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Las Cumbres Observatory Gravitational-Wave Follow-up in O3 and O4: Strengths and Weaknesses of a Rapid Response Galaxy Targeted Strategy

This paper evaluates Las Cumbres Observatory's rapid-response, galaxy-targeted strategy during gravitational-wave observing runs O3 and O4, finding that while the network offers excellent speed and depth for nearby events, its efficiency is significantly limited by larger-than-expected localization areas, necessitating better coordination between wide-field and rapid-response facilities for comprehensive follow-up.

Original authors: Ido Keinan (Tel Aviv University), Iair Arcavi (Tel Aviv University), D. Andrew Howell (Las Cumbres Observatory, University of California Santa Barbara), Curtis McCully (Las Cumbres Observatory), CraigIdo Keinan (Tel Aviv University), Iair Arcavi (Tel Aviv University), D. Andrew Howell (Las Cumbres Observatory, University of California Santa Barbara), Curtis McCully (Las Cumbres Observatory), Craig Pellegrino (NASA Goddard Space Flight Center), Ayelet Hasson (Weizmann Institute of Science), Moira Andrews (Las Cumbres Observatory, University of California Santa Barbara), Jamison Burke (Shady Side Academy), Daichi Hiramatsu (University of Florida, Harvard & Smithsonian, NSF AI Institute for Artificial Intelligence and Fundamental Interactions), Jennifer Barnes (Kavli Institute for Theoretical Physics), Sukanya Chakrabarti (University of Alabama), Joseph R. Farah (Las Cumbres Observatory, University of California Santa Barbara), Paul J. Groot (Radboud University, University of Cape Town, South African Astronomical Observatory, Inter-University Institute for Data Intensive Astronomy), Na'ama Hallakoun (Weizmann Institute of Science), Daniel Holz (University of Chicago), Saurabh W. Jha (Rutgers University), Daniel Kasen (University of California Berkeley), Chris Lidman (Australian National University), Michael J. Lundquist (W. M. Keck Observatory), Dan Maoz (Tel Aviv University), Brian D. Metzger (Flatiron Institute, Columbia University), Ehud Nakar (Tel Aviv University), Megan Newsome (University of Texas at Austin), Yuan Qi Ni (Kavli Institute for Theoretical Physics, Las Cumbres Observatory), Alexander H. Nitz (Syracuse University), Estefania Padilla Gonzalez (Space Telescope Science Institute), Tsvi Piran (Hebrew University of Jerusalem), Dovi Poznanski (Tel Aviv University, California Institute of Technology, Stanford University, Kavli Institute for Particle Astrophysics and Cosmology), Ryan Ridden-Harper (University of Canterbury), David J. Sand (University of Arizona), Brian P. Schmidt (Australian National University, ARC Centre of Excellence for All-sky Astrophysics), Giacomo Terreran (Adler Planetarium), Brad E. Tucker (Australian National University), Stefano Valenti (University of California Davis), J. Craig Wheeler (University of Texas at Austin), Samuel Wyatt (NASA Goddard Space Flight Center), Kathryn Wynn (Las Cumbres Observatory, University of California Santa Barbara)

Published 2026-04-17

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Ido Keinan (Tel Aviv University), Iair Arcavi (Tel Aviv University), D. Andrew Howell (Las Cumbres Observatory, University of California Santa Barbara), Curtis McCully (Las Cumbres Observatory), Craig Pellegrino (NASA Goddard Space Flight Center), Ayelet Hasson (Weizmann Institute of Science), Moira Andrews (Las Cumbres Observatory, University of California Santa Barbara), Jamison Burke (Shady Side Academy), Daichi Hiramatsu (University of Florida, Harvard & Smithsonian, NSF AI Institute for Artificial Intelligence and Fundamental Interactions), Jennifer Barnes (Kavli Institute for Theoretical Physics), Sukanya Chakrabarti (University of Alabama), Joseph R. Farah (Las Cumbres Observatory, University of California Santa Barbara), Paul J. Groot (Radboud University, University of Cape Town, South African Astronomical Observatory, Inter-University Institute for Data Intensive Astronomy), Na'ama Hallakoun (Weizmann Institute of Science), Daniel Holz (University of Chicago), Saurabh W. Jha (Rutgers University), Daniel Kasen (University of California Berkeley), Chris Lidman (Australian National University), Michael J. Lundquist (W. M. Keck Observatory), Dan Maoz (Tel Aviv University), Brian D. Metzger (Flatiron Institute, Columbia University), Ehud Nakar (Tel Aviv University), Megan Newsome (University of Texas at Austin), Yuan Qi Ni (Kavli Institute for Theoretical Physics, Las Cumbres Observatory), Alexander H. Nitz (Syracuse University), Estefania Padilla Gonzalez (Space Telescope Science Institute), Tsvi Piran (Hebrew University of Jerusalem), Dovi Poznanski (Tel Aviv University, California Institute of Technology, Stanford University, Kavli Institute for Particle Astrophysics and Cosmology), Ryan Ridden-Harper (University of Canterbury), David J. Sand (University of Arizona), Brian P. Schmidt (Australian National University, ARC Centre of Excellence for All-sky Astrophysics), Giacomo Terreran (Adler Planetarium), Brad E. Tucker (Australian National University), Stefano Valenti (University of California Davis), J. Craig Wheeler (University of Texas at Austin), Samuel Wyatt (NASA Goddard Space Flight Center), Kathryn Wynn (Las Cumbres Observatory, University of California Santa Barbara)

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 is a giant, dark ocean, and Gravitational Waves (GWs) are like massive ripples caused when two heavy objects (like black holes or neutron stars) crash into each other. For a long time, we could only "hear" these ripples. But in 2017, we finally "saw" the splash: a flash of light called a kilonova that happened when two neutron stars merged. This was a huge deal because it told us exactly where to look and what the crash looked like.

Since then, scientists have been trying to catch these flashes again. This paper is a report card for Las Cumbres Observatory, a global network of telescopes that acts like a rapid-response emergency team. They tried to catch the light from nine different gravitational wave events during two major observation periods (O3 and O4).

Here is the breakdown of their performance, explained simply:

1. The Strategy: "The Neighborhood Search"

When the gravitational wave detectors (LIGO and Virgo) hear a crash, they send out an alert. But the alert is vague. It's like a weather report saying, "There's a storm somewhere in this huge state," rather than "The storm is in this specific town."

To find the light, Las Cumbres used a strategy called "Galaxy Targeting."

The Analogy: Imagine you are looking for a lost dog in a massive city. Instead of driving around every single street randomly, you look at a list of all the houses (galaxies) in the city and decide to check the ones most likely to have the dog based on the dog's size and the neighborhood.
The Plan: Las Cumbres took the "storm area" from the alert, looked at a database of galaxies inside that area, and pointed their telescopes at the most promising ones first.

2. The Good News: They Were Fast and Deep

The team did an excellent job with the speed and power of their search.

Speed: Once they got the alert, they were incredibly fast. On average, they started looking within 3 hours. For some events, they were looking in less than 40 minutes!
- Analogy: If a fire alarm went off, they didn't wait for the fire truck to arrive; they were already grabbing the hose and running out the door.
Depth: Their telescopes are powerful enough to see very faint light. They could spot a flash as bright as the 2017 event (GW170817) from a distance of 250 million light-years (about 770 million light-years is the edge of the observable universe, so this is a very long way).
- Analogy: They had binoculars strong enough to read a street sign from a city away.

3. The Bad News: The "Storm" Was Too Big

Here is where the strategy hit a wall. The "Galaxy Targeting" plan worked great in 2017, but the universe changed the rules.

The Problem: In the recent observation runs (O3 and O4), the gravitational wave detectors got better at hearing the crash, but they got worse at pinpointing exactly where it happened. The "storm area" became massive—sometimes covering thousands of square degrees of the sky.
The Result: The list of "promising houses" (galaxies) grew from a few hundred to tens of thousands.
- Analogy: In 2017, you were looking for a lost dog in one neighborhood. In 2024, the police told you the dog is somewhere in the entire country. Even if you check the top 100 most likely houses, you've only covered a tiny fraction of the country.
The Outcome: Las Cumbres could only check a small percentage of the possible galaxies. For most events, they missed the vast majority of the search area.

4. The Verdict: A Great Team, But Wrong Tools for the Job

The paper concludes that Las Cumbres is a fantastic "rapid response" team. They are fast, they have powerful telescopes, and they are ready to go. However, their specific strategy of checking individual galaxies is no longer efficient because the search areas are too big.

The Solution: We need a mix of tools.
- We need Wide-Angle Cameras (like a security camera on a street corner) that can scan huge chunks of the sky quickly to find the general area.
- We need Deep, Fast Telescopes (like Las Cumbres) to zoom in and take a close look once the wide cameras spot something interesting.

Summary

Think of it like searching for a needle in a haystack.

Las Cumbres has the best tweezers in the world (fast, deep, precise).
The Problem: The haystack got 100 times bigger, and the "needle" (the light) might be anywhere in it.
The Lesson: You can't find the needle just by picking up individual straws one by one anymore. You need a giant vacuum cleaner (wide-field telescopes) to suck up the whole haystack first, and then use the tweezers to find the needle.

The paper argues that for the future, scientists need to combine different types of telescopes working together to catch these cosmic fireworks before they fade away.

1. Problem Statement

Following the historic detection of the binary neutron star (BNS) merger GW170817 and its electromagnetic (EM) counterpart, the astronomical community established strategies to rapidly identify EM counterparts to future gravitational-wave (GW) events. A primary strategy proposed by Gehrels et al. (2016) involves galaxy-targeted follow-up, where telescopes point specifically at known galaxies within the GW localization volume, assuming these are the most likely hosts for the merger.

However, during the third (O3) and fourth (O4) observing runs of the LIGO-Virgo-KAGRA (LVK) collaboration, GW localizations were often significantly larger and the distances to events greater than originally anticipated for the galaxy-targeted approach. The paper addresses the following questions:

How effective is the Las Cumbres Observatory (LCO) global network in executing this galaxy-targeted strategy under O3/O4 conditions?
What are the response times and depth capabilities of the network?
Does the galaxy-targeted strategy remain viable given the increased localization volumes and distances?

2. Methodology

The authors analyzed LCO's follow-up performance for nine GW events (seven from O3, two from O4) that had a non-negligible probability of an EM counterpart.

Facilities: The study utilized the LCO global network of 19 optical telescopes (two 2m, nine 1m, and eight 0.4m) across five sites (Australia, South Africa, Chile, Texas, Hawaii).
Strategy:
- Triggering: Upon receiving LVK alerts (via GCN/VOEvent), the team calculated the 99% localization volume.
- Target Selection: They prioritized galaxies from the GLADE catalog (Version 2.3) within this volume. Galaxies were assigned a "score" based on location, mass, distance, and limiting luminosity.
  - O3: Only galaxies brighter than the Schechter function median ( $L_B > L^*_B$ ) were prioritized.
  - O4: This luminosity restriction was relaxed to include a larger pool of galaxies.
- Observation: Telescopes observed prioritized galaxies in multiple bands ( $g, r, i, z$ ) with exposure times of 240–300 seconds.
Data Processing: Images were processed through the BANZAI pipeline and LCOGTSNpipe (which subtracts SDSS templates). Candidates were inspected manually via image cutouts.
Performance Metrics: The authors evaluated:
1. Response Time: Time from GW detection to first observation (broken down into Time to Alert, Time to Trigger, and Time to Observations).
2. Luminosity Coverage: The fraction of the total B-band luminosity of prioritized galaxies that was actually observed (distinguishing between "Total" and "Unique" coverage).
3. Follow-up Depth: The 5 $\sigma$ limiting magnitudes achieved, compared against the light curve of GW170817 to determine the maximum distance at which a similar kilonova could be detected.

3. Key Contributions

Comprehensive Performance Audit: This is the first detailed analysis of LCO's specific performance metrics (speed, depth, and coverage efficiency) across both O3 and O4 runs.
Quantification of Strategy Limitations: The paper provides empirical evidence that while LCO is fast and deep, the galaxy-targeted strategy suffers from low efficiency when localization volumes are large (containing $10^3$ – $10^5$ galaxies).
Operational Timeline Analysis: The study breaks down the latency of the entire pipeline, identifying bottlenecks in alert processing and telescope scheduling.

4. Key Results

A. Response Time

Median Total Response Time: 3.15 hours (from GW detection to first observation).
Breakdown:
- Time to Alert: 0.35 hours (median).
- Time to Trigger: 0.80 hours (median).
- Time to Observations: 2.02 hours (median).
Fastest Response: 0.68 hours (S190822c).
Slowest Response: 13.79 hours (S190510g), largely due to a long delay in the LVK issuing a high-probability alert.
Observation Speed: Once triggered, the network could begin observing within minutes (e.g., 12–20 minutes for GW190814).

B. Luminosity Coverage (Efficiency)

Low Efficiency: Due to large localization areas (median 90% area in O3 was 740 deg²; in O4 was 1021 deg²) and vast numbers of prioritized galaxies, the fraction of the galaxy population covered was minimal.
Best Case: GW190814 achieved a 37% unique luminosity coverage because it had a very small localization area (22 deg²) and was close (230 Mpc).
Worst Case: S190510g achieved only 0.075% unique coverage due to a massive localization volume containing >25,000 prioritized galaxies.
General Trend: For most events, the unique luminosity coverage was < 1%, rendering the galaxy-targeted strategy inefficient for the majority of O3/O4 events.

C. Follow-up Depth

Sensitivity: LCO observations were sufficiently deep to detect a GW170817-like kilonova out to a median distance of 250 Mpc.
GW17 Limits: More than half of the observations reached depths capable of detecting a GW170817-like event at peak brightness.
- GW190425 and S190510g had the highest fractions of "GW17 limits" (91% and 86%, respectively).
- GW190728 064510 and S190822c had zero GW17 limits, primarily because observations were either too early (before the kilonova peak) or the events were reclassified as Black Hole mergers (no EM counterpart expected).

5. Significance and Conclusions

Strengths of LCO: The Las Cumbres network is an excellent rapid-response facility. Its global distribution allows it to overcome local weather and technical outages, ensuring observations can begin within hours (or even minutes) of an alert. Its depth is sufficient to probe the most likely distances for BNS/NSBH mergers.
Weaknesses of the Strategy: The galaxy-targeted strategy is inefficient for the current generation of GW detectors (O3/O4) because the localization volumes are too large. The strategy assumes small search volumes (as in GW170817), but O3/O4 events often have volumes orders of magnitude larger, making it impossible to cover a significant fraction of the host galaxy population with a limited number of telescope hours.
Future Recommendations:
1. Hybrid Approach: A combination of wide-field tiling (to cover large localization areas quickly) and rapid-response galaxy targeting (for deep, immediate follow-up of high-probability hosts) is required.
2. Coordination: Facilities must coordinate (e.g., via platforms like Treasure Map) to leverage the strengths of different instruments. Wide-field surveys can identify candidates in large volumes, while rapid-response networks like LCO can characterize them quickly.
3. Catalog Completeness: As detector sensitivity improves and localizations shrink, galaxy-targeted strategies will become more viable, provided galaxy catalogs are complete to large distances.

In summary, while LCO proved its capability as a fast and deep follow-up instrument, the paper concludes that the specific galaxy-targeted strategy used in O3 and O4 was suboptimal due to the mismatch between the strategy's assumptions and the actual GW localization performance. Future success depends on integrating wide-field and rapid-response capabilities.