Opening
As gravitational wave astronomy matures, the next frontier lies in catching the rare events where multiple cosmic messengers align—gravitational waves, gamma rays, and light all arriving from the same cataclysm. A new study explores an even more exotic scenario: what happens when gravity itself bends these signals, creating multiple images of the same neutron star merger through gravitational lensing. The authors estimate detection rates for these "strongly lensed" multi-messenger signals from binary neutron star mergers using next-generation observatories, revealing both promising opportunities and sobering challenges for the field.
What they found
Chen, Lu, and Zeng modeled the detectability of lensed electromagnetic counterparts—short gamma-ray bursts (sGRBs), kilonovae, and afterglows—associated with gravitational wave events from binary neutron star mergers observed by future detectors like Cosmic Explorer and Einstein Telescope.
Their results paint a nuanced picture. For sGRB prompt emission, even gamma-ray telescopes with sensitivities ten times better than Fermi-GBM would detect lensed events at only ~0.1 yr⁻¹, corresponding to just ~2×10⁻³ of detectable lensed gravitational wave events. This dramatic deficit reflects the inherent rarity of lensing combined with the narrow jets of gamma-ray bursts.
The authors propose a complementary strategy: targeted follow-up observations of known galaxy-scale lens candidates within gravitational wave localization regions. This approach yields identifiable lensed-host fractions of approximately 0.15–0.30, suggesting a possible gain in sensitivity for fainter kilonovae and afterglows compared to untargeted searches.
For near-infrared observations with an RST-like facility, lensed kilonovae become more accessible, with predicted detection rates of ~0.45⁺⁰·⁸¹₋₀.₃₄ yr⁻¹ in the F106 band, 0.55⁺⁰·⁹⁸₋₀.₄₁ yr⁻¹ in F158, and 0.078⁺⁰·¹³⁹₋₀.₀₅₉ yr⁻¹ in F213. Optical and radio afterglows remain challenging, though ATHENA-like X-ray observations may detect 0.5–5 events over ten years.
Why it matters
Strongly lensed multi-messenger events offer a unique laboratory. Multiple images of the same merger, arriving at different times, can constrain the mass of the graviton and refine cosmological parameters—tests impossible with unlensed events alone. The study demonstrates that while these detections will be rare, they are not implausible with next-generation facilities, making them a worthwhile target for coordinated observational campaigns.
What's next
The authors highlight that success requires careful coordination between gravitational wave networks and electromagnetic follow-up teams. The pointed strategy targeting known lenses emerges as particularly promising, suggesting that pre-computed lens catalogs should be integrated into rapid-response protocols. Future work will likely refine these predictions as detector sensitivities improve and our understanding of neutron star merger physics deepens.
Starithm continuously monitors gravitational wave alerts and associated multi-messenger candidates—the exact events this research aims to optimize for detection.