Detecting prompt and afterglow jet emission of gravitational wave events from LIGO/Virgo/KAGRA and next generation detectors

Following the wealth of new results enabled by multimessenger observations of the binary neutron star (BNS) merger GW170817, the next goal is increasing the number of detections of electromagnetic (EM) counterparts to gravitational wave (GW) events. We study the detectability of the prompt emission and afterglows produced by the relativistic jets launched by BNS mergers that will be detected by LIGO-Virgo-KAGRA during their fifth observing run (O5), and by next generation (XG) GW detectors (Einstein Telescope and Cosmic Explorer). We quantify the impact of various BNS merger and jet afterglow parameters on the likelihood of detection, focusing on the impact of the observer’s viewing angle and the jet’s core half-opening angle. We explore detectability over a wide range of current state-of-the-art facilities (e.g., the James Webb Space Telescope, Chandra X-ray Observatory) as well as upcoming next-generation facilities (e.g., AXIS, NewAthena, ngVLA, SKA). We find that a few GW events (~0-4) per year may have a detectable afterglow component in O5, with the largest detection rates expected with SKA in the radio and JWST in the near-infrared. In the XG era, hundreds of multimessenger detections of afterglows per year may be possible with a range of instruments, such as NewAthena in the X-ray and ngVLA in the radio. While zero to a few GW events per year are expected to be accompanied by a detectable prompt emission in O5, dozens per year may be detectable in XG.

💡 Research Summary

This paper evaluates the prospects for detecting both the prompt gamma‑ray emission and the broadband afterglow from relativistic jets launched in binary neutron‑star (BNS) mergers that will be observed by the LIGO‑Virgo‑KAGRA network during its fifth observing run (O5) and by next‑generation (XG) gravitational‑wave detectors such as the Einstein Telescope (ET) and Cosmic Explorer (CE). The authors generate large Monte‑Carlo samples of merger events, applying realistic detector sensitivities, duty cycles, and signal‑to‑noise thresholds (network SNR > 8 for O5, SNR > 12 for XG). For O5 they obtain 2 403 detectable BNS events; for XG they simulate 4.9 × 10⁵ detections and select a representative subset of 3 000 events for detailed analysis.

Each event is assigned a jet launching probability based on post‑merger disk mass and remnant fate, assuming a black‑hole central engine and an accretion disk. The jet core half‑opening angle is taken to be θ_c ≈ 6°, consistent with short‑GRB observations, and the observer viewing angle θ_v is drawn isotropically. The authors use standard afterglow theory, where the peak time scales as t_peak ∝ (θ_v/θ_c)² and the peak flux as F_peak ∝ (θ_v/θ_c)⁻²p (p being the electron energy index). This framework captures the strong dependence of detectability on off‑axis geometry.

The electromagnetic detectability is assessed across a suite of current and future facilities: radio (SKA 1.4 GHz, ngVLA 8 GHz), near‑infrared (JWST F356W), optical/UV (various ground‑based imagers), and X‑ray (Chandra, Athena, NewAthena). For O5, the limited horizon (≈ 600 Mpc) and modest sensitivity of existing instruments imply that only a few (0–4) afterglows per year could be seen, primarily in radio with SKA and in the near‑IR with JWST. Prompt gamma‑ray detection is expected to be rare (zero to a few events per year) because off‑axis emission is strongly suppressed.

In the XG era, the dramatically increased volume (detectable out to redshift > 2) and the order‑of‑magnitude improvement in EM instrument sensitivity lead to a dramatic rise in expected detections. The simulations predict hundreds of afterglows per year observable in radio with ngVLA and in X‑ray with NewAthena, and dozens of prompt gamma‑ray bursts detectable by next‑generation high‑energy monitors. The authors emphasize that events with viewing angles close to the jet core dominate the detectable sample, but the larger distance reach allows even moderately off‑axis events to be observed thanks to the higher intrinsic luminosities at early times.

Beyond raw detection rates, the paper discusses optimal follow‑up strategies. Early wide‑field radio and optical surveys (e.g., SKA surveys, LSST) can identify candidate afterglows shortly after a GW trigger, after which deep pointed observations with JWST, Chandra, or NewAthena can characterize the light curves and spectra. Incorporating GW‑derived distance and inclination information can refine the expected peak times, improving scheduling efficiency. The authors also note that the lack of off‑axis short GRBs at cosmological distances may reflect reduced prompt efficiency, making afterglow‑only (“orphan”) searches essential.

Overall, the study provides a quantitative roadmap for multimessenger campaigns in the coming decade, highlighting how the synergy between next‑generation GW detectors and advanced EM facilities will transform our ability to probe jet physics, neutron‑star equations of state, and r‑process nucleosynthesis through a rich sample of BNS merger afterglows and prompt emissions.