First Light of Neutron Star Mergers: Off-axis Cocoon Cooling X-Ray Emission from Short Gamma-Ray Burst Jets
- 1Nanjing University
- 2University of California
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 postprocessing 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 ⁴⁶⁻⁴⁸ erg s ⁻¹ and durations of a few to ten seconds. The X-ray spectra are quasi-thermal with characteristic temperatures of 0.1–1 keV. For observers at θ v = 10 ∘ , the cocoon emission is detectable by the 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 can confirm their origin.
Versions
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BibTeX
@article{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},
journal = {The Astrophysical Journal Letters},
year = {2026},
month = oct,
doi = {10.3847/2041-8213/aeac25},
url = {https://iopscience.iop.org/article/10.3847/2041-8213/aeac25},
note = {CC BY 4.0},
}
RIS
TY - JOUR
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 - 2026/10/06
JO - The Astrophysical Journal Letters
DO - 10.3847/2041-8213/aeac25
UR - https://iopscience.iop.org/article/10.3847/2041-8213/aeac25
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 postprocessing 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 ⁴⁶⁻⁴⁸ erg s ⁻¹ and durations of a few to ten seconds. The X-ray spectra are quasi-thermal with characteristic temperatures of 0.1–1 keV. For observers at θ v = 10 ∘ , the cocoon emission is detectable by the 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 can confirm their origin.
SN - 2041-8205
SN - 2041-8213
ER -