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 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 ν ∝ t −α , α = 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 ∝ 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.

Eighteen Years of Kilonova Discoveries with Swift

“…The resulting thermal emission would be significantly enhanced and brighter than transients purely powered by r-process nucleosynthesis. This model, applied to GRB180618A [167], GRB050724A, and GRB061006A [162], can reproduce their luminous optical emission for reasonable values of the magnetic field, B ≈ 5 × 10 15 G, and initial spin period, P ≈ 3-5 ms. The extreme luminosities implied by some magnetar-powered models can be constrained with wide-field optical surveys [57].…”

“…All the bursts in Table 2 have an X-ray afterglow, and only 2 lack an optical counterpart. Swift/UVOT detections were reported for GRB180618A [167] and for the three closest events, namely GRB060505, GRB060614, and GRB211211A. In the latter two cases, the detection in all 6 UVOT filters (v, b, u, uvw1, uvm2, uvw2) helped constrain the GRB distance scale (z ≲ 1.5) directly from the afterglow data, and ruled out high values of dust extinction along the line of sight [42,172].…”

Searches for continuous-wave gravitational radiation

“…See Strang et al (2021), however, for an alternative study in which radiation driven from a millisecond magnetar can account for short GRB X-ray afterglows. See Dall'Osso and Stella (2022) for a recent brief review of millisecond magnetars, including evidence of their serving as central engines to create GRBs, and see Jordana-Mitjans et al (2022) for evidence of a protomagnetar remnant in the aftermath of GRB 180618A.…”

Identification of Extended Emission Gamma-Ray Burst Candidates Using Machine Learning

“…Nevertheless, it was pointed out by Ibrahim et al (2008) as a potential event from an emerging class of short bursts with EE and as a possible GRB with EE. Moreover, the EE of GRB 180618A (Fong et al 2022;Jordana-Mitjans et al 2022;O'Connor et al 2022), GRB 170728B (Fong et al 2022;O'Connor et al 2022), GRB 180805B (Fong et al 2022;O'Connor et al 2022), and GRB 200219A was suggested in the Gamma-ray Coordinates Network / Transient Astronomy Network (GCN/TAN)circulars.…”