# A kilonova associated with GRB 070809

## Abstract

For on-axis typical short gamma-ray bursts (sGRBs), the forward shock emission is usually so bright1,2 that it renders the identification of kilonovae (also known as macronovae)3,4,5,6 in the early afterglow (t < 0.5 d) phase rather challenging. This is why previously no thermal-like kilonova component has been identified at such an early time7,8,9,10,11,12,13 except in the off-axis dim GRB 170817A (refs. 14,15,16,17,18,19) associated with GW170817 (ref. 20). Here we report the identification of an unusual optical radiation component in GRB 070809 at t ~ 0.47 d, thanks plausibly to the very-weak/subdominant forward shock emission. The optical emission with a very red spectrum is well in excess of the extrapolation of the X-ray emission that is distinguished by an unusually hard spectrum, which is at odds with the forward shock afterglow prediction but can be naturally interpreted as a kilonova. Our finding supports the speculation that kilonovae are ubiquitous11, and demonstrates the possibility of revealing the neutron star merger origin with the early afterglow data of some typical sGRBs that take place well beyond the sensitive radius of the advanced gravitational wave detectors21,22 and hence the opportunity of organizing dedicated follow-up observations for events of interest.

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## Data availability

The Keck, HST, Gemini and Swift observation data analysed/used in this work are all publicly available.

## Code availability

The codes used in this analysis are standard in the community, as introduced in Methods.

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## Acknowledgements

This work was supported in part by NSFC under grants no. 11525313 (that is, Funds for Distinguished Young Scholars), no. 11433009 and no. 11773078, the Funds for Distinguished Young Scholars of Jiangsu Province (no. BK20180050), the Chinese Academy of Sciences via the Strategic Priority Research Programme (grant no. XDB23040000) and the Key Research Programme of Frontier Sciences (no. QYZDJ-SSW-SYS024). S.C. and P.D. have been supported by ASI grant I/004/11/0.

## Author information

Y.-Z.F., Z.-P.J, S.C. and D.-M.W launched the project. Z.-P.J, N.-H.L, X.L. (from PMO), S.C. and P.D. (from INAF/OAB) carried out the data analysis. Y.-Z.F. and D.-M.W. interpreted the data. Y.-Z.F. and Z.-P.J. prepared the paper and all authors participated in the discussion.

Correspondence to Yi-Zhong Fan.

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## Supplementary information

### Supplementary Information

Supplementary Fig. 1, Tables 1, 2 and refs. 1–21.

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