- GRB170817A (Abbott et al. 2017) is a short gamma-ray burst with $2.9\pm0.3$\,s soft emission over 8-70\,keV (Goldstein et al. 2017, Pozanenko et al. 2018)
detected subsequent to the double neutron star merger GW170817 at a level of confidence of 5.1$\sigma$. It is produced by prompt or continuous output from a newly formed hyper-massive neutron star or stellar mass black hole. Here, we report on a descending chirp in gravitational wave emission during GRB170817A with characteristic time scale $\tau_s=3.01\pm0.2\,$s detected in a spectrogram up to 700 Hz. Starting in the 1.7 s gap between GW170817 and GRB170817A, its significance to the central engine has a Gaussian equivalent level of confidence of 4.31$\sigma$ by causality (3.29$\sigma$) and signal-to-noise ratio in the spectrogram (2.4$\sigma$). By frequency, this indicates a hyper-massive magnetar rather than a black hole, spinning down by magnetic winds and interactions with dynamical mass ejecta.
- presented at KAGRA f2f #19
- van Putten, M.H.P.M., 2017, PTEP 093F01
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