A Short Gamma-Ray Burst from a Protomagnetar Remnant

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