TY - JOUR

T1 - A Short Gamma-Ray Burst from a Protomagnetar Remnant

AU - Jordana-Mitjans, N.

AU - Mundell, C. G.

AU - Guidorzi, C.

AU - Smith, R. J.

AU - Ramirez-Ruiz, E.

AU - Metzger, B. D.

AU - Kobayashi, S.

AU - Gomboc, A.

AU - Steele, I. A.

AU - Shrestha, M.

AU - Marongiu, M.

AU - Rossi, A.

AU - Rothberg, B.

PY - 2022/11/1

Y1 - 2022/11/1

N2 - 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 of short-duration gamma-ray bursts (GRBs) and cosmic sources of heavy r-process nuclei. However, the nature (and life span) 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 short GRB 180618A with extended gamma-ray emission-with ultraviolet and optical multicolor observations starting as soon as 1.4 minutes post-burst. The spectrum is consistent with a fast-fading afterglow and emerging thermal optical emission 15 minutes post-burst, which fades abruptly and chromatically (flux density F ( nu ) proportional to t (-alpha ), alpha = 4.6 +/- 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 (th) proportional to t (-(2.22 +/- 0.14)) to reheat the unbound merger-remnant material. These results suggest that 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.

AB - 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 of short-duration gamma-ray bursts (GRBs) and cosmic sources of heavy r-process nuclei. However, the nature (and life span) 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 short GRB 180618A with extended gamma-ray emission-with ultraviolet and optical multicolor observations starting as soon as 1.4 minutes post-burst. The spectrum is consistent with a fast-fading afterglow and emerging thermal optical emission 15 minutes post-burst, which fades abruptly and chromatically (flux density F ( nu ) proportional to t (-alpha ), alpha = 4.6 +/- 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 (th) proportional to t (-(2.22 +/- 0.14)) to reheat the unbound merger-remnant material. These results suggest that 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.

KW - NEUTRON-STAR MERGERS

KW - EXTENDED EMISSION

KW - LIGHT CURVES

KW - ACCRETION DISKS

KW - MAGNETIC-FIELDS

KW - COMPLETE SAMPLE

KW - AFTERGLOW

KW - TIME

KW - SUPERNOVA

KW - SPECTRA

U2 - 10.3847/1538-4357/ac972b

DO - 10.3847/1538-4357/ac972b

M3 - Journal article

VL - 939

JO - Astrophysical Journal

JF - Astrophysical Journal

SN - 0004-637X

IS - 2

M1 - 106

ER -