Multi-messenger Gravitational Lensing

For the first detection of a novel astrophysical phenomenon, scientific standards are particularly high. Especially in a multi-messenger context, there are also opportunity costs to follow-up observations on any detection claims. So, in searching for the still elusive lensed gravitational waves, care needs to be taken in controlling false positives (also known as false alarms). In particular, many methods for identifying strong lensing rely on some form of parameter similarity or waveform consistency, which under rapidly growing catalogue sizes exposes them to false alarms from coincident but unlensed events. And searches for waveform deformations in all lensing regimes are subject to degeneracies between lensing, intrinsic parameters, insufficiently modelled effects such as orbital eccentricity, or even deviations from General Relativity. In this talk, Dr Keitel will review these main sources of possible false positives in gravitational-wave lensing searches and the main approaches the community is pursuing to control them. The speaker will also highlight the flip-side of any hypothesis test: false negatives (or false dismissals), which we also need to control at the same time so as not to miss out on exciting science opportunities.

Dr David Keitel, University of the Balearic Islands, Spain

Dr David Keitel, University of the Balearic Islands, Spain

David Keitel focuses his research on gravitational waves (GWs) from neutron stars and black holes. A LIGO Scientific Collaboration (LSC) member since 2011, his work covers large-scale data analysis and the modelling and interpretation of astrophysical GW sources. After studying in Germany (diploma from Bonn University in 2010 and doctorate in 2014 from Leibniz University Hannover), he has worked as a postdoc at AEI Hannover, the University of the Balearic Islands (UIB), Glasgow, and Portsmouth. Since 2020 he is a tenure-track faculty member at UIB. He has leading roles in various LSC activities, including in the first searches for binary neutron star merger remnants, gravitationally lensed GWs, and GWs from glitching pulsars, as well as co-chair of the CW working group (since 2022) and coordinator of public outreach science summaries.

 

Gravitational wave (GW) error regions are large, typically 10s to 1000s of square degrees. Within this area there will be a number of electromagnetic transient sources that are detected serendipitously and that may not necessarily be related to the GW. Understanding how the light from these false positives evolves with time is important to rapidly confirm or rule out their association with the GW trigger. In this talk, Dr Oates will discuss the process of searching for the electromagnetic counterpart of GW events, what they hope to find, what they find instead and how false positives are ruled out. The author will also discuss how false positives are themselves of interest, even though they are contaminants to the GW community.

Dr Samantha Oates, Lancaster University, UK

Dr Samantha Oates, Lancaster University, UK

Sam is a lecturer at Lancaster University. She obtained her PhD in 2010 from MSSL-UCL. She went on to be a Juan de la Cierva Fellow at the IAA-CSIC in Granada, Spain; a Leverhulme Early Career Fellow at the University of Warwick and a Prize Postdoctoral Research Fellow in Gravitational Wave Astronomy at the University of Birmingham. She joined Lancaster in 2023. Her research interests cover a variety of explosive transients, including Gamma-ray bursts and the electromagnetic follow-up of gravitational wave sources. Sam is particularly interested in studying these transients in the optical/UV and is an active member of the Swift/UVOT team. 

The Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) is scheduled to begin in 2025 and will run for 10 years. It is a time-domain survey that will cover 18,000 square degrees of the sky using 6 optical filters. This survey will contribute significantly to the field of strong lensing, increasing the number of known galaxy-scale lenses by two orders of magnitude. The Strong Lensing Science Collaboration and the Dark Energy Science Collaboration's Strong Lensing Topical Team are working together to prepare the community for this groundbreaking dataset. They are setting up discovery and analysis pipelines and organizing follow-up resources. Recently, two task forces have been formed to prepare the community with discovery pipelines for lenses with static and transient sources, respectively, before the real data arrives in early 2026. The task forces are working on developing a realistic simulation pipeline, called the SLSim (Strong Lensing Simulation) pipeline. This pipeline will provide training and test datasets for machine learning algorithms for lens searches and help characterize the selection function of the discovered samples with these algorithms.

Anowar Shajib, University of Chicago, USA

Anowar Shajib, University of Chicago, USA

Anowar Shajib is an NHFP Einstein Fellow at the University of Chicago. He completed his PhD in 2020 from UCLA. He primarily works with galaxy-galaxy strong lenses and strongly lensed quasar/supernova systems to measure cosmological parameters such as the Hubble constant, and to study properties of elliptical galaxies at the intermediate redshift (z∼0.5). He is currently the co-convener of the Strong Lensing Topical Team within the Rubin Observatory LSST’s Dark Energy Science Collaboration. He is also a co-PI of the STRIDES collaboration that aims to discover strongly lensed quasars from the Dark Energy Survey and other large surveys.

Finding black holes: an unconventional multi-messenger route
Laura Uronen, The Chinese University of Hong Kong, Hong Kong
 
If, and when, lensed gravitational waves are detected by the LVK observatories, we are faced with a wealth of new science avenues. From tests of General Relativity to probing new cosmology, lensed gravitational waves have a variety of applications, and one of these is the localisation of black holes. Unseen in the dark Universe, black hole localisation is an elusive aspect of gravitational-wave physics. If these are detected, how would we aim to pin-point the home of these cosmic cataclysms? It has been suggested that both binary black holes and neutron stars can be located through a variety of means, from archival searches to identification of an electromagnetic counterpart. This talk will cover the process through which we can combine gravitational-wave and electromagnetic data in localising lensed binary black holes. For strongly-lensed gravitational waves, it is possible to observe lensed galaxies within the sky region obtained from the LVK, and through separate reconstruction of the lens both in the GW and EM regimes the host galaxy may be identified. The author, together with other researchers, further narrows down the location of the BBH within its host, as shown through a simulation completed using lenstronomy and GOLUM joint parameter estimation. This multi-pronged, multi-messenger reconstruction can provide a novel approach to determining the location of the black hole binary. 

GausSN: Bayesian time-delay estimation for strongly lensed supernovae
Erin Hayes, University of Cambridge, UK

The author presents GausSN, a Bayesian semi-parametric Gaussian Process (GP) model, for time-delay estimation with resolved gravitationally lensed supernovae (glSNe). GausSN models the underlying light curve non-parametrically using a GP. Without assuming a template light curve, GausSN fits for the time delays of all images using data in any number of filters simultaneously. The author, together with other researchers, introduces a novel time-varying magnification model to capture the effects of microlensing alongside time-delay estimation. In this analysis, the scientists model the microlensing term as both a constant, for comparison to existing approaches, and a sigmoid function. They demonstrate that GausSN provides robust time-delay estimates for simulations from the Nancy Grace Roman Space Telescope and the Vera C. Rubin Observatory's Legacy Survey of Space and Time (Rubin-LSST). The scientists find that up to 43.6% of time-delay estimates from Roman and 52.9% from Rubin-LSST have fractional errors of less than 5%. They then apply GausSN to SN Refsdal and find the time delay for the fifth image is consistent with the original analysis, regardless of microlensing treatment. GausSN maintains the precision and accuracy achieved by existing methods with fewer assumptions about the underlying shape of the light curve than template-based approaches, while incorporating microlensing into the statistical error budget.

Lensed gamma-ray bursts offer several key advantages over other lensed sources. In particular, their short duration enables precision timing of the lens to a degree not possible with quasars or even supernovae, and their short duration can plausibly probe comparably short delay times. There are now samples of ~10,000 GRBs which can be searched for lensing signals. However, most of these are poorly localised on the sky, so the actual lens cannot be identified. The search is then for identical GRBs with overlapping sky localisations but offsets in time. Several plausible examples have been found, but none are entirely compelling. In this talk, the author will review past searches for lensed GRBs and consider how to optimise future searches to improve the chances of robust discoveries. 

Professor Andrew Levan, Radboud University, The Netherlands

Professor Andrew Levan, Radboud University, The Netherlands

Andrew Levan is an observational astronomer interested in the many ways stars end their lives through explosions, implosions and disruptions. He has worked extensively on studies of gamma-ray bursts, supernovae, tidal disruption events and the electromagnetic counterparts of gravitational wave sources. Highlights from his work include the identification of the most distant gamma-ray bursts in the early Universe, the discovery of new populations of transients such as relativistic tidal disruption events and ultra-long duration gamma-ray bursts, the discovery of kilonovae powered by the production of heavy elements, and, most recently, leading the first campaigns of observations of gamma-ray bursts with the James Webb Space Telescope.

Fast Radio Bursts (FRBs) are enigmatic millisecond-duration radio transients that have been sparking curiosity in the scientific community since their recent discovery in 2007. The advent of numerous facilities conducting dedicated FRB searches has dramatically revolutionised the field: hundreds of new bursts have been detected, and some are now known to repeat. 

Using interferometry, it is now possible to localise FRBs to their host galaxies, opening up new avenues for using FRBs as astrophysical probes. One promising application is studying gravitationally lensed FRBs. As the radio waves of an FRB are deflected by a gravitational lens, they traverse different travel paths, and thus arrive with different time delays. 

In this talk, Inés Pastor-Marazuela will discuss the prospects for detecting gravitationally lensed FRBs with current and future FRB facilities. The author will outline the requirements for identifying a lensed FRB, taking into account their propagation effects and the importance of voltage data. Next, the author will explore the different lens masses that could be probed with FRBs throughout an FRB survey, from stellar masses to individual galaxies. She will distinguish the potential for discovery with repeating and non-repeating FRBs, and finally, she will highlight the unique cosmological applications of gravitationally lensed FRBs.

Inés Pastor-Marazuela, University of Manchester, UK

Inés Pastor-Marazuela, University of Manchester, UK

Inés is a Spanish researcher from Segovia. She got her bachelor’s and master’s degree in astrophysics at IRAP in Toulouse, France, where she worked on looking for X-ray transients in XMM-Newton data with Natalie Webb. She moved to the University of Amsterdam, the Netherlands, where she obtained her PhD in astrophysics in November 2022, titled 'Exploring the link between Neutron Stars and Fast Radio Bursts', supervised by Joeri van Leeuwen. In her research, she used data from Apertif and LOFAR to look for Fast Radio Bursts (FRBs) and try to understand their origin. She is now a Rubicon Research Fellow at the University of Manchester, where she works with Ben Stappers and keeps looking for FRBs using the MeerKAT radio telescope, in South Africa. 

On the feasibility of primordial black hole dark matter abundance constraints using lensing parallax of Gamma Ray Bursts
Dr Priyanka Gawade, Inter-University Centre for Astronomy and Astrophysics, India 

Primordial black holes, which could have formed during the early Universe through overdensities in primordial density fluctuations during inflation, are potential candidates for dark matter. The author, together with other scientists, explores the use of lensing parallax of Gamma ray bursts (GRBs), which results in different fluxes being observed from two different vantage points, in order to probe the abundance of primordial black holes in the unexplored window within the mass range $[10^{-15}-10^{-11}]M_\odot$. The scientists derive the optical depth for the lensing of GRBs with a distribution of source properties and realistic detector sensitivities. They comment on the ability of the proposed Indian twin satellite mission Daksha in its low earth orbit to conduct this experiment. Daksha is primarily designed for detecting electromagnetic counterparts of gravitational waves. If the two Daksha satellites observe 10000 GRBs simultaneously and the entirety of dark matter is made up of $[10^{-15}-10^{-12}]M_\odot$ black holes, Daksha will detect non-zero lensing events with a probability ranging from 80% to 50%. Non-detections will not conclusively rule out primordial black holes as dark matter in this mass range. However, the scientists show that meaningful constraints can be obtained in such a case if the two satellites are separated by at least the Earth-Moon distance.

Strongly lensed gravitational wave searches: insights from LVK O3 analysis and ongoing efforts
Srashti Goyal, Max Planck Institute for Gravitational Physics, Germany
 
No strongly lensed gravitational waves have been confidently detected to date. This presentation will outline the ongoing efforts in the search for strongly lensed gravitational waves, drawing insights from the LVK O3 analysis, which includes not only super-threshold events but also sub-threshold ones. In this talk, Srashti Goyal will discuss rapid analysis methods such as machine learning (LensID) and Bayesian model selection (posterior overlap) and delve into the challenges faced by ground-based detectors while emphasising the role of lens and source modelling. Additionally, the author will mention the follow-up strategies of O3 candidates to identify lensed host galaxies through cross-matching with electromagnetic catalogues, underlining the importance and challenges of a multi-messenger approach. If time permits, the author will briefly touch upon strategies for analysing triplets of lensed events and discuss their implications. 

With an unprecedented combination of depth and large field of view, Rubin Observatory will dramatically increase the potential for serendipitous detection of MMA counterparts while the implementation of a Target of Opportunity program will enable targeted searches for counterparts of MMA triggers. The speaker will review the Rubin Observatory's observational capabilities and current plans for contributions to MMA astronomy. 

Dr Federica Bianco, University of Delaware, USA

Dr Federica Bianco, University of Delaware, USA

Federica Bianco is an Associate Professor at the University of Delaware in the Departments of Physics and Astronomy and in the Biden School of Public Policy and Administration and a Resident Faculty in the University of Delaware Data Science Institute. She is Rubin Observatory Construction's Deputy Project Scientist and the chair of the Rubin LSST Transients and Variable Stars Science Collaboration. She works on data-driven solutions to problems that span from the nature of explosions in the sky to urban sustainability. She studies lightcurves, time series of light, in astronomy to understand stellar evolution and cosmology, and in the urban environment at the Urban Observatory, where the study of urban lightcurves enables sociological, ecologic, and economic inference. She is a TED 2019 fellow.