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First Light of Neutron Star Mergers: Off-axis Cocoon Cooling X-Ray Emission from Short Gamma-Ray Burst Jets

Year: 2026
Type: article
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.
DOI: 10.3847/2041-8213/aeac25
Language: English
Open Access status: gold
Estimated APC paid: $2,400
OpenAlex ID: W7213359995
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