🔭 astrophysics

Prospects for optical detections from binary neutron star mergers with the next-generation multi-messenger observatories

This study forecasts that the Einstein Telescope, particularly when networked with current detectors, will enable the Vera Rubin Observatory to detect tens to hundreds of kilonovae annually from binary neutron star mergers, with detection rates primarily limited by uncertainties in the local merger rate rather than neutron star microphysics.

Original authors: E. Loffredo, N. Hazra, U. Dupletsa, M. Branchesi, S. Ronchini, F. Santoliquido, A. Perego, B. Banerjee, S. Bisero, G. Ricigliano, S. Vergani, I. Andreoni, M. Cantiello, J. Harms, M. Mapelli, G. Oganes

Published 2026-06-05

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: E. Loffredo, N. Hazra, U. Dupletsa, M. Branchesi, S. Ronchini, F. Santoliquido, A. Perego, B. Banerjee, S. Bisero, G. Ricigliano, S. Vergani, I. Andreoni, M. Cantiello, J. Harms, M. Mapelli, G. Oganesyan

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a vast, dark ocean. For a long time, we've only been able to "hear" the ripples in this ocean caused by massive collisions, like two giant whirlpools smashing together. These ripples are gravitational waves, detected by giant instruments on Earth. But for a long time, we couldn't "see" what was happening at the center of the crash.

This paper is like a blueprint for a new, super-powered lighthouse system that will finally let us see the fireworks of these cosmic collisions.

Here is the story of the paper, broken down into simple parts:

1. The Players: The Ears and the Eyes

The Ears (Gravitational Wave Detectors): The paper focuses on the next generation of "ears," specifically a massive new observatory called the Einstein Telescope (ET). Think of the current detectors (like LIGO) as a single person trying to hear a whisper in a noisy room. The Einstein Telescope is like a super-sensitive microphone that can hear whispers from across the entire history of the universe. It can come in two shapes: a giant triangle or an "L" shape.
The Eyes (The Rubin Observatory): This is a new, incredibly powerful camera on a mountain in Chile. It has a very wide field of view, like a fish-eye lens that can snap a photo of a huge chunk of the sky in seconds. It's designed to spot faint, fleeting flashes of light.

2. The Event: The Cosmic Crash

When two Neutron Stars (the incredibly dense, city-sized cores of dead stars) crash into each other, they create two things:

A Sound: A gravitational wave ripple.
A Flash: A "Kilonova." This is a burst of light caused by the explosion of heavy elements (like gold and platinum) being forged in the crash. It's like a cosmic firework that fades away in a few days.

3. The Problem: Finding a Needle in a Haystack

When the current "ears" hear a crash, they often can't tell us exactly where in the sky it happened. It's like hearing a car crash in a city but only knowing it happened somewhere in a 100-square-mile area. If you have a camera that can only look at one square mile at a time, you might miss the crash entirely.

The new Einstein Telescope will be so good at "hearing" that it can narrow down the location much better. However, the sky is still huge. The paper asks: If we have a super-ear and a super-eye, how many of these crashes can we actually catch and photograph together?

4. The Simulation: Running a Cosmic Weather Forecast

The authors didn't wait for the telescopes to be built. Instead, they built a massive computer simulation.

They created a "fake universe" with thousands of neutron star collisions over 10 years.
They tested different rules for how these stars behave (like how heavy they are or how squishy their insides are).
They simulated the Einstein Telescope listening alone, or listening in a team with other detectors (like the current LIGO/Virgo or even bigger future ones called Cosmic Explorers).
They simulated the Rubin Observatory trying to take pictures of these events, accounting for real-world issues like day/night cycles and how long it takes the telescope to turn around.

5. The Results: A Golden Age of Discovery

The simulation showed some exciting numbers:

Solo Performance: Even if the Einstein Telescope works alone, the Rubin Observatory could spot 10 to 100 of these light flashes (kilonovas) every year.
Teamwork: If the Einstein Telescope works in a network with other detectors, the number of successful catches jumps by about 10 times.
The "Sweet Spot": The most important factor isn't just how powerful the telescope is, but how well we know where to look. The better the "ears" can pinpoint the location, the more likely the "eyes" are to catch the flash.

6. The Uncertainties: The Fog of War

The paper also admits that we don't know everything yet.

The Rate: We aren't sure exactly how often these crashes happen in our local neighborhood. If they happen more often than we think, we'll see more flashes. If they are rarer, we'll see fewer.
The Physics: We don't know the exact "recipe" for neutron stars. The paper tested two different "recipes" (called Equations of State). One recipe makes the stars slightly bigger and puffier, leading to brighter flashes. The other makes them smaller and denser, leading to fainter flashes. The results change slightly depending on which recipe is correct, but the overall conclusion remains the same: we will see a lot of them.

The Bottom Line

This paper is a promise of what's coming. It says that when the next generation of "ears" (Einstein Telescope) and "eyes" (Rubin Observatory) come online around the mid-2030s, we will move from hearing a few rare whispers to watching a constant, spectacular show of cosmic explosions. We will be able to catch these events often enough to study them in detail, helping us understand how the universe creates heavy elements and how space and time work.

In short: We are about to get a pair of super-hearing aids and a pair of super-glasses that will let us watch the universe's most violent and beautiful fireworks show, night after night.

Technical Summary: Prospects for optical detections from binary neutron star mergers with the next-generation multi-messenger observatories

Problem Statement
While the detection of the binary neutron star (BNS) merger GW170817 and its optical counterpart AT2017gfo established the multi-messenger nature of these events, such detections remain rare. Current gravitational wave (GW) detectors (LIGO, Virgo, KAGRA) are limited in their sky localization capabilities, often resulting in large error regions that hinder efficient optical follow-up. Furthermore, existing models for kilonova (KN) emissions are largely calibrated to the properties of GW170817, potentially failing to capture the diversity of BNS systems, such as the high-mass system GW190425 or systems undergoing prompt collapse to black holes.

With the advent of next-generation GW observatories like the Einstein Telescope (ET) and Cosmic Explorer (CE), detection rates are expected to increase by orders of magnitude, extending the observable horizon to redshifts $z \sim 4$ (ET) and $z \sim 10$ (CE). Concurrently, the Vera Rubin Observatory (LSST) is set to begin operations with unprecedented wide-field sensitivity. The central problem addressed by this study is to assess the feasibility and rates of detecting optical counterparts (KNe and short GRB afterglows) from BNS mergers using these next-generation facilities, while accounting for uncertainties in neutron star (NS) population properties, equations of state (EOS), and observational strategies.

Methodology
The authors employed a comprehensive simulation framework integrating population synthesis, GW signal analysis, and electromagnetic (EM) emission modeling:

BNS Population Synthesis: Using the sevn code, the authors generated catalogues of BNS mergers over a 10-year period (2035–2045). They explored variations in the common envelope efficiency ( $\alpha = 0.5$ and $1.0$), NS mass distributions (Gaussian and Uniform), and two microscopic EOSs (APR4 and BLh). This resulted in eight distinct population sets.
GW Detection and Parameter Estimation: GW signals were injected using the IMRPhenomD_NRTidalv2 waveform. The study simulated various detector networks, including ET alone (in both triangular and 2L configurations) and ET operating in synergy with current detectors (LVKI) and next-generation detectors (Cosmic Explorer). Parameter estimation was performed using the Fisher matrix formalism (GWFish) to determine sky localization uncertainties ( $\Omega_{90}$ ) and other source parameters.
Kilonova and Afterglow Modeling:
- Ejecta Properties: The authors developed improved fitting formulas for ejecta mass and velocity, calibrated on numerical relativity simulations targeting GW190425-like high-mass systems. They explicitly modeled the transition between prompt collapse (PC) and non-prompt collapse scenarios, accounting for the impact of the merger remnant's fate on the dynamical ejecta and disc mass.
- Light Curves: The xkn semi-analytic code was used to generate KN light curves for two-component ejecta (dynamical/spiral-wave winds and secular disc winds), incorporating cosmological corrections and recombination effects.
- Afterglows: Optical afterglows from relativistic jets were modeled using afterglowpy, with the jet production fraction ( $f_j$ ) adjusted to match local short GRB rates for the specific BNS populations.
Observational Strategy: A realistic Target-of-Opportunity (ToO) strategy for the Vera Rubin Observatory was simulated. This included constraints on slewing time, filter changes, night-day cycles, and sky accessibility. Two detection criteria were defined: a "one-epoch" (1ep) strategy and a more conservative "two-epoch" (2ep) strategy requiring detection in multiple filters over consecutive nights to distinguish KNe from contaminants.

Key Contributions

Extended KN Modeling: The study moves beyond the AT2017g70 calibration by incorporating fitting formulas specifically derived for high-mass BNS systems and prompt collapse scenarios. This allows for a more realistic assessment of KN brightness and evolution across the full parameter space of future GW detections.
Network Synergy Analysis: The paper quantifies the specific gains in joint GW-optical detection rates when ET operates in networks with current detectors (LVKI) versus next-generation detectors (CE), analyzing both triangular and 2L ET designs.
Realistic Survey Simulation: Unlike previous studies that assumed idealized conditions, this work integrates realistic Rubin Observatory constraints (duty cycles, slewing times, filter overheads) and sky localization thresholds to calculate the actual observational time required versus the number of detectable events.

Results

Detection Rates: The Einstein Telescope, operating as a single observatory, enables the detection of approximately 10 to 100 KNe per year by the Rubin Observatory, depending on the BNS merger rate and EOS. Operating in a network with current detectors (LVKI) improves this rate by a factor of $\sim 10$ . The inclusion of Cosmic Explorer further enhances the number of well-localized events.
Localization: The 2L configuration of ET generally provides better sky localization than the triangular design when operating alone. However, the presence of a network of detectors (especially current ones) significantly reduces localization uncertainties, increasing the number of events with $\Omega_{90} < 100$ deg $^2$ from hundreds to thousands per year.
Dependence on Physics: The uncertainty in the local BNS merger rate (driven by the common envelope efficiency parameter $\alpha$ ) is the dominant factor affecting detection rate predictions. The choice of NS mass distribution and EOS has a secondary but non-negligible effect; for instance, the BLh EOS generally predicts brighter KNe than APR4 due to larger ejecta masses.
Observational Feasibility: The study finds that a multi-filter, multi-epoch strategy is feasible within a reasonable fraction of Rubin's total observing time (up to 30%). The 2ep strategy, while more conservative, is crucial for identifying the unique color evolution of KNe against background transients.
Afterglow Contribution: The inclusion of short GRB afterglow emission increases the detection probability, particularly for on-axis or near-axis events, where the afterglow can outshine the faintest KNe.

Significance
The paper claims that the synergy between next-generation GW observatories (ET/CE) and the Vera Rubin Observatory will transform the field of multi-messenger astronomy. By enabling the detection of tens to hundreds of KNe annually, this combination will provide a statistically significant sample to constrain the NS equation of state, the BNS merger rate, and the properties of r-process nucleosynthesis. The study emphasizes that while the detection of GW signals will be abundant, the limiting factor for multi-messenger science will be the efficiency of optical follow-up strategies and the accuracy of sky localization, which the proposed network configurations are well-positioned to address. The authors conclude that their improved modeling of high-mass and prompt-collapse systems is essential for accurately predicting the observability of these future events.