Evidence for Gravitational-wave-dominated Emission in the Central Engine of Short GRB 200219A

GRB 200219A is a short gamma-ray burst (GRB) with extended emission (EE) lasting ∼90 s. By analyzing data observed with the Swift/BAT and Fermi/GBM, we find that a cutoff power-law model can adequately fit the spectra of the initial short pulse with ${E}_{{\rm{p}}}={1387}_{-134}^{+232}$ keV. More interestingly, together with the EE component and early X-ray data, it exhibits plateau emission smoothly connected with a ∼t−1 segment and followed by an extremely steep decay. The short GRB composed of those three segments is unique in the Swift era and is very difficult to explain with the standard internal/external shock model of a black hole central engine, but could be consistent with the prediction of a magnetar central engine from the merger of an NS binary. We suggest that the plateau emission followed by a ∼t−1 decay phase is powered by the spin-down of a millisecond magnetar, which loses its rotation energy via GW quadrupole radiation. Then, the abrupt drop decay is caused by the magnetar collapsing into a black hole before switching to EM-dominated emission. This is the first short GRB for which the X-ray emission has such an intriguing feature powered by a magnetar via GW-dominated radiation. If this is the case, one can estimate the physical parameters of a magnetar, the GW signal powered by a magnetar and the merger-nova emission are also discussed.