Late Engine Activity in Neutron Star Mergers and Its Cocoon: An Alternative Scenario for the Blue Kilonova

Author:

Hamidani HamidORCID,Kimura Shigeo S.ORCID,Tanaka MasaomiORCID,Ioka KunihitoORCID

Abstract

Abstract Follow-up observations of short gamma-ray bursts (sGRBs) have continuously unveiled late extended/plateau emissions, attributed to jet launch due to late engine activity, the nature of which remains enigmatic. Observations of GW170817 have confirmed that sGRBs are linked to neutron star (NS) mergers, and discovered a kilonova (KN) transient. Nevertheless, the origin of the early blue KN in GW170817 remains unclear. Here, we investigate the propagation of late jets in the merger ejecta. By analytically modeling jet dynamics, we determine the properties of the jet-heated cocoon, and estimate its cooling emission. Our results reveal that late jets generate significantly brighter cocoons compared to prompt jets, primarily due to reduced energy loss by adiabatic cooling. Notably, with typical late jets, emission from the cocoon trapped inside the ejecta can reproduce the blue KN emission. We estimate that the forthcoming Einstein Probe mission will detect the early cocoon emission at a rate of 2.1 1.6 + 3.2 yr−1, and that optical/UV follow-ups in the LIGO-Virgo-KAGRA O5 run will be able to detect 1.0 0.7 + 1.5 cocoon emission events. As an electromagnetic counterpart, this emission provides an independent tool to probe NS mergers in the Universe, complementing insights from sGRBs and gravitational waves.

Funder

MEXT ∣ Japan Science and Technology Agency

MEXT ∣ Japan Society for the Promotion of Science

MEXT ∣ NINS ∣ National Institute for Fusion Science

Publisher

American Astronomical Society

Cited by 4 articles. 订阅此论文施引文献 订阅此论文施引文献,注册后可以免费订阅5篇论文的施引文献,订阅后可以查看论文全部施引文献

1. GRB 211211A: The Case for an Engine-powered over r-process-powered Blue Kilonova;The Astrophysical Journal Letters;2024-08-01

2. Diversity of Early Kilonova with the Realistic Opacities of Highly Ionized Heavy Elements;The Astrophysical Journal;2024-06-01

3. A semi-analytical model for the propagation of a relativistic jet in a magnetized medium;Monthly Notices of the Royal Astronomical Society;2024-05-27

4. Possible X-ray cocoon emission from GRB 050709;Publications of the Astronomical Society of Japan;2024-05-15

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