L. V. E. Koopmans
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View article: Mitigating incoherent excess variance in high-redshift 21 cm observations with multi-output cross-Gaussian process regression
Mitigating incoherent excess variance in high-redshift 21 cm observations with multi-output cross-Gaussian process regression Open
Systematic effects that limit the achievable sensitivity of current low-frequency radio telescopes to the 21 cm signal are among the foremost challenges in observational 21 cm cosmology. The standard approach to retrieving the 21 cm signal…
View article: The impact of diffuse Galactic emission on direction-independent gain calibration in high-redshift 21 cm observations
The impact of diffuse Galactic emission on direction-independent gain calibration in high-redshift 21 cm observations Open
This study examines the impact of diffuse Galactic emission on sky-based direction-independent (DI) gain calibration using realistic forward simulations of Low-Frequency Array (LOFAR) observations of the high-redshift 21 cm signal of neutr…
View article: LenNet: direct detection and localization of strong gravitational lenses in wide-field sky survey images
LenNet: direct detection and localization of strong gravitational lenses in wide-field sky survey images Open
Strong gravitational lenses are invaluable for tackling fundamental astrophysics questions, such as the nature of dark matter and cosmic expansion. However, current sky surveys’ “crop-and-classify” lens search method faces a critical chall…
View article: First upper limits on the 21-cm signal power spectrum of neutral hydrogen at <i>z</i> = 9.16 from the LOFAR 3C 196 field
First upper limits on the 21-cm signal power spectrum of neutral hydrogen at <i>z</i> = 9.16 from the LOFAR 3C 196 field Open
The redshifted 21-cm signal of neutral hydrogen from the epoch of reionization (EoR) can potentially be detected using low-frequency radio instruments such as the Low-Frequency Array (LOFAR). So far, LOFAR upper limits on the 21-cm signal …
View article: Exploring the effect of different cosmologies on the Epoch of Reionization 21-cm signal with <scp>polar</scp>
Exploring the effect of different cosmologies on the Epoch of Reionization 21-cm signal with <span>polar</span> Open
A detection of the 21-cm signal power spectrum from the Epoch of Reionization is imminent, thanks to consistent advancements from telescopes such as LOFAR, MWA, and HERA, along with the development of SKA. In light of this progress, it is …
View article: Linear map-making with LuSEE-Night
Linear map-making with LuSEE-Night Open
LuSEE-Night is a pathfinder radio telescope on the lunar far side employing four 3-m monopole antennas arranged as two horizontal cross pseudo-dipoles on a rotational stage and sensitive to the radio sky in the 1-50 MHz frequency band. LuS…
View article: Improved upper limits on the 21-cm signal power spectrum at <i>z</i> = 17.0 and <i>z</i> = 20.3 from an optimal field observed with NenuFAR
Improved upper limits on the 21-cm signal power spectrum at <i>z</i> = 17.0 and <i>z</i> = 20.3 from an optimal field observed with NenuFAR Open
We report the deepest upper limits to date on the 21-cm signal power spectrum during the Cosmic Dawn (redshifts: $z>15$), using four nights of observations with NenuFAR. The limits are derived from two redshift bins, centred at $z=20.3$…
View article: Robust direction-dependent gain-calibration of beam-modelling errors far from the target field
Robust direction-dependent gain-calibration of beam-modelling errors far from the target field Open
Many astronomical questions require deep, wide-field observations at low radio frequencies. Phased arrays like LOFAR and SKA-Low (low band part of the Square Kilometre Array) are designed for this, but have inherently unstable element gain…
View article: Euclid Quick Data Release (Q1). A probabilistic classification of quenched galaxies
Euclid Quick Data Release (Q1). A probabilistic classification of quenched galaxies Open
Investigating what drives the quenching of star formation in galaxies is key to understanding their evolution. The Euclid mission will provide rich spatial and spectral data from optical to infrared wavelengths for millions of galaxies, an…
View article: Extracting the Epoch of Reionization Signal with 3D U-Net Neural Networks Using a Data-driven Systematic Effect Model
Extracting the Epoch of Reionization Signal with 3D U-Net Neural Networks Using a Data-driven Systematic Effect Model Open
Neutral hydrogen serves as a crucial probe for the Cosmic Dawn and the Epoch of Reionization (EoR). Actual observations of the 21 cm signal often encounter challenges such as thermal noise and various systematic effects. To overcome these …
View article: Optical+Near-IR Analysis of a Newly Confirmed Einstein Ring at <i>z </i>∼ 1 from the Kilo-Degree Survey: Dark Matter Fraction, Total and Dark Matter Density Slope, and Initial Mass Function*
Optical+Near-IR Analysis of a Newly Confirmed Einstein Ring at <i>z </i>∼ 1 from the Kilo-Degree Survey: Dark Matter Fraction, Total and Dark Matter Density Slope, and Initial Mass Function* Open
We report the spectroscopic confirmation of a bright blue Einstein ring in the Kilo-Degree Survey (KiDS) footprint: the Einstein “blue eye.” Spectroscopic data from X-Shooter at the Very Large Telescope (VLT) show that the lens is a typica…
View article: Constraints on the state of the intergalactic medium at <i>z</i>∼8 − 10 using redshifted 21 cm observations with LOFAR
Constraints on the state of the intergalactic medium at <i>z</i>∼8 − 10 using redshifted 21 cm observations with LOFAR Open
The power spectra of the redshifted 21 cm signal from the Epoch of Reionization (EoR) contain information about the ionization and thermal states of the intergalactic medium (IGM) and depend on the properties of the sources that existed du…
View article: Inferring IGM parameters from the redshifted 21-cm power spectrum using Artificial Neural Networks
Inferring IGM parameters from the redshifted 21-cm power spectrum using Artificial Neural Networks Open
The high redshift 21-cm signal promises to be a crucial probe of the state of the intergalactic medium (IGM). Understanding the connection between the observed 21-cm power spectrum and the physical quantities intricately associated with th…
View article: Broadband polarized radio emission detected from Starlink satellites below 100 MHz with NenuFAR
Broadband polarized radio emission detected from Starlink satellites below 100 MHz with NenuFAR Open
Aims . We evaluate the impact of Starlink satellites on low-frequency radio astronomy below 100 MHz. We focus on challenges of the data processing and on scientific goals. Methods . We conducted 40 hours of imaging observations using NenuF…
View article: Deeper multi-redshift upper limits on the epoch of reionisation 21 cm signal power spectrum from LOFAR between <i>z</i> = 8.3 and <i>z</i> = 10.1
Deeper multi-redshift upper limits on the epoch of reionisation 21 cm signal power spectrum from LOFAR between <i>z</i> = 8.3 and <i>z</i> = 10.1 Open
We present new upper limits on the 21 cm signal power spectrum from the epoch of reionisation (EoR), at redshifts z ≈10.1,9.1, and 8.3, based on reprocessed observations from the Low-Frequency Array (LOFAR). The analysis incorporates signi…
View article: First upper limits on the 21-cm signal power spectrum of neutral hydrogen at $z=9.16$ from the LOFAR 3C196 field
First upper limits on the 21-cm signal power spectrum of neutral hydrogen at $z=9.16$ from the LOFAR 3C196 field Open
The redshifted 21-cm signal of neutral hydrogen from the Epoch of Reionization (EoR) can potentially be detected using low-frequency radio instruments such as the Low-Frequency Array (LOFAR). So far, LOFAR upper limits on the 21-cm signal …
View article: Finding black holes: an unconventional multi-messenger
Finding black holes: an unconventional multi-messenger Open
A rather clear problem has remained for astrophysicists studying black holes: localizing black holes. One of the recent theoretical ways proposed to identify black hole mergers’ hosts is through multi-messenger gravitational lensing: match…
View article: Near-field imaging of local interference in radio interferometric data
Near-field imaging of local interference in radio interferometric data Open
Radio-frequency interference (RFI) is a major systematic limitation in radio astronomy, particularly for science cases requiring high sensitivity, such as 21 cm cosmology. Traditionally, RFI is dealt with by identifying its signature in th…
View article: The Dark Ages Explorer (DEX): a filled-aperture ultra-long wavelength radio interferometer on the lunar far side
The Dark Ages Explorer (DEX): a filled-aperture ultra-long wavelength radio interferometer on the lunar far side Open
The measurement of the spatial fluctuations of the neutral hydrogen 21 cm signal arising during the Dark Ages and Cosmic Dawn periods of our Universe (z from 200 to 10) holds the potential to resolve these still-unexplored earliest phases …
View article: Robust direction-dependent gain-calibration of beam-modelling errors far from the target field
Robust direction-dependent gain-calibration of beam-modelling errors far from the target field Open
Many astronomical questions require deep, wide-field observations at low radio frequencies. Phased arrays like LOFAR and SKA-low are designed for this, but have inherently unstable element gains, leading to time, frequency and direction-de…
View article: <i>Euclid</i>: The Early Release Observations Lens Search Experiment
<i>Euclid</i>: The Early Release Observations Lens Search Experiment Open
We investigated the ability of the Euclid telescope to detect galaxy-scale gravitational lenses. To do so, we performed a systematic visual inspection of the 0.7 deg 2 Euclid Early Release Observations data towards the Perseus cluster usin…
View article: Using Convolutional Neural Networks to Search for Strongly Lensed Quasars in KiDS DR5
Using Convolutional Neural Networks to Search for Strongly Lensed Quasars in KiDS DR5 Open
Gravitationally strongly lensed quasars (SL-QSO) offer invaluable insights into cosmological and astrophysical phenomena. With the data from ongoing and next-generation surveys, thousands of SL-QSO systems can be discovered expectedly, lea…
View article: Spectral modelling of Cygnus A between 110 and 250 MHz
Spectral modelling of Cygnus A between 110 and 250 MHz Open
Studying the redshifted 21-cm signal from the neutral hydrogen during the Epoch of Reionisation and Cosmic Dawn is fundamental for understanding the physics of the early universe. One of the challenges that 21-cm experiments face is the co…
View article: Wave Optics, Interference, and Decoherence in Strong Gravitational Lensing
Wave Optics, Interference, and Decoherence in Strong Gravitational Lensing Open
Wave-optical effects in gravitational lensing have long been predicted, and with the discovery of populations of compact transients such as gravitational wave events and fast radio bursts, may soon be observed. We present an observer’s rev…
View article: Reducing false positives in strong lens detection through effective augmentation and ensemble learning
Reducing false positives in strong lens detection through effective augmentation and ensemble learning Open
This research studies the impact of high-quality training data sets on the performance of Convolutional Neural Networks (CNNs) in detecting strong gravitational lenses. We stress the importance of data diversity and representativeness, dem…
View article: Galaxy–Galaxy Strong Lensing with U-Net (GGSL-UNet). I. Extracting Two-dimensional Information from Multiband Images in Ground and Space Observations
Galaxy–Galaxy Strong Lensing with U-Net (GGSL-UNet). I. Extracting Two-dimensional Information from Multiband Images in Ground and Space Observations Open
We present a novel deep learning method to separately extract the two-dimensional flux information of the foreground galaxy (deflector) and background system (source) of galaxy–galaxy strong-lensing events using U-Net (GGSL-UNet for short)…
View article: Automation of finding strong gravitational lenses in the Kilo Degree Survey with U-DenseLens (DenseLens + Segmentation)
Automation of finding strong gravitational lenses in the Kilo Degree Survey with U-DenseLens (DenseLens + Segmentation) Open
In the context of upcoming large-scale surveys like Euclid, the necessity for the automation of strong lens detection is essential. While existing machine learning pipelines heavily rely on the classification probability (P), this study in…
View article: Extracting the Epoch of Reionization Signal with 3D U-Net Neural Networks Using Data-driven Systematic Effect Model
Extracting the Epoch of Reionization Signal with 3D U-Net Neural Networks Using Data-driven Systematic Effect Model Open
Neutral hydrogen (HI) serves as a crucial probe for the Cosmic Dawn and the Epoch of Reionization (EoR). Actual observations of the 21-cm signal often encounter challenges such as thermal noise and various systematic effects. To overcome t…
View article: Beyond the horizon: Quantifying the full sky foreground wedge in the cylindrical power spectrum
Beyond the horizon: Quantifying the full sky foreground wedge in the cylindrical power spectrum Open
One of the main obstacles preventing the detection of the redshifted 21 cm signal from neutral hydrogen in the early Universe is the astrophysical foreground emission, which is several orders of magnitude brighter than the signal. The fore…
View article: CASCO: Cosmological and AStrophysical parameters from Cosmological simulations and Observations
CASCO: Cosmological and AStrophysical parameters from Cosmological simulations and Observations Open
Physical processes can influence the formation and evolution of galaxies in diverse ways. It is essential to validate their incorporation into cosmological simulations by testing them against real data encompassing various types of galaxie…