Evidence for gravitational-wave dominated emission in the central engine of short GRB 200219A

TL;DR

This paper presents evidence that the central engine of short GRB 200219A is likely a magnetar losing energy primarily through gravitational wave emission, challenging standard models and offering new insights into GRB mechanisms.

Contribution

It introduces a novel interpretation of the GRB's X-ray features as powered by a GW-dominated magnetar spin-down, the first such case identified.

Findings

The GRB's spectral and temporal features are consistent with a magnetar central engine.

The magnetar's collapse into a black hole explains the abrupt decay in X-ray emission.

Estimates of magnetar parameters and associated GW signals are provided.

Abstract

GRB 200219A is a short gamma-ray burst (GRB) with an extended emission (EE) lasting $\sim 90$s. By analyzing data observed with the {\em Swift}/BAT and {\em Fermi}/GBM, we find that a cutoff power-law model can adequately fit the spectra of the initial short pulse with $\rm E_{p}=1387^{+232}_{-134}$ keV. More interestingly, together with the EE component and early X-ray data, it exhibits plateau emission smoothly connected with a $\sim t^{-1}$ segment and followed by an extremely steep decay. The short GRB composed of those three segments is unique in the {\em 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 $\sim t^{-1}$ 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.