arXiv subjects

S. Awiphan

Publications and source records attributed to S. Awiphan.

At least 19 recordsLinked to original sources

arXiv2026

Discovery of the first outbursting hot subdwarf binary: ZTF J0007+4804

Hot subdwarf binaries with white dwarf companions with orbital periods of less than two hours are progenitor candidates for massive single white dwarfs as well as a variety of thermonuclear explosions. Our aim is to determine the binary properties of the hot subdwarf -- white dwarf system ZTF J000742.62+480414.51, model its future evolution, and characterize the brightening events seen in TESS photometry. Using data from ZTF and TESS, we performed a Lomb Scargle analysis to find the orbital period and the period of the brightening events. Analysis of time-resolved spectroscopy was combined with light curve modeling to determine the effective temperature, surface gravity, and radius of the primary star, the masses of both stars, and to confirm the presence of an accretion disk. X-ray observations were performed with Swift, and MESA modeling was used to find the future evolution of the system. The kinematics of the system were also calculated. ZTF J000742.62+480414.51 consists of an accreting $0.48\pm0.01\,M_\odot$ white dwarf with a $0.42\pm0.01\,M_\odot$ B-type hot subdwarf acting as a donor. The system exhibits SU UMa type dwarf nova outbursts with a recurrence time of $P_{\mathrm{out}} \approx 9$ days. No X-rays were detected, with an upper limit on the X-ray luminosity of about $3\times10^{31}$ erg/sec. The system lies in the Galactic thin disk, and has an orbital period of $P_{\mathrm{orb}} = 108.72\pm0.01$ minutes. The system has likely formed from a main sequence binary with component masses $\gtrsim2\,\mathrm{M_{\odot}}$ and will likely merge into a single white dwarf, but a thermonuclear explosion cannot be ruled out. ZTF J000742.62+480414.51 consists of a low mass white dwarf actively accreting hydrogen rich material from a B-type hot subdwarf, and is the first hot subdwarf -- white dwarf system discovered that produces dwarf nova outbursts.

astro-ph.SR↗

arXiv2026

GOTO identification and broadband modelling of the counterpart to the SVOM GRB 250818B

Rapid localisation and follow-up of gamma-ray bursts (GRBs) increasingly rely on low-latency triggers from new missions coupled to wide-field robotic optical facilities. We present the discovery and multi-wavelength follow-up of GRB 250818B, detected by the Space Variable Objects Monitor (SVOM) and localised optically by the Gravitational-wave Optical Transient Observer (GOTO). We compile and homogenise X-ray, optical/NIR, and radio data to build broadband light curves and spectral energy distributions. The afterglow is unusually luminous for a nominal short GRB, lying on the bright end of the short-GRB population in X-rays and optical and among the most luminous high-redshift short-GRB afterglows in the radio. MeerKAT detects the source at 3.1 GHz, while ALMA provides deep higher-frequency limits. Keck/LRIS spectroscopy shows continuum and metal absorption (Fe II, Mg II, Mg I), giving $z=1.216$. Synchrotron forward-shock modelling favours a constant-density medium and strongly prefers refreshed (energy-injection) emission, well described by a two-component jet with $E_{K,iso} \sim 4\times10^{52}$ erg, $n_0 \sim 3.6$ cm$^{-3}$, $\theta_j \simeq 0.10$ rad ($\sim 5.7$ deg), and $p \simeq 1.64$. The host association is ambiguous: the nearest LS DR10 galaxy candidate ($r_{AB} \sim 24.7$) is offset by $\sim 4$ arcsec ($\sim 34$ kpc) with chance-alignment probability $P_{cc} \sim 0.2$, and current imaging does not exclude a fainter, near-coincident host. SED fitting of the candidate host suggests a low-mass galaxy. GRB 250818B highlights the power of rapid wide-field counterpart identification in the SVOM era, while host-association uncertainty can still limit offset-based interpretation.

astro-ph.HE↗

arXiv2026

Gaia20fnr: A binary-lens microlensing event with full orbital motion revealed by four space telescopes

The microlensing event Gaia20fnr is a long-duration, non-caustic-crossing binary-lens event at high Galactic latitude. Triggered by a photometric rise detected by the Gaia space mission, the event was followed up with observations from multiple ground-based facilities and four space telescopes: Gaia, NEOWISE, Swift, and TESS. We characterize the Gaia20fnr microlensing system by determining the physical and orbital properties of the binary lens, the nature of the luminous source, and the kinematics of both the source and the lens. We employed a binary-lens microlensing model including full Keplerian orbital motion and annual microlens parallax to fit the photometric data. The event is best explained by a K2 giant source at $D_{\rm S} = 3.10 \pm 0.10\,\mathrm{kpc}$ lensed by a stellar binary composed of $M_{\rm L,1} = 0.46 \pm 0.06\,M_\odot$ and $M_{\rm L,2} = 0.52 \pm 0.06\,M_\odot$ at a distance of $D_{\rm L} = 0.54 \pm 0.05\,\mathrm{kpc}$. The light curve exhibits strong signatures of orbital motion and requires a full Keplerian model with a period of $P = 0.67 \pm 0.04\,\mathrm{yr}$ and a radial-velocity semi-amplitude of $K_1 = 16.9 \pm 0.9\,\mathrm{km\,s^{-1}}$. Gaia20fnr is one of the few microlensing events for which a complete Keplerian binary-lens solution has been derived. The model can be tested with follow-up radial-velocity and high-resolution imaging observations as well as forthcoming Gaia DR4 and DR5 astrometric time-series data. Its long duration, multi-peak structure, and extensive coverage make it a benchmark for studying faint nearby low-mass binaries through microlensing.

astro-ph.SR↗

arXiv2025

Constraining Lens Masses in Moderately to Highly Magnified Microlensing Events from Gaia

Microlensing events provide a unique way to detect and measure the masses of isolated, non-luminous objects, particularly dark stellar remnants. Under certain conditions, it is possible to measure the mass of these objects using photometry alone, specifically when a microlensing light curve displays a finite source (FS) effect. This effect generally occurs in highly magnified light curves, i.e. when the source and the lens are very well aligned. In this study, we analyse Gaia Alerts and Gaia Data Release 3 datasets, identifying four moderate-to-high-magnification microlensing events without a discernible FS effect. The absence of this effect suggests a large Einstein radius, implying substantial lens masses. In each event, we constrained the FS effect, and therefore established lower limits for the angular Einstein radius and lens mass. Additionally, we used the DarkLensCode software to obtain the mass, distance, and brightness distribution for the lens based on the Galactic model. Our analysis established lower mass limits of $\sim 0.7$ $M_{\odot}$ for one lens and $\sim 0.3-0.5$ $M_{\odot}$ for two others. A DarkLensCode analysis supports these findings, estimating lens masses in the range of $\sim 0.42-1.70$ $M_{\odot}$ and dark lens probabilities exceeding 80%. These results strongly indicate that the lenses are stellar remnants, such as white dwarfs or neutron stars. While further investigations are required to confirm the nature of these lenses, we demonstrate a straightforward yet effective approach to identifying stellar remnant candidates.

astro-ph.GA↗

arXiv2024

$\textit{Kilonova Seekers}$: the GOTO project for real-time citizen science in time-domain astrophysics

Time-domain astrophysics continues to grow rapidly, with the inception of new surveys drastically increasing data volumes. Democratised, distributed approaches to training sets for machine learning classifiers are crucial to make the most of this torrent of discovery -- with citizen science approaches proving effective at meeting these requirements. In this paper, we describe the creation of and the initial results from the $\textit{Kilonova Seekers}$ citizen science project, built to find transient phenomena from the GOTO telescopes in near real-time. $\textit{Kilonova Seekers}$ launched in July 2023 and received over 600,000 classifications from approximately 2,000 volunteers over the course of the LIGO-Virgo-KAGRA O4a observing run. During this time, the project has yielded 20 discoveries, generated a `gold-standard' training set of 17,682 detections for augmenting deep-learned classifiers, and measured the performance and biases of Zooniverse volunteers on real-bogus classification. This project will continue throughout the lifetime of GOTO, pushing candidates at ever-greater cadence, and directly facilitate the next-generation classification algorithms currently in development.

astro-ph.IM↗

arXiv2024

A close binary lens revealed by the microlensing event Gaia20bof

During the last 25 years, hundreds of binary stars and planets have been discovered towards the Galactic Bulge by microlensing surveys. Thanks to a new generation of large-sky surveys, it is now possible to regularly detect microlensing events across the entire sky. The OMEGA Key Projet at the Las Cumbres Observatory carries out automated follow-up observations of microlensing events alerted by these surveys with the aim of identifying and characterizing exoplanets as well as stellar remnants. In this study, we present the analysis of the binary lens event Gaia20bof. By automatically requesting additional observations, the OMEGA Key Project obtained dense time coverage of an anomaly near the peak of the event, allowing characterization of the lensing system. The observed anomaly in the lightcurve is due to a binary lens. However, several models can explain the observations. Spectroscopic observations indicate that the source is located at $\le2.0$ kpc, in agreement with the parallax measurements from Gaia. While the models are currently degenerate, future observations, especially the Gaia astrometric time series as well as high-resolution imaging, will provide extra constraints to distinguish between them.

astro-ph.EP↗

arXiv2023

Lens mass estimate in the Galactic disk extreme parallax microlensing event Gaia19dke

We present the results of our analysis of Gaia19dke, an extraordinary microlensing event in the Cygnus constellation that was first spotted by the {\gaia} satellite. This event featured a strong microlensing parallax effect, which resulted in multiple peaks in the light curve. We conducted extensive photometric, spectroscopic, and high-resolution imaging follow-up observations to determine the mass and the nature of the invisible lensing object. Using the Milky Way priors on density and velocity of lenses, we found that the dark lens is likely to be located at a distance of $D_L =(3.05^{+4.10}_{-2.42})$kpc, and has a mass of $M_L =(0.51^{+3.07}_{-0.40}) M_\odot$. Based on its low luminosity and mass, we propose that the lens in Gaia19dke event is an isolated white dwarf.

astro-ph.SR↗

arXiv2022

Self-Supervised Clustering on Image-Subtracted Data with Deep-Embedded Self-Organizing Map

Developing an effective automatic classifier to separate genuine sources from artifacts is essential for transient follow-ups in wide-field optical surveys. The identification of transient detections from the subtraction artifacts after the image differencing process is a key step in such classifiers, known as real-bogus classification problem. We apply a self-supervised machine learning model, the deep-embedded self-organizing map (DESOM) to this "real-bogus" classification problem. DESOM combines an autoencoder and a self-organizing map to perform clustering in order to distinguish between real and bogus detections, based on their dimensionality-reduced representations. We use 32x32 normalized detection thumbnails as the input of DESOM. We demonstrate different model training approaches, and find that our best DESOM classifier shows a missed detection rate of 6.6% with a false positive rate of 1.5%. DESOM offers a more nuanced way to fine-tune the decision boundary identifying likely real detections when used in combination with other types of classifiers, for example built on neural networks or decision trees. We also discuss other potential usages of DESOM and its limitations.

cs.CV↗

arXiv2022

Earth through the looking glass: how frequently are we detected by other civilisations through photometric microlensing?

Microlensing is proving to be one of the best techniques to detect distant, low-mass planets around the most common stars in the Galaxy. In principle, Earth's microlensing signal could offer the chance for other technological civilisations to find the Earth across Galactic distances. We consider the photometric microlensing signal of Earth to other potential technological civilisations and dub the regions of our Galaxy from which Earth's photometric microlensing signal is most readily observable as the "Earth Microlensing Zone" (EMZ). The EMZ can be thought of as the microlensing analogue of the Earth Transit Zone (ETZ) from where observers see Earth transit the Sun. Just as for the ETZ, the EMZ could represent a game-theoretic Schelling point for targeted searches for extra-terrestrial intelligence (SETI). To compute the EMZ, we use the Gaia DR2 catalogue with magnitude G<20 to generate Earth microlensing probability and detection rate maps to other observers. Whilst our Solar system is a multi-planet system, we show that Earth's photometric microlensing signature is almost always well approximated by a binary lens assumption. We then show that the Earth is in fact well-hidden to observers with technology comparable to our own. Specifically, even if observers are located around every Gaia DR2 star with G<20, we expect photometric microlensing signatures from the Earth to be observable on average only tens per year by any of them. In addition, the EMZs overlap with the ETZ near the Galactic centre which could be the main areas for future SETI searches.

astro-ph.EP↗

arXiv2021

Single-lens mass measurement in the high-magnification microlensing event Gaia19bld located in the Galactic disc

We present the photometric analysis of Gaia19bld, a high-magnification ($A\approx60$) microlensing event located in the southern Galactic plane, which exhibited finite source and microlensing parallax effects. Due to a prompt detection by the Gaia satellite and the very high brightness of $I = 9.05~$mag at the peak, it was possible to collect a complete and unique set of multi-channel follow-up observations, which allowed us to determine all parameters vital for the characterisation of the lens and the source in the microlensing event. Gaia19bld was discovered by the Gaia satellite and was subsequently intensively followed up with a network of ground-based observatories and the Spitzer Space Telescope. We collected multiple high-resolution spectra with Very Large Telescope (VLT)/X-Shooter to characterise the source star. The event was also observed with VLT Interferometer (VLTI)/PIONIER during the peak. Here we focus on the photometric observations and model the light curve composed of data from Gaia, Spitzer, and multiple optical, ground-based observatories. We find the best-fitting solution with parallax and finite source effects. We derived the limit on the luminosity of the lens based on the blended light model and spectroscopic distance. We compute the mass of the lens to be $1.13 \pm 0.03~M_{\odot}$ and derive its distance to be $5.52^{+0.35}_{-0.64}~\mathrm{kpc}$. The lens is likely a main sequence star, however its true nature has yet to be verified by future high-resolution observations. Our results are consistent with interferometric measurements of the angular Einstein radius, emphasising that interferometry can be a new channel for determining the masses of objects that would otherwise remain undetectable, including stellar-mass black holes.

astro-ph.GA↗

arXiv2021

The Gravitational-wave Optical Transient Observer (GOTO): prototype performance and prospects for transient science

The Gravitational-wave Optical Transient Observer (GOTO) is an array of wide-field optical telescopes, designed to exploit new discoveries from the next generation of gravitational wave detectors (LIGO, Virgo, KAGRA), study rapidly evolving transients, and exploit multi-messenger opportunities arising from neutrino and very high energy gamma-ray triggers. In addition to a rapid response mode, the array will also perform a sensitive, all-sky transient survey with few day cadence. The facility features a novel, modular design with multiple 40-cm wide-field reflectors on a single mount. In June 2017 the GOTO collaboration deployed the initial project prototype, with 4 telescope units, at the Roque de los Muchachos Observatory (ORM), La Palma, Canary Islands. Here we describe the deployment, commissioning, and performance of the prototype hardware, and discuss the impact of these findings on the final GOTO design. We also offer an initial assessment of the science prospects for the full GOTO facility that employs 32 telescope units across two sites.

astro-ph.IM↗

arXiv2021

Eclipse timing variations in the WD+dM eclipsing binary RR Cae

We present the binary model and the eclipse timing variations of the eclipsing binary RR Cae, which consists of a white dwarf eclipsed by an M-type dwarf companion. The multi-wavelength optical photometry from the Transiting Exoplanet Survey Satellite (TESS), the 0.6-m PROMPT-8 telescope, and the 0.7-m Thai Robotic Telescope at Spring Brook Observatory, combined with archive H-alpha radial velocities from the Very Large Telescope (VLT) are analysed. From the data, the physical parameters of the system are obtained along with 430 new times of minima. The TESS light curves in 2018 and 2020 show out-of-eclipse variations, which might be caused by a large spot on the secondary component. The light travel time effect models due to the gravitational interaction of one or two circumbinary objects are adopted to fit the cyclic variations in the RR Cae's O-C curve. The fitting solution of the O-C curve with one circumbinary object model shows a periodic variation with a period of $16.6\pm0.2$ yr and an amplitude of $14\pm1$ s, which can be caused by a planet with a minimum mass of $3.4\pm0.2$ M$_{Jup}$. When we consider the model with two circumbinary objects, the O-C curve shows cyclic variations with periods of $15.0\pm0.5$ yr and $39\pm5$ yr and amplitudes of $12\pm1$ s and $20\pm5$ s, respectively, corresponding to minimum masses of $3.0\pm0.3$ M$_{Jup}$ and $2.7\pm0.7$ M$_{Jup}$.

astro-ph.EP↗

arXiv2021

Searching For Fermi GRB Optical Counterparts With The Prototype Gravitational-Wave Optical Transient Observer (GOTO)

The typical detection rate of $\sim1$ gamma-ray burst (GRB) per day by the \emph{Fermi} Gamma-ray Burst Monitor (GBM) provides a valuable opportunity to further our understanding of GRB physics. However, the large uncertainty of the \emph{Fermi} localization typically prevents rapid identification of multi-wavelength counterparts. We report the follow-up of 93 \emph{Fermi} GRBs with the Gravitational-wave Optical Transient Observer (GOTO) prototype on La Palma. We selected 53 events (based on favourable observing conditions) for detailed analysis, and to demonstrate our strategy of searching for optical counterparts. We apply a filtering process consisting of both automated and manual steps to 60\,085 candidates initially, rejecting all but 29, arising from 15 events. With $\approx3$ GRB afterglows expected to be detectable with GOTO from our sample, most of the candidates are unlikely to be related to the GRBs. Since we did not have multiple observations for those candidates, we cannot confidently confirm the association between the transients and the GRBs. Our results show that GOTO can effectively search for GRB optical counterparts thanks to its large field of view of $\approx40$ square degrees and its depth of $\approx20$ mag. We also detail several methods to improve our overall performance for future follow-up programs of \emph{Fermi} GRBs.

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arXiv2021

Light curve classification with recurrent neural networks for GOTO: dealing with imbalanced data

The advent of wide-field sky surveys has led to the growth of transient and variable source discoveries. The data deluge produced by these surveys has necessitated the use of machine learning (ML) and deep learning (DL) algorithms to sift through the vast incoming data stream. A problem that arises in real-world applications of learning algorithms for classification is imbalanced data, where a class of objects within the data is underrepresented, leading to a bias for over-represented classes in the ML and DL classifiers. We present a recurrent neural network (RNN) classifier that takes in photometric time-series data and additional contextual information (such as distance to nearby galaxies and on-sky position) to produce real-time classification of objects observed by the Gravitational-wave Optical Transient Observer (GOTO), and use an algorithm-level approach for handling imbalance with a focal loss function. The classifier is able to achieve an Area Under the Curve (AUC) score of 0.972 when using all available photometric observations to classify variable stars, supernovae, and active galactic nuclei. The RNN architecture allows us to classify incomplete light curves, and measure how performance improves as more observations are included. We also investigate the role that contextual information plays in producing reliable object classification.

astro-ph.IM↗

arXiv2021

Processing GOTO data with the Rubin Observatory LSST Science Pipelines II: Forced Photometry and light curves

We have adapted the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) Science Pipelines to process data from the Gravitational-Wave Optical Transient Observer (GOTO) prototype. In this paper, we describe how we used the Rubin Observatory LSST Science Pipelines to conduct forced photometry measurements on nightly GOTO data. By comparing the photometry measurements of sources taken on multiple nights, we find that the precision of our photometry is typically better than 20~mmag for sources brighter than 16 mag. We also compare our photometry measurements against colour-corrected PanSTARRS photometry, and find that the two agree to within 10~mmag (1$\sigma$) for bright (i.e., $\sim14^{\rm th}$~mag) sources to 200~mmag for faint (i.e., $\sim18^{\rm th}$~mag) sources. Additionally, we compare our results to those obtained by GOTO's own in-house pipeline, {\sc gotophoto}, and obtain similar results. Based on repeatability measurements, we measure a $5\sigma$ L-band survey depth of between 19 and 20 magnitudes, depending on observing conditions. We assess, using repeated observations of non-varying standard SDSS stars, the accuracy of our uncertainties, which we find are typically overestimated by roughly a factor of two for bright sources (i.e., $<15^{\rm th}$~mag), but slightly underestimated (by roughly a factor of 1.25) for fainter sources ($>17^{\rm th}$~mag). Finally, we present lightcurves for a selection of variable sources, and compare them to those obtained with the Zwicky Transient Factory and GAIA. Despite the Rubin Observatory LSST Science Pipelines still undergoing active development, our results show that they are already delivering robust forced photometry measurements from GOTO data.

astro-ph.IM↗

arXiv2021

TransitFit: combined multi-instrument exoplanet transit fitting for JWST, HST and ground-based transmission spectroscopy studies

We present TransitFit, a package designed to fit exoplanetary transit light-curves. TransitFit offers multi-epoch, multi-wavelength fitting of multi-telescope transit data. TransitFit allows per-telescope detrending to be performed simultaneously with transit parameter fitting, including custom detrending. Host limb darkening can be fitted using prior conditioning from stellar atmosphere models. We demonstrate TransitFit in a number of contexts. We model multi-telescope broadband optical data from the ground-based SPEARNET survey of the low-density hot-Neptune WASP-127b and compare results to a previously published higher spectral resolution GTC/OSIRIS transmission spectrum. Using TransitFit, we fit 26 transit epochs by TESS to recover improved ephemeris of the hot-Jupiter WASP-91b and a transit depth determined to a precision of 111 ppm. We use TransitFit to conduct an investigation into the contested presence of TTV signatures in WASP-126b using 180 transits observed by TESS, concluding that there is no statistically significant evidence for such signatures from observations spanning 27 TESS sectors. We fit HST observations of WASP-43b, demonstrating how TransitFit can use custom detrending algorithms to remove complex baseline systematics. Lastly, we present a transmission spectrum of the atmosphere of WASP-96b constructed from simultaneous fitting of JWST NIRISS Early Release Observations and archive HST WFC3 transit data. The transmission spectrum shows generally good correspondence between spectral features present in both datasets, despite very different detrending requirements.

astro-ph.EP↗

arXiv2021

Deep Contrast and Companion Detection Using the EvWaCo Testbed Equipped with an Achromatic Focal Plane Mask and an Adjustable Inner Working Angle

The evanescent wave coronagraph uses the principle of frustrated total internal reflection (FTIR) to suppress the light coming from the star and study its close environment. Its focal plane mask is composed of a lens and a prism placed in contact with each other to produce the coronagraphic effect. In this paper, we present the experimental results obtained using an upgraded focal plane mask of the Evanescent Wave Coronagraph (EvWaCo). These experimental results are also compared to the theoretical performance of the coronagraph obtained through simulations. Experimentally, we reach a raw contrast equal to a few $10^{-4}$ at a distance equal to 3 ${\lambda}/D$ over the full I-band (${\lambda}_c =$ $800$ $nm$, ${\Delta}{\lambda}/{\lambda} \approx 20\%$) and equal to 4 ${\lambda}/D$ over the full R-band (${\lambda}_c =$ $650$ $nm$, ${\Delta}{\lambda}/{\lambda} \approx 23\%$) in unpolarized light. However, our simulations show a raw contrast close to $10^{-4}$ over the full I-band and R-band at the same distance, thus, confirming the theoretical achromatic advantage of the coronagraph. We also verify the stability of the mask through a series of contrast measurements over a period of 8 months. Furthermore, we measure the sensitivity of the coronagraph to the lateral and longitudinal misalignment of the focal plane mask, and to the lateral misalignment of the Lyot stop.

astro-ph.IM↗

arXiv2021

Transient-optimised real-bogus classification with Bayesian Convolutional Neural Networks -- sifting the GOTO candidate stream

Large-scale sky surveys have played a transformative role in our understanding of astrophysical transients, only made possible by increasingly powerful machine learning-based filtering to accurately sift through the vast quantities of incoming data generated. In this paper, we present a new real-bogus classifier based on a Bayesian convolutional neural network that provides nuanced, uncertainty-aware classification of transient candidates in difference imaging, and demonstrate its application to the datastream from the GOTO wide-field optical survey. Not only are candidates assigned a well-calibrated probability of being real, but also an associated confidence that can be used to prioritise human vetting efforts and inform future model optimisation via active learning. To fully realise the potential of this architecture, we present a fully-automated training set generation method which requires no human labelling, incorporating a novel data-driven augmentation method to significantly improve the recovery of faint and nuclear transient sources. We achieve competitive classification accuracy (FPR and FNR both below 1%) compared against classifiers trained with fully human-labelled datasets, whilst being significantly quicker and less labour-intensive to build. This data-driven approach is uniquely scalable to the upcoming challenges and data needs of next-generation transient surveys. We make our data generation and model training codes available to the community.

astro-ph.IM↗