arXiv subjects

Paul Chote

Publications and source records attributed to Paul Chote.

At least 19 recordsLinked to original sources

arXiv2026

Mode Instability and a Massive, Isolated Outburst in the Pulsating White Dwarf GD 1212

We analyze a large brightening event that lasted for roughly half a day in the pulsating hydrogen-atmosphere white dwarf GD 1212 during K2 Campaign 12 of the extended Kepler mission. For the other 80 days of K2 observations, GD 1212 exhibited a rich spectrum of long-period (~1100 s) pulsations that underwent rapid variations in frequency and amplitude but did not exhibit any additional outbursts. We refine previous attempts at mode identification and find a likely sequence of dipole and quadrupole splittings that reveal an overall rotation rate of roughly 17.0 hr. The outburst at Day 61 is fully resolved by the 60-second-cadence K2 data, with the entire white dwarf becoming up to 17.5% brighter overall, from an approximately 850 K increase in effective temperature, with pulsational variability during the outburst showing shorter periods and higher amplitudes. Outbursts are believed to be the result of nonlinear mode coupling via parametric instability, whereby energy stored in linearly excited parent modes is rapidly transferred to damped child modes that dissipate near the surface. Additionally, we characterize a "failed" outburst that caused correlated pulsation frequency changes, an approximately 5 microHz increase, with a small approximately 0.35% corresponding brightness increase. GD 1212 is now the eighth pulsating hydrogen-atmosphere DAV white dwarf to show outburst behavior, although it exhibited the largest outburst yet and has the longest inferred recurrence timescale. This high-signal-to-noise record tracing pulsations through both large and small temperature excursions in GD 1212 provides unique insights into parametric resonance and nonlinear mode coupling in white dwarf pulsations.

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arXiv2026

DebrisWatch II: Digging deeper for geosynchronous debris

The geosynchronous (GSO) debris environment is continually evolving. Regular monitoring of the region is consequently of great importance, though the trade-off between coverage and sensitivity makes this challenging for the population of optically faint debris, where collecting area becomes a pivotal factor. Surveys conducted with large-aperture telescopes have provided crucial insights into the nature of this largely uncharacterised population. In this paper, we revisit a survey conducted with the 2.54 m Isaac Newton Telescope (INT), presenting an overhaul of the astrometric calibration and object detection stages of the original analysis pipeline. We apply a blind stacking technique to boost target recovery, unearthing 25 tracklets previously missed by single-frame extraction methods, and pushing the sensitivity limit fainter by 1 magnitude. The same algorithm is applied to a contemporaneous dataset, captured with a 36 cm astrograph, enabling performance benchmarking through the attempted recovery of INT detections from commercial-off-the-shelf observations. We achieve sub-arcsecond astrometric accuracy through a combination of improved star trail centroiding and iterative distortion fitting, allowing short arc initial orbit solutions to be obtained. High-cadence light curves extracted for trailing detections indicate that faint fragments are proportionally more variable than bright derelicts, with many exhibiting photometric signatures of rapid tumbling, often straddling the image noise floor. Lastly, we present preliminary findings from a follow-up multi-national observation campaign, utilising telescopes in Australia, Japan and La Palma. As space traffic management concerns begin to extend beyond GSO altitudes, scientifically-driven surveys of high-altitude orbits have an important role to play in characterising the faint debris environment.

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arXiv2026

Brightness, Colour, Polarisation: A Multi-Instrument Observation Campaign of the Gorizont-6 Satellite

We present a coordinated multi-instrument photometric and polarimetric study of the defunct geosynchronous satellite, Gorizont-6. This observation campaign combined wide-field multi-colour observations with simultaneous multi-site photometry and linear polarimetry. Our results demonstrate that the combined simultaneous colour and polarimetric measurements aid in breaking the degeneracy between the fundamental spin period and its harmonics, enabling light curve features to be associated with specific reflecting surfaces. Using phase-dispersion minimisation with bootstrap resampling, we then measure a steadily increasing rotation period across six epochs spanning nine months, well described by a damped exponential curve consistent with Yarkovsky O'Keefe Radzievskii Paddack (YORP) driven spin-down. A geometrical analysis of the multi-site observations provides additional constraints on the spin-axis orientation based on paired glint events. Overall, multi-colour photometry yields efficient, robust period measurements compared with single-band data, while polarimetry supplies the decisive constraints needed for unambiguous rotation-state determination, highlighting the value of combined photometric-polarimetric strategies for characterising defunct satellites.

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arXiv2026

High-Precision Photometry with a scientific CMOS Camera: II On-Sky Testing of the Marana camera at the NGTS facility

Modern scientific CMOS cameras offer very fast readout speeds and low read noise. In this study, we evaluate the performance of the Andor Marana CMOS camera through on-sky testing carried out at the NGTS facility at the ESO Paranal Observatory in Chile. We mount the Marana camera to an NGTS telescope, and conduct photometric observations of bright stars. In particular, we target transit events around eight known bright exoplanet host stars. Simultaneous observations are carried out using an existing Andor iKon-L CCD camera on a neighbouring NGTS telescope. This allows for a direct comparison of the photometric precision between the CMOS and CCD cameras. We find that the Marana CMOS exhibits a similar level of photometric performance to the CCD camera, achieving 500\,ppm at a 30-minute timescale for a T $=10$\,mag star. Although the CCD has a slightly better quantum efficiency over the NGTS filter range (520-890\,nm), we find that the faster readout speed of the CMOS compared to the CCD means that the CMOS camera detects 20\,\% more photons per unit time for a solar-type star in our standard 10\,s exposure time operation mode. This results in the CMOS performing slightly better photometry in the photon-limited regime. We conclude that modern CMOS cameras, such as the Marana, are very well-suited for astronomical time-series photometry applications.

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arXiv2025

A search for photometric variability towards the globular cluster M3 with the TWenty Inch Survey Telescope

We present the commissioning results and first scientific observations from the newly installed TWIST observatory - a 50 cm telescope equipped with an sCMOS camera providing a $36.1\times24.1$ arcmin$^2$ field of view - housed in the former SuperWASP-North enclosure. We conducted a 67-night, 199-day baseline white-light monitoring campaign centred on the globular cluster M3 aimed at characterizing stellar variability within the cluster while also assessing the photometric performance of the newly commissioned system. We report the discovery of four new SX Phoenicis variables (V301-304), confirm their cluster membership, and identify fundamental-mode pulsation in one, allowing an independent period-luminosity-based distance estimate to M3. We revisited 231 previously known RR Lyrae stars, providing updated period measurements for 203 and white-light amplitudes for 198. We detected Blazhko-like modulation in 53 stars and characterized the modulation parameters for 28. Notably, we measure periods and amplitudes for the unclassified variables V286 and V287 for the first time. We also identify three foreground flaring M dwarfs, and assess the feasibility of detecting microlensing events in M3, concluding that expected rates are negligible. Alongside the scientific results, we introduce a new correction technique for flat-field images affected by scattered light and present a full characterization of the observatory's photometric capabilities. These results demonstrate the scientific utility of TWIST for high-cadence time-domain surveys using modest-aperture instrumentation.

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arXiv2025

Lord of the (sub-)Rings : Mapping the surface reflectance and spin-axis of Ajisai

Active debris removal techniques are posed to become an important tool in maintaining the safety of the near-Earth space environment. These techniques rely on a clear understanding of the rotational motion of the debris targets, which is challenging to constrain from unresolved imaging. The Ajisai satellite provides an ideal test case for developing and demonstrating these techniques due to its simple geometry and well constrained spin behaviour. We present four observations of the Ajisai satellite taken with SuperWASP in August of 2019, where high cadence photometry was extracted from streaked images as a part of a larger survey of Low Earth Orbit. We develop an MCMC-driven method to determine the spin-state of Ajisai by comparing the alignment between a map of modelled mirror positions and a novel derived map of surface reflectivity. We generally find good agreement within the expectation and uncertainties set by empirical models and our determined spin-state solutions align the surface reflectivity map and modelled mirror location well. Our results show that streak photometry can be used to recover the spin-axis and rotation period of fast-spinning objects such as Ajisai using modest ground-based instrumentation, making it readily scalable to a wider range of targets and observatories.

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arXiv2025

High-Precision Photometry with a scientific CMOS Camera: I Lab Testing of the Marana camera

Scientific CMOS cameras are becoming increasingly prevalent in modern observational astronomy. We assess the ability of CMOS image sensors technology to perform high-precision photometry with a detailed laboratory characterization of the Marana 4.2BV-11 CMOS camera. We characterise the camera in the Fastest Frame Rate (FFR) and High Dynamic Range (HDR) modes. Our evaluation includes read noise, dark current, photo response and dark signal non-uniformities, quantum efficiency and window transmittance. The read noise is found to be 1.577\,e$^-$ for the FFR mode. For the HDR mode the read noise floor is measured to be 1.571\,e$^-$ for signal levels below approximately 1800\,e$^-$. The bias level shows dark signal non-uniformities with values of 0.318\,e$^-$ and 0.232\,e$^-$ for FFR and HDR mode, respectively. Pixel well capacity reached 2366 e$^-$pix$^{-1}$ for the FFR mode and 69026 e$^-$pix$^{-1}$ with a dynamic range of 93\,dB for the HDR mode. The camera demonstrates good linearity, yielding linearity errors of 0.099\,\% for FFR mode and 0.122\,\% for HDR mode. The uniformity across the image arrays show a photo response non-uniformity of 0.294\,\% for the FFR mode and 0.131\,\% for the HDR mode. The dark current displays a noticeable glow pattern, resulting in mean dark current levels of $1.674\pm0.011$\, \eps\, for the FFR mode and $1.617\pm0.008$\,\eps\, for the HDR mode at a constant temperature of -25\,$^\circ$C. We measured the quantum efficiency across the visible spectrum, with a peak of of >95\,\% at 560\,nm. Our tests indicate that the Marana CMOS camera is potentially capable of performing precise photometry.

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arXiv2025

High-precision light curves of geostationary objects: The PHANTOM ECHOES 2 RPO campaign

We present results from an extensive optical observation campaign that monitored the Geostationary satellites Intelsat 10-02, Mission Extension Vehicle 2, Thor 5, Thor 6, Thor 7, and Meteosat 11 over a 14 week period that covered the proximity operations and docking of Mission Extension Vehicle 2 with Intelsat 10-02. High-cadence single-color photometric observations are supplemented with targeted multi-color observations, high resolution imaging, and passive radio frequency positioning obtained using complementary facilities. The photometric signatures of the six targets are presented in the form of two-dimensional color maps. A selection of interesting features are investigated in further detail, including a rapid glinting behavior in Thor 6; a brightening event from Meteosat 11; using glints to constrain the unresolved positions of Intelsat 10-02 and MEV-2; changes in the photometric signature of Intelsat 10-02 before and after docking; and signatures of attitude changes and maneuvering in the light curves of MEV-2. A detailed description of the photometric data reduction pipeline is also presented, with a focus on details that must be considered when aiming for sub-percent photometric precision.

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arXiv2025

Classification of LEO Satellites Using Occultations of Background Stars

We present the result of a proof-of-concept simulation designed to classify LEO satellites based on their occultations of background stars. We generate satellite shapes drawn from two broad shape classes, 'boxwing' and 'square'. We then simulate the resulting occultation photometry that would be caused by these satellites orbiting in LEO and intersecting with background stars. The resulting data is then inverted to attempt to recover the input shape and classify the satellite correctly. We find that the technique is theoretically sound, but ambitious with current telescope capabilities. We construct an equation for the required success rate of the method, as a function of exposure time and density of background stars. We find that successful classification requires short exposure times and high background stellar densities. For success rates in excess of 75%, we find a required exposure time of $\sim2.0\times10^{-3}$s, and $\sim500$ stars along the satellites path. Results are presented in terms of these two key parameters, and are discussed in the context of current observational capabilities and alternative satellite characterisation methods.

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arXiv2025

A comprehensive survey of the GEO-belt using simultaneous four-colour observations with STING

Colour light curves of resident space objects (RSOs) encapsulate distinctive features that can offer insights into an object's structure and design, making them an invaluable tool for classification and characterisation. We present the results of the first large systematic colour survey of the GEO belt in which we obtain full-night multi-colour light curves for 112 active geostationary objects between April and May 2023. Colour light curve maps were created to compare and contrast the colours between different satellites and bus configurations. We find that satellites with BSS-702 and STAR-2 buses can be effectively distinguished from the colour measurements on these maps, but comparing the average colour of individual satellites within given solar equatorial phase angle ranges shows that it is difficult to distinguish between bus configurations based on colour alone. We also find tentative evidence to suggest that there is a relationship between colour and time spent on orbit for the Eurostar-3000 class satellites, which is unseen behaviour within other bus configuration classes. The satellites in our sample exhibit `redder' colours than the Sun, which is in agreement with previous findings. We found common light curve features such as symmetrical colour changes as well as unique regions of short timescale glinting which are `bluer' than other regimes within the colour light curves. If these features are indeed seasonal, this would be a powerful characterisation tool. We are able to detect and resolve features in the light curve of the LDPE-3A satellite related to manoeuvres being performed. Finally, we measured the solar panel offsets of 54 satellites in our sample and found variation in the type of colour response. The majority of which did not exhibit any colour change across the solar panel glints compared to them shifting towards 'redder' or 'bluer' colours.

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arXiv2024

Predicting RSO Populations Using a Neighbouring Orbits Technique

The determination of the full population of Resident Space Objects (RSOs) in Low Earth Orbit (LEO) is a key issue in the field of space situational awareness that will only increase in importance in the coming years. We endeavour to describe a novel method of inferring the population of RSOs as a function of orbital height and inclination for a range of magnitudes. The method described uses observations of an orbit of known height and inclination to detect RSOs on neighbouring orbits. These neighbouring orbit targets move slowly relative to our tracked orbit, and are thus detectable down to faint magnitudes. We conduct simulations to show that, by observing multiple passes of a known orbit, we can infer the population of RSOs within a defined region of orbital parameter space. Observing a range of orbits from different orbital sites will allow for the inference of a population of LEO RSOs as a function of their orbital parameters and object magnitude.

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arXiv2023

Simulated recovery of LEO objects using sCMOS blind stacking

We present the methodology and results of a simulation to determine the recoverability of LEO objects using a blind stacking technique. The method utilises sCMOS and GPU technology to inject and recover LEO objects in real observed data. We explore the target recovery fraction and pipeline run-time as a function of three optimisation parameters; number of frames per data-set, exposure time, and binning factor. Results are presented as a function of magnitude and velocity. We find that target recovery using blind stacking is significantly more complete, and can reach fainter magnitudes, than using individual frames alone. We present results showing that, depending on the combination of optimisation parameters, recovery fraction is up to 90% of detectable targets for magnitudes up to 13.5, and then falls off steadily up to a magnitude limit around 14.5. Run-time is shown to be a few multiples of the observing time for the best combinations of optimisation parameters, approaching real-time processing.

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arXiv2020

Developing the GOTO telescope control system

The Gravitational-wave Optical Transient Observer (GOTO) is a wide-field telescope project focused on detecting optical counterparts to gravitational-wave sources. The GOTO Telescope Control System (G-TeCS) is a custom robotic control system which autonomously manages the GOTO telescope hardware and nightly operations. Since the commissioning the GOTO prototype on La Palma in 2017, development of the control system has focused on the alert handling and scheduling systems. These allow GOTO to receive and process transient alerts and then schedule and carry out observations, all without the need for human involvement. GOTO is ultimately anticipated to include multiple telescope arrays on independent mounts, both on La Palma and at a southern site in Australia. When complete these mounts will be linked to form a single multi-site observatory, requiring more advanced scheduling systems to best optimise survey and follow-up observations.

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arXiv2020

The pulsating white dwarf G117-B15A: still the most stable optical clock known

The pulsating hydrogen atmosphere white dwarf star G 117-B15A has been observed since 1974. Its main pulsation period at 215.19738823(63) s, observed in optical light curves, varies by only (5.12+/-0.82)x10^{-15} s/s and shows no glitches, as pulsars do. The observed rate of period change corresponds to a change of the pulsation period by 1 s in 6.2 million years. We demonstrate that this exceptional optical clock can continue to put stringent limits on fundamental physics, such as constraints on interaction from hypothetical dark matter particles, as well as to search for the presence of external substellar companions.

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arXiv2020

Machine Learning for Transient Recognition in Difference Imaging With Minimum Sampling Effort

The amount of observational data produced by time-domain astronomy is exponentially in-creasing. Human inspection alone is not an effective way to identify genuine transients fromthe data. An automatic real-bogus classifier is needed and machine learning techniques are commonly used to achieve this goal. Building a training set with a sufficiently large number of verified transients is challenging, due to the requirement of human verification. We presentan approach for creating a training set by using all detections in the science images to be thesample of real detections and all detections in the difference images, which are generated by the process of difference imaging to detect transients, to be the samples of bogus detections. This strategy effectively minimizes the labour involved in the data labelling for supervised machine learning methods. We demonstrate the utility of the training set by using it to train several classifiers utilizing as the feature representation the normalized pixel values in 21-by-21pixel stamps centered at the detection position, observed with the Gravitational-wave Optical Transient Observer (GOTO) prototype. The real-bogus classifier trained with this strategy can provide up to 95% prediction accuracy on the real detections at a false alarm rate of 1%.

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arXiv2020

DebrisWatch I: A survey of faint geosynchronous debris

Recent anomalies exhibited by satellites and rocket bodies have highlighted that a population of faint debris exists at geosynchronous (GEO) altitudes, where there are no natural removal mechanisms. Despite previous optical surveys probing to around 10-20 cm in size, regular monitoring of faint sources at GEO is challenging, thus our knowledge remains sparse. It is essential that we continue to explore the faint debris population using large telescopes to better understand the risk posed to active GEO satellites. To this end, we present photometric results from a survey of the GEO region carried out with the 2.54 m Isaac Newton Telescope in La Palma, Canary Islands. We probe to 21st visual magnitude (around 10 cm, assuming Lambertian spheres with an albedo of 0.1), uncovering 129 orbital tracks with GEO-like motion across the eight nights of dark-grey time comprising the survey. The faint end of our brightness distribution continues to rise until the sensitivity limit of the sensor is reached, suggesting that the modal brightness could be even fainter. We uncover a number of faint, uncatalogued objects that show photometric signatures of rapid tumbling, many of which straddle the limiting magnitude of our survey over the course of a single exposure, posing a complex issue when estimating object size. This work presents the first instalment of DebrisWatch, an ongoing collaboration between the University of Warwick and the Defence Science and Technology Laboratory (UK) investigating the faint population of GEO debris.

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arXiv2020

Two transiting hot Jupiters from the WASP survey: WASP-150b and WASP-176b

We report the discovery of two transiting exoplanets from the WASP survey, WASP-150b and WASP-176b. WASP-150b is an eccentric ($e$ = 0.38) hot Jupiter on a 5.6 day orbit around a $V$ = 12.03, F8 main-sequence host. The host star has a mass and radius of 1.4 $\rm M_{\odot}$ and 1.7 $\rm R_{\odot}$ respectively. WASP-150b has a mass and radius of 8.5 $\rm M_J$ and 1.1 $\rm R_J$, leading to a large planetary bulk density of 6.4 $\rm ρ_J$. WASP-150b is found to be $\sim3$ Gyr old, well below its circularisation timescale, supporting the eccentric nature of the planet. WASP-176b is a hot Jupiter planet on a 3.9 day orbit around a $V$ = 12.01, F9 sub-giant host. The host star has a mass and radius of 1.3 $\rm M_{\odot}$ and 1.9 $\rm R_{\odot}$. WASP-176b has a mass and radius of 0.86 $\rm M_J$ and 1.5 $\rm R_J$ respectively, leading to a planetary bulk density of 0.23 $\rm ρ_J$.

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arXiv2020

An ultra-massive white dwarf with a mixed hydrogen-carbon atmosphere as a likely merger remnant

White dwarfs are dense, cooling stellar embers consisting mostly of carbon and oxygen, or oxygen and neon (with a few percent carbon) at higher initial stellar masses. These stellar cores are enveloped by a shell of helium which in turn is usually surrounded by a layer of hydrogen, generally prohibiting direct observation of the interior composition. However, carbon is observed at the surface of a sizeable fraction of white dwarfs, sometimes with traces of oxygen, and it is thought to be dredged-up from the core by a deep helium convection zone. In these objects only traces of hydrogen are found as large masses of hydrogen are predicted to inhibit hydrogen/helium convective mixing within the envelope. We report the identification of WDJ055134.612+413531.09, an ultra-massive (1.14 $M_\odot$) white dwarf with a unique hydrogen/carbon mixed atmosphere (C/H=0.15 in number ratio). Our analysis of the envelope and interior indicates that the total hydrogen and helium mass fractions must be several orders of magnitude lower than predictions of single star evolution: less than $10^{-9.5}$ and $10^{-7.0}$, respectively. Due to the fast kinematics ($129\pm5$ km/s relative to the local standard of rest), large mass, and peculiar envelope composition, we argue that WDJ0551+4135 is consistent with formation from the merger of two white dwarfs in a tight binary system.

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