arXiv subjects

Paul Groot

Publications and source records attributed to Paul Groot.

At least 19 recordsLinked to original sources

arXiv2025

The final stages of binary evolution using multi-messenger studies

Ultracompact Galactic binaries with orbital periods below an hour are among the strongest persistent gravitational-wave (GW) sources in the mHz band and will constitute the dominant population detected by the Laser Interferometer Space Antenna (LISA). Tens of thousands are predicted to be individually resolved, with a substantial fraction bright enough for electromagnetic (EM) follow-up. This opens an unprecedented multi-messenger window on compact binary evolution, tidal interactions, mass transfer, and the progenitors of Type Ia supernovae. We highlight key science enabled by joint GW + EM constraints and emphasize the critical need for rapid, high-cadence spectroscopic capabilities in the 2040s. In particular, the most compact (<10 min) binaries detected by LISA will require read-noise-free, zero-dead-time spectroscopic facilities, potentially realized through coordinated arrays of telescopes with time-staggered exposures, to measure radial velocities, tidal heating signatures, and orbital evolution with the precision needed for transformative multi-messenger studies.

astro-ph.IM↗

arXiv2025

GRBs and Relativistic Transients in the 2040s

Relativistic transients such as gamma-ray bursts (GRBs), jetted tidal disruption events, luminous fast blue optical transients, and fast X-ray transients, represent the brightest explosions in the Universe and serve dual roles as laboratories for extreme physics and as cosmic lighthouses probing the earliest epochs of the Universe. The 2040s will bring transformative capabilities: wide-field optical surveys discovering tens of thousands of optical transients nightly, proposed high-energy missions like THESEUS providing 10-100x improved high-energy monitoring, and third-generation gravitational wave detectors identifying $\mathcal{O}(10^5)$ compact object mergers annually, many accompanied by relativistic jets. This industrial-scale discovery rate will enable population studies addressing fundamental questions such as jet launching mechanisms, nucleosynthesis, the first stars, and how progenitor environments shape these transients across cosmic time. However, realizing this science requires overcoming a critical bottleneck: these transients evolve on timescales of seconds to days, with their physics encoded in rapidly-changing multi-wavelength signatures demanding immediate spectroscopic characterization down to m ~ 25. Current facilities, optimized for classical/queue scheduling, do not provide the rapid, flexible, multi-target response necessary for industrial-scale follow-up. This white paper demonstrates that without a dedicated large-aperture (10-30 m effective collecting area) time-domain facility with robotic scheduling and optical-NIR spectroscopic capabilities, the transformative potential of relativistic transient science in the 2040s will be considerably limited.

astro-ph.HE↗

arXiv2025

Nova Explosions in 2040

Novae are thermonuclear explosions on the surface of accreting white dwarfs and are key laboratories for studying explosive nucleosynthesis, particle acceleration, shock physics, and binary evolution. Despite major progress driven by wide-field time-domain surveys and multi-wavelength facilities, our understanding of nova explosions remains limited by incomplete temporal coverage, heterogeneous spectroscopic follow-up, and poorly constrained ejecta properties. In this white paper we outline the open scientific questions that will define nova research in the 2040s, focusing on the mass, composition, geometry, and dynamics of the ejecta, the role of the underlying binary system, and the connection between nuclear burning, shocks, and emission across the electromagnetic spectrum. We argue that decisive progress requires rapid-response, high-cadence, multi-wavelength observations, anchored by systematic high-resolution optical and near-infrared spectroscopy from eruption to quiescence. Finally, we identify key technological requirements needed to enable transformative advances in the physics of nova explosions over the coming decades.

astro-ph.IM↗

arXiv2025

Multi-messenger and time-domain astronomy in the 2040s

Multi-messenger astronomy will be transformed in the 2040s by an unprecedented volume of detections from next-generation gravitational wave, high-energy, and ultra-high energy neutrino, cosmic ray, and time domain observatories. This white paper, prepared for the European Southern Observatory (ESO) Expanding Horizons call, outlines the key science questions enabled by this emerging multi-messenger ecosystem, ranging from nucleosynthesis and dense matter physics to cosmology, fundamental physics, and the growth of black holes across cosmic time. We demonstrate that fully exploiting these discoveries requires a step change in optical to near infrared spectroscopic capability, including low latency response, high throughput, and flexible time domain operations across both hemispheres. We argue that without a dedicated large-aperture time domain facility, the scientific return of multi-messenger astronomy in the 2040s will be considerably limited.

astro-ph.IM↗

arXiv2025

How Mass Flows Through Accretion Discs: A Spectral-Timing Vision for the 2040s

Understanding how mass and angular momentum flow through accretion discs remains a fundamental unsolved problem in astrophysics. Accreting white dwarfs offer an ideal laboratory for addressing this question: their variability occurs on accessible timescales of seconds to minutes, and their optical spectra contain continuum and emission-line components that trace distinct disc regions. Broad-band timing studies have revealed time-lags similar to those observed in X-ray binaries and active galactic nuclei, suggesting propagating fluctuations and possible coupling to an inner hot flow. However, the blending of line and continuum light in broad filters prevents a physical interpretation of these signals. The 2040s will bring an unprecedented number of disc-accreting systems discovered by Rubin-LSST, space-based gravitational-wave observatories, and third-generation ground and space-based detectors. To extract disc physics from these sources, high-cadence optical spectral-timing, simultaneously resolving continuum and individual lines, is essential. Such measurements would directly map how variability propagates through discs, determine how the outer disc responds to changes in the inner flow, and test whether accretion physics is scale-invariant from white dwarfs to supermassive black holes. This white paper outlines the scientific motivation and observational capabilities required to realise this vision. It highlights the opportunity for ESO to enable a transformative new window on accretion physics in the coming decade.

astro-ph.IM↗

arXiv2024

The Near-Ultraviolet eXplorer (NUX): a ground-based wide-field near-UV telescope to search for near-UV transients

We present the Near-Ultraviolet eXplorer (NUX), which will consist out of 4 small (36 cm diameter) ground-based telescopes that are optimized for the shortest wavelengths that are detectable from Earth (i.e., the near-UV [NUV] wavelength range of 300-350 nm). Each telescope will have a field-of-view of ~17 square degrees sampled at ~2.6"/pixel, and will reach a NUV magnitude (AB) of 20 in 2.5 minutes exposures (in dark time). The goal of NUX is to improve our understanding of the physical processes that power fast (days) to very fast (hours) hot transients, such as shock-breakout and shock-cooling emission of supernovae and the electromagnetic counterparts of gravitational wave events. Each telescope will be an off-the-shelf 14" Celestron RASA telescope, retrofitted with NUV optics. We have already demonstrated that the normal Schmidt corrector of this telescope can be replaced by a custom made one consisting of NUV transparent glass. Currently, a prototype NUX telescope is being fully assembled to demonstrate the technical and scientific feasibility of the NUX concept. Site tests will be held (in 2025/2026) at La Silla, Chile, to determine the NUV characteristics of the atmosphere at this site.

astro-ph.IM↗

arXiv2023

A Radio Flare in the Long-Lived Afterglow of the Distant Short GRB 210726A: Energy Injection or a Reverse Shock from Shell Collisions?

We present the discovery of the radio afterglow of the short $\gamma$-ray burst (GRB) 210726A, localized to a galaxy at a photometric redshift of $z\sim 2.4$. While radio observations commenced $\lesssim 1~$day after the burst, no radio emission was detected until $\sim11~$days. The radio afterglow subsequently brightened by a factor of $\sim 3$ in the span of a week, followed by a rapid decay (a "radio flare"). We find that a forward shock afterglow model cannot self-consistently describe the multi-wavelength X-ray and radio data, and underpredicts the flux of the radio flare by a factor of $\approx 5$. We find that the addition of substantial energy injection, which increases the isotropic kinetic energy of the burst by a factor of $\approx 4$, or a reverse shock from a shell collision are viable solutions to match the broad-band behavior. At $z\sim 2.4$, GRB 210726A is among the highest redshift short GRBs discovered to date as well as the most luminous in radio and X-rays. Combining and comparing all previous radio afterglow observations of short GRBs, we find that the majority of published radio searches conclude by $\lesssim 10~$days after the burst, potentially missing these late rising, luminous radio afterglows.

astro-ph.HE↗

arXiv2023

Bursts from Space: MeerKAT - The first citizen science project dedicated to commensal radio transients

The newest generation of radio telescopes are able to survey large areas with high sensitivity and cadence, producing data volumes that require new methods to better understand the transient sky. Here we describe the results from the first citizen science project dedicated to commensal radio transients, using data from the MeerKAT telescope with weekly cadence. Bursts from Space: MeerKAT was launched late in 2021 and received ~89000 classifications from over 1000 volunteers in 3 months. Our volunteers discovered 142 new variable sources which, along with the known transients in our fields, allowed us to estimate that at least 2.1 per cent of radio sources are varying at 1.28 GHz at the sampled cadence and sensitivity, in line with previous work. We provide the full catalogue of these sources, the largest of candidate radio variables to date. Transient sources found with archival counterparts include a pulsar (B1845-01) and an OH maser star (OH 30.1-0.7), in addition to the recovery of known stellar flares and X-ray binary jets in our observations. Data from the MeerLICHT optical telescope, along with estimates of long time-scale variability induced by scintillation, imply that the majority of the new variables are active galactic nuclei. This tells us that citizen scientists can discover phenomena varying on time-scales from weeks to several years. The success both in terms of volunteer engagement and scientific merit warrants the continued development of the project, whilst we use the classifications from volunteers to develop machine learning techniques for finding transients.

astro-ph.HE↗

arXiv2022

SXP 15.6 -- an accreting pulsar close to spin equilibrium?

SXP 15.6 is a recently established Be star X-ray binary system (BeXRB) in the Small Magellanic Cloud (SMC). Like many such systems the variable X-ray emission is driven by the underlying behaviour of the mass donor Be star. It is shown here that the neutron star in this system is exceptionally close to spin equilibrium averaged over several years, with the angular momentum gain from mass transfer being almost exactly balanced by radiative losses. This makes SXP 15.6 exceptional compared to all other known members of its class in the SMC, all of whom exhibit much higher spin period changes. In this paper we report on X-ray observations of the brightest known outburst from this system. These observations are supported by contemporaneous optical and radio observations, as well as several years of historical data.

astro-ph.HE↗

arXiv2022

Serendipitous discovery of radio flaring behaviour from a nearby M dwarf with MeerKAT

We report on the detection of MKT J174641.0$-$321404, a new radio transient found in untargeted searches of wide-field MeerKAT radio images centred on the black hole X-ray binary H1743$-$322. MKT J174641.0$-$321404 is highly variable at 1.3 GHz and was detected three times during 11 observations of the field in late 2018, reaching a maximum flux density of 590 $\pm$ 60 $\mu$Jy. We associate this radio transient with a high proper motion, M dwarf star SCR~1746$-$3214 12 pc away from the Sun. Multiwavelength observations of this M dwarf indicate flaring activity across the electromagnetic spectrum, consistent with emission expected from dMe stars, and providing upper limits on quiescent brightness in both the radio and X-ray regimes. \textit{TESS} photometry reveals a rotational period for SCR~1746$-$3214 of $0.2292 \pm 0.0025$ days, which at its estimated radius makes the star a rapid rotator, comparable to other low mass systems. Dedicated spectroscopic follow up confirms the star as a mid-late spectral M dwarf with clear magnetic activity indicated by strong H$\alpha$ emission. This transient's serendipitous discovery by MeerKAT, along with multiwavelength characterisation, make it a prime demonstration of both the capabilities of the current generation of radio interferometers and the value of simultaneous observations by optical facilities such as MeerLICHT. Our results build upon the literature of of M dwarfs' flaring behaviour, particularly relevant to the habitability of their planetary systems.

astro-ph.SR↗

arXiv2022

Astrophysics with the Laser Interferometer Space Antenna

The Laser Interferometer Space Antenna (LISA) will be a transformative experiment for gravitational wave astronomy, and, as such, it will offer unique opportunities to address many key astrophysical questions in a completely novel way. The synergy with ground-based and space-born instruments in the electromagnetic domain, by enabling multi-messenger observations, will add further to the discovery potential of LISA. The next decade is crucial to prepare the astrophysical community for LISA's first observations. This review outlines the extensive landscape of astrophysical theory, numerical simulations, and astronomical observations that are instrumental for modeling and interpreting the upcoming LISA datastream. To this aim, the current knowledge in three main source classes for LISA is reviewed; ultracompact stellar-mass binaries, massive black hole binaries, and extreme or intermediate mass ratio inspirals. The relevant astrophysical processes and the established modeling techniques are summarized. Likewise, open issues and gaps in our understanding of these sources are highlighted, along with an indication of how LISA could help making progress in the different areas. New research avenues that LISA itself, or its joint exploitation with upcoming studies in the electromagnetic domain, will enable, are also illustrated. Improvements in modeling and analysis approaches, such as the combination of numerical simulations and modern data science techniques, are discussed. This review is intended to be a starting point for using LISA as a new discovery tool for understanding our Universe.

gr-qc↗

arXiv2021

MeerCRAB: MeerLICHT Classification of Real and Bogus Transients using Deep Learning

Astronomers require efficient automated detection and classification pipelines when conducting large-scale surveys of the (optical) sky for variable and transient sources. Such pipelines are fundamentally important, as they permit rapid follow-up and analysis of those detections most likely to be of scientific value. We therefore present a deep learning pipeline based on the convolutional neural network architecture called $\texttt{MeerCRAB}$. It is designed to filter out the so called 'bogus' detections from true astrophysical sources in the transient detection pipeline of the MeerLICHT telescope. Optical candidates are described using a variety of 2D images and numerical features extracted from those images. The relationship between the input images and the target classes is unclear, since the ground truth is poorly defined and often the subject of debate. This makes it difficult to determine which source of information should be used to train a classification algorithm. We therefore used two methods for labelling our data (i) thresholding and (ii) latent class model approaches. We deployed variants of $\texttt{MeerCRAB}$ that employed different network architectures trained using different combinations of input images and training set choices, based on classification labels provided by volunteers. The deepest network worked best with an accuracy of 99.5$\%$ and Matthews correlation coefficient (MCC) value of 0.989. The best model was integrated to the MeerLICHT transient vetting pipeline, enabling the accurate and efficient classification of detected transients that allows researchers to select the most promising candidates for their research goals.

astro-ph.IM↗

arXiv2020

Anomalous gas in ESO 149-G003: A MeerKAT-16 View

ESO 149-G003 is a close-by, isolated dwarf irregular galaxy. Previous observations with the ATCA indicated the presence of anomalous neutral hydrogen (HI) deviating from the kinematics of a regularly rotating disc. We conducted follow-up observations with the MeerKAT radio telescope during the 16-dish Early Science programme as well as with the MeerLICHT optical telescope. Our more sensitive radio observations confirm the presence of anomalous gas in ESO 149-G003, and further confirm the formerly tentative detection of an extraplanar HI component in the galaxy. Employing a simple tilted-ring model, in which the kinematics is determined with only four parameters but including morphological asymmetries, we reproduce the galaxy's morphology, which shows a high degree of asymmetry. By comparing our model with the observed HI, we find that in our model we cannot account for a significant (but not dominant) fraction of the gas. From the differences between our model and the observed data cube we estimate that at least 7%-8% of the HI in the galaxy exhibits anomalous kinematics, while we estimate a minimum mass fraction of less than 1% for the morphologically confirmed extraplanar component. We investigate a number of global scaling relations and find that, besides being gas-dominated with a neutral gas-to-stellar mass ratio of 1.7, the galaxy does not show any obvious global peculiarities. Given its isolation, as confirmed by optical observations, we conclude that the galaxy is likely currently acquiring neutral gas. It is either re-accreting gas expelled from the galaxy or accreting pristine intergalactic material.

astro-ph.GA↗

arXiv2020

A spectroscopic, photometric, polarimetric and radio study of the eclipsing polar UZ Fornacis: the first simultaneous SALT and MeerKAT observations

We present phase-resolved spectroscopy, photometry and circular spectropolarimetry of the eclipsing polar UZ Fornacis. Doppler tomography of the strongest emission lines using the inside-out projection revealed the presence of three emission regions: from the irradiated face of the secondary star, the ballistic stream and the threading region, and the magnetically confined accretion stream. The total intensity spectrum shows broad emission features and a continuum that rises in the blue. The circularly polarized spectrum shows the presence of three cyclotron emission harmonics at $\sim$4500 \AA{}, 6000 \AA{} and 7700 \AA{}, corresponding to harmonic numbers 4, 3, and 2, respectively. These features are dominant before the eclipse and disappear after the eclipse. The harmonics are consistent with a magnetic field strength of $\sim$57 MG. We also present phase-resolved circular and linear photopolarimetry to complement the spectropolarimetry around the times of eclipse. MeerKAT radio observations show a faint source which has a peak flux density of 30.7 $\pm$ 5.4 $\mu$Jy/beam at 1.28 GHz at the position of UZ For.

astro-ph.SR↗

arXiv2018

Multi-Messenger Astrophysics: Harnessing the Data Revolution

The past year has witnessed discovery of the first identified counterparts to a gravitational wave transient (GW 170817A) and a very high-energy neutrino (IceCube-170922A). These source identifications, and ensuing detailed studies, have realized longstanding dreams of astronomers and physicists to routinely carry out observations of cosmic sources by other than electromagnetic means, and inaugurated the era of "multi-messenger" astronomy. While this new era promises extraordinary physical insights into the universe, it brings with it new challenges, including: highly heterogeneous, high-volume, high-velocity datasets; globe-spanning cross-disciplinary teams of researchers, regularly brought together into transient collaborations; an extraordinary breadth and depth of domain-specific knowledge and computing resources required to anticipate, model, and interpret observations; and the routine need for adaptive, distributed, rapid-response observing campaigns to fully exploit the scientific potential of each source. We argue, therefore, that the time is ripe for the community to conceive and propose an Institute for Multi-Messenger Astrophysics that would coordinate its resources in a sustained and strategic fashion to efficiently address these challenges, while simultaneously serving as a center for education and key supporting activities. In this fashion, we can prepare now to realize the bright future that we see, beyond, through these newly opened windows onto the universe.

astro-ph.IM↗

arXiv2018

Chemical Abundances of Globular Clusters in NGC 5128 (Centaurus A)

We perform a detailed abundance analysis on integrated-light spectra of 20 globular clusters (GCs) in the early-type galaxy NGC 5128 (Centaurus A). The GCs were observed with X-Shooter on the VLT. The cluster sample spans a metallicity range of $-1.92 < $ [Fe/H] $< -0.13$ dex. Using theoretical isochrones we compute synthetic integrated-light spectra and iterate the individual abundances until the best fit to the observations is obtained. We measured abundances of Mg, Ca, and Ti, and find a slightly higher enhancement in NGC 5128 GCs with metallicities [Fe/H] < $-$0.75 dex, of the order of $\sim$0.1 dex, than in the average values observed in the MW for GCs of the same metallicity. If this $\alpha$-enhancement in the metal-poor GCs in NGC 5128 is genuine, it could hint at a chemical enrichment history different than that experienced by the MW. We also measure Na abundances in 9 out of 20 GCs. We find evidence for intra-cluster abundance variations in 6 of these clusters where we see enhanced [Na/Fe] > $+$0.25 dex. We obtain the first abundance measurements of Cr, Mn, and Ni for a sample of the GC population in NGC 5128 and find consistency with the overall trends observed in the MW, with a slight enhancement ($<$0.1 dex) in the Fe-peak abundances measured in the NGC 5128.

astro-ph.GA↗

arXiv2017

Metallicities of Young Massive Clusters in NGC 5236 (M83)

We present integrated-light spectra of 8 Young Massive Clusters (YMCs) in the metal-rich spiral galaxy NGC 5236 (M 83). The observations were taken with the X-Shooter spectrograph on the ESO Very Large Telescope. Through the use of theoretical isochrones and synthetic integrated-light (IL) spectra we derive metallicities and study the radial metallicity gradient observed through these young populations. For the inner regions of the galaxy we observe a relatively shallow metallicity gradient of $-$0.37 $\pm$0.29 dex R$_{25}^{-1}$, agreeing with chemical evolution models with an absence of infall material and a relatively low mass loss due to winds in the inner parts of the disk. We estimate a central metallicity of [$Z$] = $+$0.17 $\pm$ 0.12 dex, finding excellent agreement with that obtained via other methods (e.g. blue supergiants and J-band). We infer a metallicity of 12+log(O/H) = 8.75 $\pm$ 0.08 dex at R/R$_{25}$ = 0.4, which fits the stellar mass-metallicity relation (MZR) compilation of blue supergiants and IL studies.

astro-ph.GA↗

arXiv2017

Chemical abundances of two extragalactic young massive clusters

We use integrated-light spectroscopic observations to measure metallicities and chemical abundances for two extragalactic young massive star clusters (NGC1313-379 and NGC1705-1). The spectra were obtained with the X-Shooter spectrograph on the ESO Very Large Telescope. We compute synthetic integrated-light spectra, based on colour-magnitude diagrams for the brightest stars in the clusters from Hubble Space Telescope photometry and theoretical isochrones. Furthermore, we test the uncertainties arising from the use of Colour Magnitude Diagram (CMD) +Isochrone method compared to an Isochrone-Only method. The abundances of the model spectra are iteratively adjusted until the best fit to the observations is obtained. In this work we mainly focus on the optical part of the spectra. We find metallicities of [Fe/H] = $-$0.84 $\pm$ 0.07 and [Fe/H] = $-$0.78 $\pm$ 0.10 for NGC1313-379 and NGC1705-1, respectively. We measure [$\alpha$/Fe]=$+$0.06 $\pm$ 0.11 for NGC1313-379 and a super-solar [$\alpha$/Fe]=$+$0.32 $\pm$ 0.12 for NGC1705-1. The roughly solar [$\alpha$/Fe] ratio in NGC1313-379 resembles those for young stellar populations in the Milky Way (MW) and the Magellanic Clouds, whereas the enhanced [$\alpha$/Fe] ratio in NGC1705-1 is similar to that found for the cluster NGC1569-B by previous studies. Such super-solar [$\alpha$/Fe] ratios are also predicted by chemical evolution models that incorporate the bursty star formation histories of these dwarf galaxies. Furthermore, our $\alpha$-element abundances agree with abundance measurements from H II regions in both galaxies. In general we derive Fe-peak abundances similar to those observed in the MW and Large Magellanic Cloud (LMC) for both young massive clusters. For these elements, however, we recommend higher-resolution observations to improve the Fe-peak abundance measurements.

astro-ph.GA↗