First Light of Neutron Star Mergers: Off-axis Cocoon Cooling X-ray Emission from Short Gamma-Ray Burst Jets
Abstract
Neutron star mergers (NSMs) are confirmed gravitational wave sources. Identifying an early electromagnetic counterpart for these events is therefore crucial for rapid localization and multimessenger follow-up. However, the associated Gamma-Ray Bursts (GRBs) are highly collimated and are therefore easily missed by off-axis observers. An early, less beamed counterpart is essential for identifying the majority of mergers. In this Letter, we investigate the cooling emission from jet-driven cocoons produced by short gamma-ray burst jets propagating through merger ejecta. We perform hydrodynamic simulations and radiative post-processing to calculate the early X-ray emission over a wide range of viewing angles. We find that the mildly relativistic cocoon produces bright soft X-ray transients for off-axis observers, with luminosities of 1046−48erg s−1 and durations of a few to ten seconds. The X-ray spectra are quasi-thermal with characteristic temperatures of $0.1−−1\,{\rm keV}$. For observers at θv=10∘, the cocoon emission is detectable by Einstein Probe (EP) out to z≃ 0.3. For nearby events like GW170817, it remains detectable up to θv≃ 45∘. The predicted detection rate for EP is 0.5 yr−1 in the canonical model. In future multimessenger campaigns, rapid UV/optical/IR follow-up of such X-ray triggers can subsequently identify the associated kilonova and jet afterglow emission, which will confirm the origin.
Versions
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BibTeX
@misc{zheng2026first,
title = {{First Light of Neutron Star Mergers: Off-axis Cocoon Cooling X-ray Emission from Short Gamma-Ray Burst Jets}},
author = {Jian-He Zheng and Wenbin Lu},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2609.14508},
eprint = {2609.14508},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.14508},
note = {CC BY-NC-SA 4.0},
}
RIS
TY - UNPB
TI - First Light of Neutron Star Mergers: Off-axis Cocoon Cooling X-ray Emission from Short Gamma-Ray Burst Jets
AU - Jian-He Zheng
AU - Wenbin Lu
PY - 2026
DA - 2026/10/02
PB - arXiv
DO - 10.48550/arxiv.2609.14508
UR - https://arxiv.org/abs/2609.14508
AB - Neutron star mergers (NSMs) are confirmed gravitational wave sources. Identifying an early electromagnetic counterpart for these events is therefore crucial for rapid localization and multimessenger follow-up. However, the associated Gamma-Ray Bursts (GRBs) are highly collimated and are therefore easily missed by off-axis observers. An early, less beamed counterpart is essential for identifying the majority of mergers. In this Letter, we investigate the cooling emission from jet-driven cocoons produced by short gamma-ray burst jets propagating through merger ejecta. We perform hydrodynamic simulations and radiative post-processing to calculate the early X-ray emission over a wide range of viewing angles. We find that the mildly relativistic cocoon produces bright soft X-ray transients for off-axis observers, with luminosities of 10^46−48erg s^−1 and durations of a few to ten seconds. The X-ray spectra are quasi-thermal with characteristic temperatures of $0.1−−1\,{\rm keV}$. For observers at θ_v=10^∘, the cocoon emission is detectable by Einstein Probe (EP) out to z≃ 0.3. For nearby events like GW170817, it remains detectable up to θ_v≃ 45^∘. The predicted detection rate for EP is 0.5 yr^−1 in the canonical model. In future multimessenger campaigns, rapid UV/optical/IR follow-up of such X-ray triggers can subsequently identify the associated kilonova and jet afterglow emission, which will confirm the origin.
ER -