arXiv subjects

Luka Vujeva

Publications and source records attributed to Luka Vujeva.

16 recordsLinked to original sources

arXiv2026

Probability of gravitational-wave lensing by intermediate-mass black holes and globular clusters

Strongly lensed gravitational waves (GWs) are powerful probes of substructure in the lens. Intermediate-mass black holes (IMBHs) are postulated to be efficient lenses that may distort the lensed waveforms of currently detectable stellar-mass compact binaries, as hinted by GW231123. Assuming that IMBHs are located in globular clusters (GCs), we compute the rate at which they would affect strongly lensed repeated chirps produced by galaxy-scale lenses, considering a compound lens system. Exploring different astrophysical model assumptions and lensing criteria, we find that the relative rate is at most 1/1000 and decays to 1/10,000 for our fiducial optimistic scenario. At high magnifications, $\mu$ > 100, the relative rate approaches 1%, but these cases are intrinsically rare in absolute value. Our results imply that GW lensing by IMBHs and GCs is unlikely, disfavoring such an interpretation for GW231123. In turn, they point towards lensed GWs being a clean probe of dark matter substructures and primordial black holes.

astro-ph.CO↗

arXiv2025

Discovering gravitational waveform distortions from lensing: A deep dive into GW231123

Gravitational waves (GWs) are unique messengers as they travel through the Universe without alteration except for gravitational lensing. Their long wavelengths make them susceptible to diffraction by cosmic structures, providing an unprecedented opportunity to map dark matter substructures. Identifying lensed events requires the analysis of thousands to millions of simulated events to reach high statistical significances. This is computationally prohibitive with standard GW parameter estimation methods. We exploit DINGO-lensing, a deep-learning algorithm that accelerates the inference from CPU days to minutes to thoroughly reanalyze GW231123, the most promising lensing candidate to date. By performing more than 200,000 simulations with 3 different waveform models, we find that its statistical significance is below 4$\sigma$ and the event cannot be claimed as lensed. We observe that 8% of GW231123-like nonlensed simulations favor lensing, which could be explained by the self-similarity of short-duration signals. Still, 58% of GW231123-like lensed simulations have larger support for lensing, showing that higher detection statistics are possible. We show that analyzing simulations with different waveform models only lowers the significance, highlighting the relevance of waveform systematics. Although GW231123 exposes the challenges of claiming the first GW lensing detection, our deep-learning methods have demonstrated to be powerful enough to enable the upcoming discovery of lensed GWs.

gr-qc↗

arXiv2025

Identification and characterization of distorted gravitational waves by lensing using deep learning

Gravitational waves (GWs) can be distorted by intervening mass distributions while propagating, leading to frequency-dependent modulations that imprint a distinct signature on the observed waveforms. Bayesian inference for GW lensing with conventional sampling methods is costly, and the problem is exacerbated by the rapidly growing GW catalog. Moreover, assessing the statistical significance of lensed candidates requires thousands, if not millions, of simulations to estimate the background from noise fluctuations and waveform systematics, which is infeasible with standard samplers. We present a novel method, DINGO-lensing, for performing inference on lensed GWs, extending the neural posterior estimation framework DINGO. By comparing our results with those using conventional samplers, we show that the compute time of parameter estimation of lensed GWs can be reduced from weeks to seconds, while preserving accuracy both in the posterior distributions and the evidence ratios. We train our neural networks with LIGO detector noise at design sensitivity and a lens model that accommodates two overlapping images with a constant $\pi/2$ phase shift. We show that the lensing parameters are recovered with millisecond precision for the time delays. We also demonstrate that our network can identify signals diffracted by point masses, highlighting its flexibility for searches. By simulating thousands of lensed and nonlensed events, we determine how the detectability changes with different source properties. DINGO-lensing provides a scalable and efficient avenue for identifying and characterizing gravitationally lensed GW events in the upcoming observing runs.

gr-qc↗

arXiv2025

Dark Matter Subhalos and Higher Order Catastrophes in Gravitational Wave Lensing

Gravitational lensing is an invaluable probe of the nature of dark matter, and the structures it forms. Lensed gravitational waves in particular allow for unparalleled sensitivity to small scale structures within the lenses, due to the precise time resolution in combination with the continuous monitoring of the entire sky. In this work, we show two distinct ways of using strongly lensed gravitational waves to identify the presence of dark matter subhalos: \emph{{i)}} through higher order caustics generating high relative magnification ($\mu_r > 2$), short time delay image pairs that break the caustic universality relations of single dark matter halos, which occur for $\sim 1-10$ percent of strongly lensed events in our cold dark matter models, and \emph{ii)} through the presence of more than three highly magnified images, which occur for $\sim 0.01-1$ percent of the same simulated events. We find that these results are highly sensitive to the concentrations of subhalos in our simulations, and more mildly to their number densities. The presence of low-mass subhalos increases the probability of observing wave-optics lensing in lensed gravitational waves, which is studied by solving the diffraction integral with the stationary phase approximation, as well as numerically. We also report distinct quantitative and qualitative differences in the distributions of relative magnifications and time delays for subhalo populations with increased number densities or concentrations. With the upcoming detection of strongly lensed events by ground- and space- based detectors, comparisons against these simulated distributions will provide insight into the nature of dark matter.

astro-ph.CO↗

arXiv2025

Diffraction around caustics in gravitational wave lensing

Gravitational lensing magnification is maximal around caustics. At these source locations, an incoming wave from a point source would formally experience an infinite amplification in the high-frequency or geometric optics limit. This divergence reflects the break-down of the mathematical formalism, which is regularized by either the finite size of the source or its wavelength. We explore diffraction around caustics and their implications for the distortion of waveforms from point sources, focusing on three types of caustics: point singularities, folds, and cusps. We derive analytical results for the amplitude and phase of the diffracted waves, and compare those against the stationary phase approximation. We then study the observational signatures and detectability of these distortions on gravitational waves. We find that the lensing distortions could be detectable, but that the stationary phase approximation is still a good description of the system even close to the caustic, when the repeated gravitational wave chirps interfere with each other. We also quantify the possibility of distinguishing lensed signals from different caustics by performing Bayesian parameter estimation on simulated signals. Our results demonstrate that the universal distortions due to diffraction around caustics could be used to single out a gravitational wave event as lensed.

gr-qc↗

arXiv2025

Multi-messenger Gravitational Lensing

We introduce the rapidly emerging field of multi-messenger gravitational lensing - the discovery and science of gravitationally lensed phenomena in the distant universe through the combination of multiple messengers. This is framed by gravitational lensing phenomenology that has grown since the first discoveries in the 20th century, messengers that span 30 orders of magnitude in energy from high energy neutrinos to gravitational waves, and powerful "survey facilities" that are capable of continually scanning the sky for transient and variable sources. Within this context, the main focus is on discoveries and science that are feasible in the next 5-10 years with current and imminent technology including the LIGO-Virgo-KAGRA network of gravitational wave detectors, the Vera C. Rubin Observatory, and contemporaneous gamma/X-ray satellites and radio surveys. The scientific impact of even one multi-messenger gravitational lensing discovery will be transformational and reach across fundamental physics, cosmology and astrophysics. We describe these scientific opportunities and the key challenges along the path to achieving them. This article is the introduction to the Theme Issue of the Philosophical Transactions of The Royal Society A on the topic of Multi-messenger Gravitational Lensing, and describes the consensus that emerged at the associated Theo Murphy Discussion Meeting in March 2024.

astro-ph.HE↗

arXiv2025

Lensing and wave optics in the strong field of a black hole

Gravitational waves (GWs) are lensed by matter, offering a unique probe of both the large-scale structure of the Universe and the fundamental properties of GW propagation. GWs can also be affected by wave optics effects when their wavelength is comparable to the size of the lens. While this regime has been well studied in the Newtonian approximation, the role of strong gravitational fields remains largely unexplored. This is particularly relevant for lensing by intermediate and supermassive black holes (BHs), which can occur near active galactic nuclei or in compact triple systems. In this work, we analyze the lensing of GWs by a non-rotating BH and compare our results to the Newtonian point-mass approximation. We construct frequency-dependent amplification factors that incorporate strong-field effects, revealing explicit polarization mixing and absorption by the event horizon. Using a fiducial GW event, we explore key phenomenological signatures of BH lensing, highlighting new observational opportunities to probe strong gravitational fields through GW lensing.

gr-qc↗

arXiv2025

Constraining Proper Motion of Strongly Lensed Eccentric Binary Mergers using Doppler Triangulation

Strong lensing of gravitational wave (GW) sources allows the observer to see the GW source from different lines-of-sight (LOS) through the corresponding images, which provides a way for constraining the relative proper motion of the GW source. This is possible as the GW signals received from each image will have slightly different projected velocity components, from which one can `Doppler-Triangulate' for the GW source velocity vector. The difference in projected velocity between the different images can be observationally inferred through pairwise GW phase measurements that accumulate over the time-of-observation. In this paper we study lensed eccentric GW sources and explore how the observable GW phase shift between images evolve as a function of time, eccentricity, lens- and binary parameters. Next generation GW observatories, including the Einstein Telescope and Cosmic Explorer, will see $\sim $hundreds/year of lensed GW sources, where a significant fraction of these are expected to be eccentric. We discuss the expected unique observables for such eccentric lensed GW sources, and the relation to their observable relative linear motion, which otherwise is exceedingly difficult to constrain in general.

astro-ph.HE↗

arXiv2025

Effects of Galaxy Cluster Structure on Lensed Gravitational Waves

Strong gravitational lenses come in many forms, but are typically divided into two populations: galaxies, and groups and clusters of galaxies. The largest objects in the Universe (i.e. galaxy clusters) are highly irregular and composed of many components due to a history of (or active) hierarchical mergers. In this work, we analyze the discrepancies in the observables of strongly lensed gravitational wave transients in both scenarios, namely relative magnifications, time delays, and image multiplicities. We compare the detection rates between the single spherical dark matter halo models found in the literature, and publicly available state-of-the-art cluster lens models. We find there to be approximately an order of magnitude fewer detection of strongly lensed transients in the realistic model case, likely caused by their loss of overall strong lensing optical depth. We also report detection rates in the weak lensing or single-image regime. Additionally, we find a systemic shift towards lower time delays between the brightest image pairs in the cases of the realistic models, as well as higher fractions of positive versus negative parity images, which was previously reported in the literature. This deviation in the joint relative magnification factor-time delay distribution will hinder the feasibility of the reconstruction of cluster-scale lenses through gravitational wave transients alone, but can still provide a lower limit on the lens mass.

astro-ph.CO↗

arXiv2024

Measuring the Transverse Velocity of Strongly Lensed Gravitational Wave Sources with Ground Based Detectors

Observations of strongly gravitationally lensed gravitational wave (GW) sources provide a unique opportunity for constraining their transverse motion, which otherwise is exceedingly hard for GW mergers in general. Strong lensing makes this possible when two or more images of the lensed GW source are observed, as each image essentially allows the observer to see the GW source from different directional lines-of-sight. If the GW source is moving relative to the lens and observer, the observed GW signal from one image will therefore generally appear blue- or redshifted compared to GW signal from the other image. This velocity induced differential Doppler shift gives rise to an observable GW phase shift between the GW signals from the different images, which provides a rare glimpse into the relative motion of GW sources and their host environment across redshift. We illustrate that detecting such GW phase shifts is within reach of next-generation ground-based detectors such as Einstein Telescope, that is expected to detect $\sim$hundreds of lensed GW mergers per year. This opens up completely new ways of inferring the environment of GW sources, as well as studying cosmological velocity flows across redshift.

astro-ph.HE↗

arXiv2024

What is the nature of GW230529? An exploration of the gravitational lensing hypothesis

On the 29th of May 2023, 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 $M_\odot$ 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, disfavoring this option. Moreover, when the magnification is assumed to be strong enough to bring the mass of the heavier binary component below the standard limits on neutron star masses, we find high probability for the lighter object to have a sub-solar mass, making the binary even more exotic than a mass-gap neutron star-black hole system. Even when the secondary is not sub-solar, 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.

gr-qc↗

arXiv2024

Observational Signatures of Highly Magnified Gravitational Waves from Compact Binary Coalescence

Gravitational lensing has empowered telescopes to discover astronomical objects that are otherwise out of reach without being highly magnified by foreground structures. While we expect gravitational waves (GWs) from compact binary coalescences to also experience lensing, the phenomenology of highly magnified GWs has not been fully exploited. In this Letter, we fill this gap and explore the observational signatures of these highly magnified GWs. We find that these signatures are robust against modeling details and can be used as smoking-gun evidence to confirm the detection of lensing of GWs without any electromagnetic observation. Additionally, diffraction becomes important in some cases, which limits the maximum possible magnification and gives waveform signatures of lensing that can only be observed by GW detectors. Even with current-generation observatories, we are already sensitive to these rare, highly magnified GWs and could use them to probe the high-redshift Universe beyond the usual horizon.

gr-qc↗

arXiv2023

Mapping a lower limit on the mass fraction of the cold neutral medium using Fourier transformed HI 21cm emission line spectra: Application to the DRAO Deep Field from DHIGLS and the HI4PI survey

We develop a new method for spatially mapping a lower limit on the mass fraction of the cold neutral medium by analyzing the amplitude structure of $\hat T_b(k_v)$, the Fourier transform of $T_b(v)$, the spectrum of the brightness temperature of HI 21cm line emission with respect to the radial velocity $v$. This advances a broader effort exploiting 21cm emission line data alone (without absorption line data, $τ$) to extract integrated properties of the multiphase structure of the HI gas and to map each phase separately. Using toy models, we illustrate the origin of interference patterns seen in $\hat T_b(k_v)$. Building on this, a lower limit on the cold gas mass fraction is obtained from the amplitude of $\hat T_b$ at high $k_v$. Tested on a numerical simulation of thermally bi-stable turbulence, the lower limit from this method has a strong linear correlation with the "true" cold gas mass fraction from the simulation for relatively low cold gas mass fraction. At higher mass fraction, our lower limit is lower than the "true" value, because of a combination of interference and opacity effects. Comparison with absorption surveys shows a similar behavior, with a departure from linear correlation at $N_{\rm HI}\gtrsim 3-5\times10^{20}$ cm$^{-2}$. Application to the DRAO Deep Field (DF) from DHIGLS reveals a complex network of cold filaments in the Spider, an important structural property of the thermal condensation of the HI gas. Application to the HI4PI survey in the velocity range $-90 < v < 90$ km/s produces a full sky map of a lower limit on the mass fraction of the cold neutral medium at 16'.2 resolution. Our new method has the ability to extract a lower limit on the cold gas mass fraction for massive amounts of emission line data alone with low computing time and memory, pointing the way to new approaches suitable for the new generation of radio interferometers.

astro-ph.GA↗

arXiv2023

Efficient survey design for finding high-redshift galaxies with JWST

Several large JWST blank field observing programs have not yet discovered the first galaxies expected to form at $15 \leq z \leq 20$. This has motivated the search for more effective survey strategies that will be able to effectively probe this redshift range. Here, we explore the use of gravitationally lensed cluster fields, that have historically been the most effective discovery tool with HST. In this paper, we analyze the effectiveness of the most massive galaxy clusters that provide the highest median magnification factor within a single JWST NIRCam module in uncovering this population. The results of exploiting these lensing clusters to break the $z > 15$ barrier are compared against the results from large area, blank field surveys such as JADES and CEERS in order to determine the most effective survey strategy for JWST. We report that the fields containing massive foreground galaxy clusters specifically chosen to occupy the largest fraction of a single NIRCam module with high magnification factors in the source plane, whilst containing all multiple images in the image plane within a single module provide the highest probability of both probing the $15 \leq z \leq 20$ regime, as well as discovering the highest redshift galaxy possible with JWST. We also find that using multiple massive clusters in exchange for shallower survey depths is a more time efficient method of probing the $z > 15$ regime.

astro-ph.GA↗

arXiv2023

Mapping the multiphase structure of H I in the Low-Latitude Intermediate-Velocity Arch 1

We have analyzed the thermal and turbulent properties of the Low-Latitude Intermediate-Velocity Arch 1 (LLIV1). This was accomplished using archival H I emission and absorption data from two 21,cm line surveys: GHIGLS at $9.^\prime$4 resolution and DHIGLS at $1^\prime$ resolution. The spectral decomposition code $\tt{ROHSA}$ was used to model the column density of different thermal phases and also to analyze an absorption measurement against the radio source 4C~+66.09. From the latter we found spin temperature $T_{\mathrm{s}} \sim 75$K, cold gas mass fraction $f\sim0.5$, and turbulent sonic Mach number $M_t\sim3.4$. Similar to the absorption line modeling against 4C~+66.09, our best emission line decomposition model has no unstable gas across the whole field of view, suggesting that the thermal condensation and phase transition are not on-going but rather have reached an equilibrium state. The cold phase of LLIV1 appears as a collection of elongated filaments that forms a closed structure within the field decomposed. These substructures follow the orientation of the overall large scale cloud, along the diagonal of the GHIGLS field from north-west to south-east (in Galactic coordinates). The angular power spectrum of the cold phase is slightly shallower than that of the warm phase, quantifying that the cold phases have relatively more structure on small scales. Our spatially resolved map of the cold gas mass fraction in LLIV1 from DHIGLS reveals significant variations spanning the possible range of $f$, with mean and standard deviation 0.33 and 0.19, respectively.

astro-ph.GA↗

arXiv2022

Mapping the Thermal Condensation of Diffuse H I in the North Celestial Pole Loop

The North Celestial Pole Loop (NCPL) provides a unique laboratory for studying the early stages of star formation, in particular the condensation of the neutral interstellar medium (ISM). Understanding the physical properties that control the evolution of its contents is key to uncovering the origin of the NCPL. Archival data from the NCPL region of the GHIGLS 21 cm line survey (9'4) are used to map its multiphase content with ${\tt ROHSA}$, a Gaussian decomposition tool that includes spatial regularization. Column density and mass fraction maps of each phase were extracted along with their uncertainties. Archival data from the DHIGLS 21 cm (1') survey are used to further probe the multiphase content of the NCPL. We have identified four spatially (and dynamically) coherent components in the NCPL, one of which is a remarkably well-defined arch moving at about $14\ {\rm km s^{-1}}$ away from us that could be a relic of the large-scale organized dynamical process at the origin of the phase transition. The cold and lukewarm phases together dominate the mass content of the neutral gas along the loop. Using absorption measurements, we find that the cold phase exhibits slightly supersonic turbulence.

astro-ph.GA↗