Two Step Localization Method for Electromagnetic Followup of LIGO-Virgo-KAGRA Gravitational-Wave Triggers

TL;DR

This paper tackles the latency bottleneck in identifying electromagnetic counterparts to gravitational-wave triggers by proposing a Two-Step Localization strategy that deploys a wide-field auxiliary telescope to monitor evolving GW sky localizations and a narrow-field main telescope for prompt high-resolution follow-up. The authors implement a modular simulation pipeline that generates dynamic skymaps, computes SNR-weighted localizations, and evaluates three coordination strategies across four pre-merger update times and multiple telescope configurations. Results show that Two-Step Localization consistently speeds up the first EM detection, with larger gains when using very wide auxiliary fields (e.g., 1000 deg^2), and indicate that next-generation faster slews and truly wide-field auxiliary instruments will further enhance early-time EM discovery. The findings inform observing strategies and instrument design for future multi-messenger campaigns, highlighting the practical value of real-time coordination and cross-instrument handoffs in reducing EM follow-up latency.

Abstract

Rapid detection and follow-up of electromagnetic (EM) counterparts to gravitational wave (GW) signals from binary neutron star (BNS) mergers are essential for constraining source properties and probing the physics of relativistic transients. Observational strategies for these early EM searches are therefore critical, yet current practice remains suboptimal, motivating improved, coordination-aware approaches. We propose and evaluate the Two-Step Localization strategy, a coordinated observational protocol in which one wide-field auxiliary telescope and one narrow-field main telescope monitor the evolving GW sky localization in real time. The auxiliary telescope, by virtue of its large field of view, has a higher probability of detecting early EM emission. Upon registering a candidate signal, it triggers the main telescope to slew to the inferred location for prompt, high-resolution follow-up. We assess the performance of Two-Step Localization using large-scale simulations that incorporate dynamic sky-map updates, realistic telescope parameters, and signal-to-noise ratio (SNR)-weighted localization contours. For context, we compare Two-Step Localization to two benchmark strategies lacking coordination. Our results demonstrate that Two-Step Localization significantly reduces the median detection latency, highlighting the effectiveness of targeted cooperation in the early-time discovery of EM counterparts. Our results point to the most impactful next step: next-generation faster telescopes that deliver drastically higher slew rates and shorter scan times, reducing the number of required tiles; a deeper, truly wide-field auxiliary improves coverage more than simply adding more telescopes.

Figures (9)

• Figure 1: Overall simulation flowchart. Each run proceeds from GW event generation and SNR computation to sky-map updates and detection-time comparison across three coordination strategies. The green, dotted-frame box links to the telescope movement logic in Figure \ref{['fig:tel_movement']}.
• Figure 2: Telescope movement and coordination logic, expanding the green dotted-frame box in Figure \ref{['fig:sim_flowchart']}. The purple, dashed-background box connects back to event simulation. The blue dashed-frame box sets communication rules, and the red,dotted, path marks the Two Step Localization sequence.
• Figure 3: Localization metrics vs. time before merger (LVK O4).Top panel: SNR (blue, left linear axis) and 90% credible localization area (green, right logarithmic axis) as functions of the time before merger, $t_{\rm pre\text{-}merge}$. The SNR rises toward coalescence, while the localization area correspondingly decreases by orders of magnitude as additional in-band cycles accumulate. We truncate the 90% area curve once it reaches $A_{90} \simeq 5\,\mathrm{deg}^2$, which is already smaller than the narrowest field of view of the telescopes considered in this work, which is $A_{90} \simeq 7\,\mathrm{deg}^2$ for Swope telescope. Bottom panel: Gravitational-wave frequency $f(t)$ versus $t_{\rm pre\text{-}merge}$. The increasing $f(t)$ (the GW chirp) drives the SNR growth and thus the improvement—i.e., reduction—of the 90% area. All curves are generated from our simulations using the O4 noise curve, with SNR computed from the strain model described in this subsection and localization areas obtained from our numerical mapping between SNR, update time $t_u$, and sky-map size, using the SNR–area fits of Sachdeve2020.
• Figure 4: Telescope motion and event detection using the Two Step Localization Method, with "BIG" as the auxiliary telescope configured with a $1000~\text{deg}^2$ FOV. The event is detected $153.5~\text{sec}$ after the first alarm, which was triggered $51.2~\text{sec}$ before the merger—i.e., $102.3~\text{sec}$ after the merger. Localization is progressively refined through subsequent sky-map updates. The third sky-map update—providing the first meaningful localization (see Sec. \ref{['Simulation']})—marks the start of telescope motion. The time color-bar begins at $28.4~\text{sec}$, representing the time elapsed from the first to the third sky-map update. In this timeline, the merger occurs at $51.2~\text{sec}$.
• Figure 5: Telescope motion and event detection using the Partial Communication Method, with "BIG" as the auxiliary telescope configured with a $1000~\text{deg}^2$ FOV. The event is detected $250.0~\text{sec}$ after the first alarm, which was triggered $51.2~\text{sec}$ before the merger—i.e., $198.8~\text{sec}$ after the merger. Localization is progressively refined through subsequent sky-map updates. In contrast to the Two Step Localization Method, the auxiliary telescope does not signal the main telescope upon detection, resulting in a longer search before the main telescope detects the event.