satellite, Dysnomia. In this Letter, we present new light curve data for Eris, taken with ∼1–2 m-class ground based telescopes and with the TESS and Gaia space telescopes. The TESS data did not provide a well-defined light curve period, but it could be used to constrain light curve variations to a maximum possible light curve amplitude of Δm ≤ 0.03 mag (1-σ) for P ≤ 24 h periods. Both the combined ground-based data and Gaia measurements unambiguously point to a light curve period equal to the orbital period of Dysnomia, P = 15.8 d, with a light curve amplitude of Δm ≈ 0.03 mag, indicating that the rotation of Eris is tidally locked. Assuming that Dysnomia has a collisional origin, calculations with a simple tidal evolution model show that Dysnomia must be relatively massive (mass ratio of q = 0.01–0.03) and large (radius of Rs ≥ 300 km) to have the potential to slow Eris down to a synchronised rotation. These simulations also indicate that (assuming tidal parameters usually considered for trans-Neptunian objects) the density of Dysnomia should be 1.8–2.4 g cm−3. This is an exceptionally high value among similarly sized trans-Neptunian objects, setting important constraints on their formation conditions.

Study of X-Ray Intraday Variability of HBL Blazars Based on Observations Obtained with XMM-Newton

We present an extensive study on the X-ray intraday variability of 10 teraelectronvolt-emitting high synchrotron peaked blazars (HBLs): 1ES 0229+200, 1ES 0414+009, PKS 0548-322, 1ES 1101-232, 1H 1219+301, H 1426+428, Mrk 501, 1ES 1959+650, PKS 2005-489, and 1ES 2344+514 made with 25 XMM-Newton pointed observations during its operational period. Intraday variability has been estimated in three energy bands: soft (0.3–2 keV), hard (2–10 keV), and total (0.3–10 keV). Although seven out of 10 teraelectronvolt HBLs exhibited some intraday variability at 3σ levels, no major variations exceeding 6% were detected. We explored the spectral properties of the sample by extracting the hardness ratio from the soft and hard bands; no significant variations in the hardness ratio were observed in any source. We performed power spectral density analyses on the variable light curves by fitting power laws, yielding slopes lying in the range of 1.11–2.93 for different HBLs. We briefly discuss possible emission mechanisms and carry out rough estimates for magnetic fields, electron Lorentz factors, and emission region sizes for seven of these HBLs.

The first photometric and orbital period investigation of an extremely low mass ratio contact binary with a sudden period change TYC 4002-2628-1

ABSTRACT Photometric observations for the totally eclipsing binary system TYC 4002-2628-1, were obtained between November 2020 and November 2021. To determine the stellar atmospheric parameters, a spectral image was taken with the 2.16 m telescope at National Astronomical Observatory of China (NAOC). TYC 4002-2628-1 is a low-amplitude (about 0.15 mag for Vband) short-period (0.3670495 d) contact eclipsing binary with a total secondary eclipse. Intrinsic light curve variations and the reversal of the O’Connell effect are detected in the light curves, which may be due to spot activity. Based on the photometric solutions derived from the multiband time series light curves, TYC 4002-2628-1 is an extremely low-mass ratio contact binary with a mass ratio of q ∼ 0.0482 and a fill-out factor of $f\sim 5{{\ \rm per\ cent}}$. By analysing the O− C variations, we find that its orbital period remains unchanged when BJD < 2458321. Then the orbital period changed suddenly around BJD 2458743 and has an increasing rate of $\mathrm{ d}P/\mathrm{ d}t=1.62\times {10^{-5}}\,\mathrm{ d}\cdot \mathrm{ yr}^{-1}=140\, \mathrm{ s}\cdot \mathrm{ century}^{-1}$. If confirmed, TYC 4002-2628-1 would be the contact binary with the highest orbital period increasing rate so far. By investigating the ratio of orbital angular momentum to the spin angular momentum (Jorb/$J_{\mathrm{ spin}}\, \lt\, 3$), the instability mass ratio (qinst/q = 1.84) and the instability separation (Ainst/A = 1.35), TYC 4002-2628-1 can be regarded as a merger candidate.

Analysis of eight magnetic chemically peculiar stars with rotational modulation

ABSTRACT Since the end of 2018, the Transiting Exoplanet Survey Satellite (TESS) has provided stellar photometry to the astronomical community. We have used TESS data to study rotational modulation in the light curves of a sample of chemically peculiar stars with measured large-scale magnetic fields (mCP stars). In general, mCP stars show inhomogeneous distributions of elements in their atmospheres that lead to spectroscopic (line profile) and photometric (light curve) variations commensurate with the rotational period. We analyzed the available TESS data from 50 sectors for eight targets after post-processing them in order to minimize systematic instrumental trends. Analysis of the light curves allowed us to determine rotational periods for all eight of our targets. For each star, we provide a phase diagram calculated using the derived period from the light curves and from the available measurements of the disc-averaged longitudinal magnetic field 〈Bz〉. In most cases, the phased light curve and 〈Bz〉 measurements show consistent variability. Using our rotation periods, and global stellar parameters derived from fitting Balmer line profiles, and from Geneva and Strömgren–Crawford photometry, we determined the equatorial rotational velocities and calculated the respective critical rotational fractions veq/vcrit. We have shown from our sample that the critical rotational fraction decreases with stellar age, at a rate consistent with the magnetic braking observed in the larger population of mCP stars.

Dust reverberation mapping and light-curve modelling of Zw229-015

ABSTRACT Multiwavelength variability studies of active galactic nuclei can be used to probe their inner regions that are not directly resolvable. Dust reverberation mapping (DRM) estimates the size of the dust emitting region by measuring the delays between the infrared (IR) response to variability in the optical light curves. We measure DRM lags of Zw229-015 between optical ground-based and Kepler light curves and concurrent IR Spitzer 3.6 and 4.5 µm light curves from 2010 to 2015, finding an overall mean rest-frame lag of 18.3 ± 4.5 d. Each combination of optical and IR light curve returns lags that are consistent with each other within 1σ, which implies that the different wavelengths are dominated by the same hot dust emission. The lags measured for Zw229-015 are found to be consistently smaller than predictions using the lag–luminosity relationship. Also, the overall IR response to the optical emission actually depends on the geometry and structure of the dust emitting region as well, so we use Markov chain Monte Carlo modelling to simulate the dust distribution to further estimate these structural and geometrical properties. We find that a large increase in flux between the 2011–2012 observation seasons, which is more dramatic in the IR light curve, is not well simulated by a single dust component. When excluding this increase in flux, the modelling consistently suggests that the dust is distributed in an extended flat disc, and finds a mean inclination angle of 49$^{+3}_{-13}$ deg.

Quasars with Periodic Variability: Capabilities and Limitations of Bayesian Searches for Supermassive Black Hole Binaries in Time-domain Surveys

Supermassive black hole binaries (SMBHBs) are an inevitable consequence of galaxy mergers. At sub-parsec separations, they are practically impossible to resolve, and the most promising technique is to search for quasars with periodic variability. However, searches for quasar periodicity in time-domain data are challenging due to the stochastic variability of quasars. In this paper, we used Bayesian methods to disentangle periodic SMBHB signals from intrinsic damped random walk (DRW) variability in active galactic nuclei light curves. We simulated a wide variety of realistic DRW and DRW+sine light curves. Their observed properties are modeled after the Catalina Real-time Transient Survey (CRTS) and expected properties of the upcoming Legacy Survey of Space and Time (LSST) from the Vera C. Rubin Observatory. Through a careful analysis of parameter estimation and Bayesian model selection, we investigated the range of parameter space for which binary systems can be detected. We also examined which DRW signals can mimic periodicity and be falsely classified as binary candidates. We found that periodic signals are more easily detectable if the period is short or the amplitude of the signal is large compared to the contribution of the DRW noise. We saw similar detection rates both in the CRTS and LSST-like simulations, while the false-detection rate depends on the quality of the data and is minimal in LSST. Our idealized simulations provide an excellent way to uncover the intrinsic limitations in quasar periodicity searches and set the stage for future searches for SMBHBs.

The Lyα Emission in a C1.4 Solar Flare Observed by the Extreme Ultraviolet Imager aboard Solar Orbiter

The hydrogen Lyα (H i Lyα) emission during solar flares has rarely been studied in spatially resolved images, and its physical origin has not been fully understood. In this paper, we present novel Lyα images for a C1.4 solar flare (SOL2021-08-20T22:00) from the Extreme Ultraviolet Imager aboard Solar Orbiter, together with multi-wave-band and multiperspective observations from the Solar Terrestrial Relations Observatory Ahead and the Solar Dynamics Observatory spacecraft. It is found that the Lyα emission has a good temporal correlation with the thermal emissions at 1–8 Å and 5–7 keV, indicating that the flaring Lyα is mainly produced by a thermal process in this small event. However, nonthermal electrons play a minor role in generating Lyα at flare ribbons during the rise phase of the flare, as revealed by the hard X-ray imaging and spectral fitting. Besides originating from flare ribbons, the Lyα emission can come from flare loops, likely caused by plasma heating and also cooling that happen in different flare phases. It is also found that the Lyα emission shows fairly similar features to the He ii λ304 emission in light curve and spatiotemporal variation, along with small differences. These observational results improve our understanding of the Lyα emission in solar flares and also provide some insights for investigating the Lyα emission in stellar flares.