Search

On 2023 May 29, the LIGO–Virgo–KAGRA Collaboration observed a compact binary coalescence event consistent with a neutron star–black hole merger, though the heavier object of mass |$2.5-4.5\, {\rm M}_{\odot }$| would fall into the purported lower mass gap. An alternative explanation for apparent observations of events in this mass range has been suggested as strongly gravitationally lensed binary neutron stars. In this scenario, magnification would lead to the source appearing closer and heavier than it really is. Here, we investigate the chances and possible consequences for the GW230529 event to be gravitationally lensed. We find this would require high magnifications and we obtain low rates for observing such an event, with a relative fraction of lensed versus unlensed observed events of |$2\times 10^{-3}$| at most. When comparing the lensed and unlensed hypotheses accounting for the latest rates and population model, we find a |$1/58$| chance of lensing, disfavouring this option. Moreover, when the magnification is assumed to be strong enough to bring the mass of the heavier binary component below the standard upper limits on neutron star masses, we find high probability for the lighter object to have a subsolar mass, making the binary even more exotic than a mass-gap neutron star–black hole system. Even when the secondary is not subsolar, its tidal deformability would likely be measurable, which is not the case for GW230529. Finally, we do not find evidence for extra lensing signatures such as the arrival of additional lensed images, type-II image dephasing, or microlensing. Therefore, we conclude it is unlikely for GW230529 to be a strongly gravitationally lensed binary neutron star signal. [ABSTRACT FROM AUTHOR]

By the end of the next decade, we hope to have detected strongly lensed gravitational waves by galaxies or clusters. Although there exist optimal methods for identifying lensed signal, it is shown that machine learning (ML) algorithms can give comparable performance but are orders of magnitude faster than non-ML methods. We present the SLICK pipeline which comprises a parallel network based on deep learning. We analyse the Q-transform maps (QT maps) and the Sine-Gaussian projection (SGP) maps generated for the binary black hole signals injected in Gaussian as well as real noise. We compare our network performance with the previous work and find that the efficiency of our model is higher by a factor of 5 at a false positive rate of 0.001. Further, we show that including SGP maps with QT maps data result in a better performance than analysing QT maps alone. When combined with sky localization constraints, we hope to get unprecedented accuracy in the predictions than previously possible. We also evaluate our model on the real events detected by the LIGO–Virgo collaboration and find that, at a threshold of 0.75 our network correctly classifies all of them, consistent with non-detection of lensing. [ABSTRACT FROM AUTHOR]

Efficient algorithms are being developed to search for strong gravitational lens systems owing to increasing large imaging surveys. Neural networks have been successfully used to discover galaxy-scale lens systems in imaging surveys such as the Kilo Degree Survey, Hyper-Suprime Cam (HSC) Survey, and Dark Energy Survey over the last few years. Thus, it has become imperative to understand how some of these networks compare, their strengths and the role of the training data sets which are essential in supervised learning algorithms used commonly in neural networks. In this work, we present the first-of-its-kind systematic comparison and benchmarking of networks from four teams that have analysed the HSC Survey data. Each team has designed their training samples and developed neural networks independently but coordinated a priori in reserving specific data sets strictly for test purposes. The test sample consists of mock lenses, real (candidate) lenses, and real non-lenses gathered from various sources to benchmark and characterize the performance of each of the network. While each team's network performed much better on their own constructed test samples compared to those from others, all networks performed comparable on the test sample with real (candidate) lenses and non-lenses. We also investigate the impact of swapping the training samples among the teams while retaining the same network architecture. We find that this resulted in improved performance for some networks. These results have direct implications on measures to be taken for lens searches with upcoming imaging surveys such as the Rubin-Legacy Survey of Space and Time, Roman, and Euclid. [ABSTRACT FROM AUTHOR]

We report the discovery of new lensed quasar candidates in the imaging data of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) Data Release 4, covering |$1\, 310\deg ^2$| of the sky with median seeing in the i band of ≈0.6 arcsec. In addition to two catalogues of Million Quasars Catalog v7.2 and AllWISE Catalog of Mid-Infra Red Active Galactic Nuclei, containing confirmed and candidate quasars, we preselect quasar sources using colour cuts from the HSC (grizy) and unWISE (W 1 + W 2) photometric data based on SDSS spectroscopic catalogues. We search for the presence of multiple point sources with similar colour through the convolution of the Laplacian of the preselected quasar image cutouts with the Laplacian of the point spread function, resulting in a reduction of lens candidates from 1 652 329 to 121 511 (7.4 per cent). After visual binary classification, we grade 6 199 (0.4 per cent) potential lenses on a scale of 0 to 3, with 3 indicating a lens and 0 indicating a non-lens. Finally we obtain 162 lens candidates with an average grade of ≥2, and among them, we successfully recover 18 known lenses. By fitting the light distribution and removing the known contaminants, we discover that 57 new systems contain at least two point sources and a galaxy in between, including 10 possible quadruply lensed quasars. This new sample exhibits a median separation of 1.26 arcsec and a magnitude limit of i ≈ 22. Spectroscopic or high-resolution imaging follow up on these newly discovered lensed quasar candidates will further allow their natures to be confirmed. [ABSTRACT FROM AUTHOR]

When travelling from their source to the observer, gravitational waves can get deflected by massive objects along their travel path. For a massive lens and a good source-lens alignment, the wave undergoes strong lensing, leading to several images with the same frequency evolution. These images are separated in time, magnified, and can undergo an overall phase shift. Searches for strongly lensed gravitational waves look for events with similar masses, spins, and sky location and linked through so-called lensing parameters. However, the agreement between these quantities can also happen by chance. To reduce the overlap between background and foreground, one can include lensing models. When doing realistic searches, one does not know which model is the correct one to be used. Using an incorrect model could lead to the non-detection of genuinely lensed events. In this work, we investigate how one can reduce the false alarm probability when searching for strongly lensed events. We focus on the impact of the addition of a model for the lens density profile and investigate the effect of potential errors in the modelling. We show that the risks of false alarm are high without the addition of a lens model. We also show that slight variations in the profile of the lens model are tolerable, but a model with an incorrect assumption about the underlying lens population causes significant errors in the identification process. We also suggest some strategies to improve confidence in the detection of strongly lensed gravitational waves. [ABSTRACT FROM AUTHOR]

A double source plane (DSP) system is a precious probe for the density profile of distant galaxies and cosmological parameters. However, these measurements could be affected by the surrounding environment of the lens galaxy. Thus, it is important to evaluate the cluster-scale mass for detailed mass modelling. We observed the Eye of Horus , a DSP system discovered by the Hyper Suprime-Cam Subaru Strategic Survey (HSC–SSP), with XMM–Newton. We detected two X-ray extended emissions, originating from two clusters, one centred at the Eye of Horus , and the other located ∼100 arcsec north-east to the Eye of Horus. We determined the dynamical mass assuming hydrostatic equilibrium, and evaluated their contributions to the lens mass interior of the Einstein radius. The contribution of the former cluster is |$1.1^{+1.2}_{-0.5}\times 10^{12}\, \mathrm{M}_{\odot }$|⁠ , which is |$21\!-\!76{{\ \rm per\ cent}}$| of the total mass within the Einstein radius. The discrepancy is likely due to the complex gravitational structure along the line of sight. On the other hand, the contribution of the latter cluster is only |$\sim 2{{\ \rm per\ cent}}$| on the Eye of Horus. Therefore, the influence associated with this cluster can be ignored. [ABSTRACT FROM AUTHOR]

Strong lensing of active galactic nuclei in the radio can result in razor-thin arcs, with a thickness of less than a milliarcsecond, if observed at the resolution achievable with very long baseline interferometry (VLBI). Such razor-thin arcs provide a unique window on the coarseness of the matter distribution between source and observer. In this paper, we investigate to what extent such razor-thin arcs can constrain the number density and mass function of 'free-floating' black holes, defined as black holes that do not, or no longer, reside at the centre of a galaxy. These can be either primordial in origin or arise as by-products of the evolution of supermassive black holes in galactic nuclei. When sufficiently close to the line of sight, free-floating black holes cause kink-like distortions in the arcs, which are detectable by eye in the VLBI images as long as the black hole mass exceeds ∼1000 Solar masses. Using a crude estimate for the detectability of such distortions, we analytically compute constraints on the matter density of free-floating black holes resulting from null-detections of distortions along a realistic, fiducial arc, and find them to be comparable to those from quasar milli-lensing. We also use predictions from a large hydrodynamical simulation for the demographics of free-floating black holes that are not primordial in origin and show that their predicted mass density is roughly four orders of magnitude below the constraints achievable with a single razor-thin arc. [ABSTRACT FROM AUTHOR]

We report modelling follow-up of recently discovered gravitational-lens candidates in the Canada France Hawaii Telescope Legacy Survey. Lens modelling was done by a small group of specially interested volunteers from the SpaceWarps citizen-science community who originally found the candidate lenses. Models are categorized according to seven diagnostics indicating (a) the image morphology and how clear or indistinct it is, (b) whether the mass map and synthetic lensed image appear to be plausible, and (c) how the lens-model mass compares with the stellar mass and the abundance-matched halo mass. The lensing masses range from ~1011 to >1013M☉. Preliminary estimates of the stellar masses show a smaller spread in stellar mass (except for two lenses): a factor of a few below or above ~1011M☉. Therefore, we expect the stellar-to-total mass fraction to decline sharply as lensing mass increases. The most massive system with a convincing model is J1434+522 (SW 05). The two low-mass outliers are J0206-095 (SW 19) and J2217+015 (SW 42); if these two are indeed lenses, they probe an interesting regime of very low star formation efficiency. Some improvements to the modelling software (SpaghettiLens), and discussion of strategies regarding scaling to future surveys with more and frequent discoveries, are included. [ABSTRACT FROM AUTHOR]

We report the discovery of 13 confirmed two-image quasar lenses from a systematic search for gravitationally lensed quasars in the SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS). We adopted a methodology similar to that used in the SDSS Quasar Lens Search (SQLS). In addition to the confirmed lenses, we report 11 quasar pairs with small angular separations (≲2 arcsec) confirmed from our spectroscopy, which are either projected pairs, physical binaries, or possibly quasar lens systems whose lens galaxies have not yet been detected. The newly discovered quasar lens system, SDSS J1452+4224 at zs ≈ 4.8 is one of the highest redshift multiply imaged quasars found to date. Furthermore, we have over 50 good lens candidates yet to be followed up. Owing to the heterogeneous selection of BOSS quasars, the lens sample presented here does not have a well-defined selection function. [ABSTRACT FROM AUTHOR]

We report the discovery of 29 promising (and 59 total) new lens candidates from the Canada- France-Hawaii Telescope Legacy Survey (CFHTLS) based on about 11 million classifications performed by citizen scientists as part of the first SPACEWARPS lens search. The goal of the blind lens search was to identify lens candidates missed by robots (the RINGFINDER on galaxy scales and ARCFINDER on group/cluster scales) which had been previously used to mine the CFHTLS for lenses. We compare some properties of the samples detected by these algorithms to the SPACE WARPS sample and find them to be broadly similar. The image separation distribution calculated from the SPACE WARPS sample shows that previous constraints on the average density profile of lens galaxies are robust. SPACE WARPS recovers about 65 per cent of known lenses, while the new candidates show a richer variety compared to those found by the two robots. This detection rate could be increased to 80 per cent by only using classifications performed by expert volunteers (albeit at the cost of a lower purity), indicating that the training and performance calibration of the citizen scientists is very important for the success of SPACE WARPS. In this work we present the SIMCT pipeline, used for generating in situ a sample of realistic simulated lensed images. This training sample, along with the false positives identified during the search, has a legacy value for testing future lens-finding algorithms. We make the pipeline and the training set publicly available. [ABSTRACT FROM AUTHOR]

We describe SPACE WARPS, a novel gravitational lens discovery service that yields samples of high purity and completeness through crowdsourced visual inspection. Carefully produced colour composite images are displayed to volunteers via a web-based classification interface, which records their estimates of the positions of candidate lensed features. Images of simulated lenses, as well as real images which lack lenses, are inserted into the image stream at random intervals; this training set is used to give the volunteers instantaneous feedback on their performance, as well as to calibrate a model of the system that provides dynamical updates to the probability that a classified image contains a lens. Low-probability systems are retired from the site periodically, concentrating the sample towards a set of lens candidates. Having divided 160 deg² of Canada-France-Hawaii Telescope Legacy Survey imaging into some 430 000 overlapping 82 by 82 arcsec tiles and displaying them on the site, we were joined by around 37 000 volunteers who contributed 11 million image classifications over the course of eight months. This stage 1 search reduced the sample to 3381 images containing candidates; these were then refined in stage 2 to yield a sample that we expect to be over 90 per cent complete and 30 per cent pure, based on our analysis of the volunteers performance on training images. We comment on the scalability of the SPACE WARPS system to the wide field survey era, based on our projection that searches of 105 images could be performed by a crowd of 105 volunteers in 6 d. [ABSTRACT FROM AUTHOR]

Luminous red galaxies (LRG) from the Sloan Digital Sky Survey are among the best understood samples of galaxies and are employed in a broad range of cosmological studies. In this paper, we study how LRGs occupy massive haloes via counts in clusters and reveal several unexpected trends. Using the red-sequence Matched-filter Probabilistic Percolation (redMaPPer) cluster catalogue, we derive the central occupation of LRGs as a function richness. We show that clusters contain a significantly lower fraction of central LRGs than predicted from the two-point correlation function. At halo masses of 1014.5M⊙, we find Ncen = 0.73 compared to Ncen = 0.89 from correlation studies. Our central occupation function for LRGs converges to 0.95 at large halo masses. A strong anticorrelation between central luminosity and cluster mass at fixed richness is required to reconcile our results with those based on clustering studies. We derive the probability that the brightest cluster member is not the central galaxy. We find PBNC ≈ 20-30 per cent which is a factor of ~2 lower than the value found by Skibba et al. Finally, we study the radial offsets of bright non-central LRGs from cluster centres and show that bright non-central LRGs follow a different radial distribution compared to red cluster members. This work demonstrates that even the most massive clusters do not always have an LRG at the centre, and that the brightest galaxy in a cluster is not always the central galaxy. [ABSTRACT FROM AUTHOR]