Observational evidence for Extended Emission to GW170817

Abstract

The recent LIGO event GW170817 is the merger of a double neutron star system with an associated short GRB170817A with $2.9\pm0.3$\,s soft emission over 8-70\,keV. This association has a Gaussian equivalent level of confidence of 5.1$\sigma$. The merger produced a hyper-massive neutron star of stellar mass black hole with prompt or continuous energy output powering GRB170817A. {Here, we report on a possible detection of} Extended Emission (EE) in gravitational radiation {\em during} GRB170817A: a descending chirp with characteristic time scale $\tau_s=3.01\pm0.2\,$s in a (H1,L1)-spectrogram up to 700\,Hz with {Gaussian equivalent level of confidence greater than 3.3$\sigma$ based on causality alone following edge detection applied to (H1,L1)-spectrograms merged by frequency coincidences. Additional confidence derives from the strength of this EE.} The observed frequencies below 1kHz indicate a hyper-massive magnetar rather than a black hole, spinning down by magnetic winds and interactions with dynamical mass ejecta.