A short gamma-ray burst from a proto-magnetar remnant

Abstract

The contemporaneous detection of gravitational waves and gamma rays from the GW170817/GRB 170817A, followed by kilonova emission a day after, confirmed compact binary neutron-star mergers as progenitors of short-duration gamma-ray bursts (GRBs), and cosmic sources of heavy r-process nuclei. However, the nature (and lifespan) of the merger remnant and the energy reservoir powering these bright gamma-ray flashes remains debated, while the first minutes after the merger are unexplored at optical wavelengths. Here, we report the earliest discovery of bright thermal optical emission associated with the short GRB 180618A with extended gamma-ray emission, with ultraviolet and optical multicolour observations starting as soon as 1.4 minutes post-burst. The spectrum is consistent with a fast-fading afterglow and emerging thermal optical emission at 15 minutes post-burst, which fades abruptly and chromatically (flux density $F_{\nu }\propto t^{-\alpha }$, $\alpha =4.6\pm 0.3$) just 35 minutes after the GRB. Our observations from gamma rays to optical wavelengths are consistent with a hot nebula expanding at relativistic speeds, powered by the plasma winds from a newborn, rapidly-spinning and highly magnetized neutron star (i.e. a millisecond magnetar), whose rotational energy is released at a rate $L_{\mathrm{th}}\propto t^{-(2.22\pm 0.14)}$ to reheat the unbound merger-remnant material. These results suggest such neutron stars can survive the collapse to a black hole on timescales much larger than a few hundred milliseconds after the merger, and power the GRB itself through accretion. Bright thermal optical counterparts to binary merger gravitational wave sources may be common in future wide-field fast-cadence sky surveys.

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• Identification of Extended Emission Gamma-Ray-Bursts Candidates using Machine Learning
• Jetlike structures in low-mass binary neutron star merger remnants
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• Postmerger multimessenger analysis of binary neutron stars: Effect of the magnetic field strength and topology
• Comparison of the Origin of Short Gamma-Ray Bursts with or without Extended Emission
• GRB minimum variability timescales with Fermi/GBM
• Exploring Radial Oscillations in Slow Stable and Hybrid Neutron Stars
• Constraining the $f$-mode oscillations frequency in Neutron Stars through Universal Relations in the realm of Energy-Momentum Squared Gravity
• Spacetime Curvature as a Probe of Exotic Core Phases in Neutron Stars within Modified Gravity
• Science with a large field-of-view polarization survey: The Large Array Survey Telescope Polarization Node (LAST-P)
• Revise thermal winds of remnant neutron stars in gamma-ray bursts