Short-duration Gamma-ray Bursts Research Articles

On non-detection of gamma-ray bursts in three compact binary merger events detected by LIGO

ABSTRACT The joint detection of the gravitational wave (GW) event GW170817 and the short-duration gamma-ray burst (SGRB) event GRB 170817A, marked the beginning of GW multimessenger astronomy and confirmed that binary neutron star mergers are progenitors of at least some SGRBs. An estimated joint detection rate of 0.3–1.7 per year between the LIGO-Hanford, LIGO-Livingston, and Virgo GW network at design sensitivity, and the Fermi Gamma-ray Burst Monitor was predicted. However, to date, the GW170817/GRB 170817A joint detection has been the only event of its kind so far. Taking into account that SGRBs are narrowly beamed and are emitted perpendicular to the orbital plane of the binary system, we propose that previous mergers involving neutron stars, were orientated such that observation of the emitted SGRB along this narrow jet was not possible. To support this hypothesis we have estimated the inclination of the binary systems for previously detected Binary Neutron Star (BNS) and Black Hole Neutron Star (BHNS) mergers through GW analysis. This analysis was performed using bilby, a python based Bayesian inference library, to estimate the inclination of the BNS events GW170817 and GW190425, and the BHNS events GW190917_114630 and GW200115_042309. The results obtained in this study indicate that these binaries may have had inclinations greater than 33° with respect to the line of sight from Earth, an upper limit on the viewing angle set from observations of GRB 170817A. This then suggests that the observation of the emitted SGRB from these past mergers might not have been possible.

Neutron star mergers as the dominant contributor to the production of heavy r-process elements

ABSTRACT The discovery of the radioactively powered kilonova AT2017gfo, associated with the short-duration gamma-ray burst GRB 170817A and the gravitational wave source GW170817, has provided the first direct evidence supporting binary neutron star mergers as crucial astrophysical sites for the synthesis of heavy elements beyond iron through r-process nucleosysthesis in the universe. However, recent identification of kilonovae following long-duration gamma-ray bursts, such as GRB 211211A and GRB 230307A, has sparked discussions about the potential of neutron star–white dwarf mergers to also produce neutron-rich ejecta and contribute to the production of heavy r-process elements. In this work, we estimate the contribution of binary neutron star mergers to the total mass of r-process elements in the Milky Way and investigate the possibility of neutron star–white dwarf mergers as alternative astrophysical sites for r-process nucleosynthesis through an analysis of the total mass of the r-process elements in the Milky Way. Our results reveal that binary neutron star mergers can sufficiently account for the Galactic heavy r-process elements, suggesting that these events are the dominant contributor to the production of heavy r-process elements in the Milky Way. Considering the total mass of r-process elements in the Milky Way and the higher occurrence rate of neutron star–white dwarf mergers, it is unlikely that such mergers can produce a significant amount of neutron-rich ejecta, with the generated mass of r-process elements being lower than $0.005\, {\rm M}_{\odot }$.

A Population of Short-duration Gamma-Ray Bursts with Dwarf Host Galaxies

We present a population of 11 of the faintest (>25.5 AB mag) short gamma-ray burst (GRB) host galaxies. We model their sparse available observations using the stellar population inference code Prospector-β and develop a novel implementation to incorporate the galaxy mass–radius relation. Assuming these hosts are randomly drawn from the galaxy population and conditioning this draw on their observed flux and size in a few photometric bands, we determine that these hosts have dwarf galaxy stellar masses of 7.0≲log(M*/M⊙)≲9.1 . This is striking as only 14% of short GRB hosts with previous inferred stellar masses had M * ≲ 109 M ⊙. We further show these short GRBs have smaller physical and host-normalized offsets than the rest of the population, suggesting that the majority of their neutron star (NS) merger progenitors were retained within their hosts. The presumably shallow potentials of these hosts translate to small escape velocities of ∼5.5–80 km s−1, indicative of either low postsupernova systemic velocities or short inspiral times. While short GRBs with identified dwarf host galaxies now comprise ≈14% of the total Swift-detected population, a number are likely missing in the current population, as larger systemic velocities (observed from the Galactic NS population) would result in highly offset short GRBs and less secure host associations. However, the revelation of a population of short GRBs retained in low-mass host galaxies offers a natural explanation for the observed r-process enrichment via NS mergers in Local Group dwarf galaxies, and has implications for gravitational-wave follow-up strategies.

Anisotropic energy injection from magnetar central engines in short GRBs

ABSTRACT A long-lived magnetar, potentially originating from a binary neutron star system, has been proposed to explain the extended emission observed in certain short-duration gamma-ray bursts (sGRBs), and is posited as a potential central engine to power the engine-fed kilonovae. Previously, the process by which energy is injected into the surrounding ejecta/jet was widely believed to be nearly isotropic. In this study, we employ special relativity magnetohydrodynamic (SRMHD) simulations to investigate the wind injection process from a magnetar central engine. We explore the dynamics and energy distribution within the system and found that the parameter α = uA/uMWN can be used to indicate the collimation of the magnetar wind energy injection, where uA is the local Alfven four-speed and uMWN is the four-speed of the magnetar wind nebular (MWN) formed from wind-ejecta collision. A significant portion of the injected energy from the magnetar spin-down wind will be channeled to the jet axis due to collimation within the MWN. Achieving isotropic energy injection requires a significantly small α that necessitates either an ultra-relativistic expanding MWN or an extremely low magnetization MWN, both of which are challenging to attain in sGRBs. Consequently, a considerably reduced energy budget (i.e. energy per solid angle reduced by a factor of up to 10 with respect to the value under isotropic assumption) is anticipated to be injected into the ejecta for engine-fed kilonovae. Engine-fed kilonovae would appear fainter than originally anticipated.

Searching for Associations between Short Gamma-Ray Bursts and Fast Radio Bursts

The physical origin of fast radio bursts (FRBs) is still unclear. However, young magnetars associated with short-duration gamma-ray bursts (SGRBs) have been thought to be possible central engines for some FRBs. In this paper, we perform a systematic search for SGRBs that are associated with FRBs in a sample including 623 FRBs (601 one-off bursts and 22 repeaters) and 168 SGRBs with precise localizations. We find that FRB 190309A is spatially associated with GRB 060502B, with a chance probability of 0.05 when temporal and redshift information is taken into account. Considering the high chance probability (the statistical significance is <3σ), we examine other observational properties such as the host galaxy, the dispersion measure, and the energy budget of the central engine to check the possibility of their association. Although the available observational information is insufficient to determine whether they are physically associated, it does not rule out such a possibility. As the only pair of FRB and GRB that are spatially associated, it remains an interesting case worthy of further attention.

Contributions of the Swift/UV Optical Telescope to the Study of Short Gamma-ray Bursts

Before the Neil Gehrels Swift Observatory, we knew little about short-duration Gamma-ray bursts (sGRBs). Their briefness led to the suspicion that they resulted from mergers of compact objects, e.g., two neutron stars or a neutron star and a black hole. However, proof was lacking. sGRB post-prompt emission, or afterglow, was undetected; thus, we could not apply essential investigation tools. Swift was the first to pinpoint sGRB afterglows. sGRBs were found to differ from long GRBs in terms of host galaxies, offset from host, environment, energy and progenitors. The Swift UV/Optical Telescope (UVOT) has greatly contributed to these discoveries with its unique combination of fast repointing capabilities and UV sensitivity. But the long-sought proof of the sGRB–merger connection arrived in 2017. The gravitational signal GW 170817A caused by two NSs collision was associated with sGRB 170817A. Swift/UVOT discovered that its early optical emission was—unusually for GRB afterglows—thermal. It was interpreted as an emission from the merger’s hot debris: the kilonova. Kilonovae have seemingly been found in other sGRBs and—puzzingly—in long GRBs. Over almost 20 years, Swift/UVOT observations have also been pivotal to understanding peculiar events. In this review, I will summarize UVOT’s major contributions in the fields highlighted.

Black hole growths in gamma-ray bursts driven by the Blandford–Znajek mechanism

ABSTRACT The Blandford–Znajek (BZ) mechanism in stellar-mass black hole (BH) hyperaccretion systems is generally considered to power gamma-ray bursts (GRBs). Based on observational GRB data, we use the BZ mechanism driven by the BH hyperaccretion disc to investigate the evolution of the BH mass and spin after the jets break out from the progenitors. We find that the BH growths are almost independent of initial BH masses. Meanwhile, the BH growths will be more efficient with smaller initial spin parameters. We conclude that (i) the BZ mechanism is efficient for triggering BH growths for only 1 of 206 typical long-duration GRBs; (ii) the mean BH mass growths of ultra-long GRBs are marginal for all 7 samples collected; (iii) for the short-duration GRBs, the results that BHs show minimal growths is consistent with the mass supply limitation in the scenario of compact object mergers.

Quasiperiodic Oscillation in Short Gamma-Ray Bursts from Black Hole–Neutron Star Mergers

Short-duration gamma-ray bursts (sGRBs) are commonly attributed to the mergers of double neutron stars (NSs) or the mergers of a neutron star with a black hole (BH). While the former scenario was confirmed by the event GW170817, the latter remains elusive. Here, we consider the latter scenario in which an NS is tidally disrupted by a fast-spinning low-mass BH and the accretion onto the BH launches a relativistic jet and hence produces an sGRB. The merging binary’s orbit is likely misaligned with the BH’s spin. Hence, the Lense–Thirring precession around the BH may cause a hyperaccreting thick disk to precess in a solid-body manner. We propose that a jet, initially aligned with the BH spin, is deflected and collimated by the wind from the disk, therefore being forced to precess along with the disk. This would result in a quasiperiodic oscillation or modulation in the gamma-ray light curve of the sGRB, with a quasi-period of ∼0.01–0.1 s. The appearance of the modulation may be delayed respective to the triggering of the light curve. This feature, unique to the BH–NS merger, may have already revealed itself in a few observed sGRBs (such as GRB 130310A), and it carries the spin–orbit orientation information of the merging system. Identification of this feature would be a new approach to reveal spin–orbit misaligned merging BH–NS systems, which are likely missed by the current gravitational-wave searching strategy that is principally targeting aligned systems.

Striped Jets in Post–Neutron Star Merger Systems

Models invoking magnetic reconnection as the particle acceleration mechanism within relativistic jets often adopt a gradual energy dissipation profile within the jet. However, such a profile has yet to be reproduced in first-principles simulations. Here we perform a suite of 3D general relativistic magnetohydrodynamic simulations of post–neutron star merger disks with an initially purely toroidal magnetic field. We explore the variations in both the microphysics (e.g., nuclear recombination, neutrino emission) and system parameters (e.g, disk mass). In all of our simulations, we find the formation of magnetically striped jets. The stripes result from the reversals in the poloidal magnetic flux polarity generated in the accretion disk. The simulations display large variations in the distributions of stripe duration, τ, and power, 〈P Φ〉. We find that more massive disks produce more powerful stripes, the most powerful of which reaches 〈P Φ〉 ∼ 1049 erg s−1 at τ ∼ 20 ms. The power and variability that result from the magnetic reconnection of the stripes agree with those inferred in short-duration gamma-ray bursts. We find that the dissipation profile of the cumulative energy is roughly a power law in both radial distance, z, and τ, with a slope in the range of ∼1.7–3; more massive disks display larger slopes.

Possible Alternate Scenario for short Duration GRBs invoking Dark Matter Objects

In recent works we had discussed the possibility of primordial planets composed entirely of dark matter (DM), and considered this to be a possible reason for not detecting DM particles in the various ongoing experiments. It has been suggested that such primordial DM objects could have formed in the early universe. Here we look at these DM objects as possible candidates for short duration gamma ray bursts. This model has the advantage of eliminating the as yet unresolved baryon load problem in the usual scenario. These could also provide a possible mechanism for the formation of sub-stellar black holes, distinct from the usual Hawking (primordial) black holes.

Joint analysis of gravitational-wave and electromagnetic data of mergers: breaking an afterglow model degeneracy in GW170817 and in future events

ABSTRACT On 2017 August 17, Advanced LIGO and Virgo observed GW170817, the first gravitational-wave (GW) signal from a binary neutron star merger. It was followed by a short-duration gamma-ray burst, GRB 170817A, and by a non-thermal afterglow emission. In this work, a combined simultaneous fit of the electromagnetic (EM, specifically, afterglow) and GW domains is implemented, both using the posterior distribution of a GW standalone analysis as prior distribution to separately process the EM data, and fitting the EM and GW domains simultaneously. These approaches coincide mathematically, as long as the actual posterior of the GW analysis, and not an approximation, is used as prior for the EM analysis. We treat the viewing angle, θv, as shared parameter across the two domains. In the afterglow modelling with a Gaussian structured jet this parameter and the jet core angle, θc, are correlated, leading to high uncertainties on their values. The joint EM + GW analysis relaxes this degeneracy, reducing the uncertainty compared to an EM-only fit. We also apply our methodology to hypothetical GW170817-like events occurring in the next GW observing run at ∼140 and 70 Mpc. At 70 Mpc the existing EM degeneracy is broken, thanks to the inclusion of the GW domain in the analysis. At 140 Mpc, the EM-only fit cannot constrain θv nor θc because of the lack of detections in the afterglow rising phase. Folding the GW data into the analysis leads to tighter constraints on θv, still leaving θc unconstrained, requiring instruments with higher sensitivities, such as Athena.

Eighteen Years of Kilonova Discoveries with Swift

Swift has now completed 18 years of mission, during which it discovered thousands of gamma-ray bursts as well as new classes of high-energy transient phenomena. Its first breakthrough result was the localization of short duration GRBs, which enabled for redshift measurements and kilonova searches. Swift, in synergy with the Hubble Space Telescope and a wide array of ground-based telescopes, provided the first tantalizing evidence of a kilonova in the aftermath of a short GRB. In 2017, Swift observations of the gravitational wave event GW170817 captured the early UV photons from the kilonova AT2017gfo, opening a new window into the physics of kilonovae. Since then, Swift has continued to expand the sample of known kilonovae, leading to the surprising discovery of a kilonova in a long duration GRB. This article will discuss recent advances in the study of kilonovae driven by the fundamental contribution of Swift.

Predicting Short-duration GRB Rates in the Advanced LIGO Volume

Starting with models for the compact object merger event rate, the short-duration gamma-ray burst (sGRB) luminosity function, and the Swift-BAT detector, we calculate the observed Swift sGRB rate and its uncertainty. Our probabilistic sGRB world model reproduces the observed number distributions in redshift and flux for 123 Swift-BAT detected sGRBs and can be used to predict joint sGRB/LIGO detection rates. We discuss the dependence of the rate predictions on the model parameters and explore how they vary with increasing experimental sensitivity. In particular, the number of bursts in the LIGO volume depends strongly on the parameters that govern sGRB beaming. Our results suggest that nearby sGRBs should be observed to have broader jets on average (θ jet ≳ 30°), as compared to the narrowly beamed appearance of cosmological sGRBs due to the detection-selection effect driving the observed jet angle. Assuming all sGRBs are due to compact object mergers, within a D < 200 Mpc aLIGO volume, we predict sGRB–gravitational wave (GW) associations all sky per year for on-axis events at Swift sensitivities, increasing to with the inclusion of off-axis events. We explore the consistency of our model with GW170817/GRB 170817A in the context of structured jets. Predictions for future experiments are made.

A Collapsar Origin for GRB 211211A Is (Just Barely) Possible

Gamma-ray bursts (GRBs) have historically been divided into two classes. Short-duration GRBs are associated with binary neutron star mergers (NSMs), while long-duration bursts are connected to a subset of core-collapse supernovae (SNe). GRB 211211A recently made headlines as the first long-duration burst purportedly generated by an NSM. The evidence for an NSM origin was excess optical and near-infrared emission consistent with the kilonova observed after the gravitational-wave-detected NSM GW170817. Kilonovae derive their unique electromagnetic signatures from the properties of the heavy elements synthesized by rapid neutron capture (the r-process) following the merger. Recent simulations suggest that the “collapsar” SNe that trigger long GRBs may also produce r-process elements. While observations of GRB 211211A and its afterglow rule out an SN typical of those that follow long GRBs, an unusual collapsar could explain both the duration of GRB 211211A and the r-process-powered excess in its afterglow. We use semianalytic radiation transport modeling to evaluate low-mass collapsars as the progenitors of GRB 211211A–like events. We compare a suite of collapsar models to the afterglow-subtracted emission that followed GRB 211211A, and find the best agreement for models with high kinetic energies and an unexpected pattern of 56Ni enrichment. We discuss how core-collapse explosions could produce such ejecta, and how distinct our predictions are from those generated by more straightforward kilonova models. We also show that radio observations can distinguish between kilonovae and the more massive collapsar ejecta we consider here.

Flares, Jets, and Quasiperiodic Outbursts from Neutron Star Merger Remnants

Using numerical relativity simulations with a subgrid dynamo prescription to generate strong initial magnetic fields, we investigate the possibility of launching a jet-like outflow from the hypermassive neutron star (HMNS) during the early stages of the merger, prior to the remnant’s collapse to a black hole. We demonstrate that buoyant instabilities in the strongly magnetized HMNS can lead to a periodic emission of powerful electromagnetic flares shortly after the merger. These are followed by a collimated mildly relativistic outflow. Both types of outflows feature quasiperiodic kilohertz substructure. These early-time outflows may power precursors to short-duration gamma-ray bursts (sGRBs) or in some cases the entire sGRB. While the overall temporal power spectrum we find broadly agrees with the one recently reported for quasiperiodic oscillations in the sGRB GRB910711, our simulations suggest that the periodic electromagnetic substructure is dominated by magnetohydrodynamic shearing processes rather than correlating with the corresponding postmerger gravitational-wave signal.

Studying Postmerger Outflows from Magnetized-neutrino-cooled Accretion Disks

Neutrino-cooled accretion flow around a spinning black hole, produced by a compact binary merger, is a promising scenario for jet formation and launching magnetically driven outflows. Based on GW170817 gravitational wave detection by LIGO and Virgo observatories, followed by electromagnetic counterparts, this model can explain the central engine of the short-duration gamma-ray bursts and kilonova radiations. Using the open-source general relativistic magnetohydrodynamic HARM-COOL code, we evolved several 2D magnetized accretion disk–black hole models with a realistic equation of state in the fixed curved spacetime background. We applied the particle tracer technique to measure the properties of the outflows. The disk and black hole's initial parameters are chosen in a way to represent different possible postmerger scenarios of the merging compact objects. Our simulations show a strong correlation between the black hole's spin and ejected mass. Generally, mergers producing massive disks and rapidly spinning black holes launch stronger outflows. We observed our models generate winds with moderate velocity (v/c ∼ 0.1–0.2) and a broad range of electron fractions. We use these results to estimate the luminosity and light curves of possible radioactively powered transients emitted by such systems. We found that the luminosity peaks within the range of 1040–1042 erg s−1, which agrees with previous studies for disk wind outflows.

Multi-scale VLBI observations of the candidate host galaxy of GRB 200716C

We present the discovery and the subsequent follow up of radio emission from SDSS J130402.36+293840.6 (J1304+2938), the candidate host galaxy of the gamma-ray burst (GRB) GRB 200716C. The galaxy is detected in the RACS (0.89 GHz), the NVSS, the Apertif imaging survey, and the FIRST (1.4 GHz), the VLASS (3 GHz), and in public LOFAR (130–170 MHz), WISE (3.4–22 μm), and SDSS (z, i, r, g, u filters) data. The luminosity inferred at 1.4 GHz is (5.1 ± 0.2) × 1030 erg s−1 Hz−1. To characterise the emission and distinguish between different components within the galaxy, we performed dedicated, high-sensitivity and high-resolution observations with the European VLBI Network (EVN) + e-MERLIN at 1.6 and 5 GHz. We did not detect any emission from a compact core, suggesting that the presence of a radio-loud active galactic nucleus (AGN) is unlikely, and therefore we ascribe the emission observed in the public surveys to star-forming regions within the galaxy. We confirm and refine the redshift estimate, z = 0.341 ± 0.004, with a dedicated Telescopio Nazionale Galileo (TNG) spectroscopic observation. Finally, we compiled a list of all the known hosts of GRB afterglows detected in radio and computed the corresponding radio luminosity: if GRB 200716C belongs to J1304+2938, this is the third most radio-luminous host of a GRB, implying one of the highest star-formation rates (SFRs) currently known, namely SFR ∼ 324±61 M⊙ yr−1. On the other hand, through the analysis of the prompt emission light curve, recent works suggest that GRB 200716C might be a short-duration GRB located beyond J1304+2938 and gravitationally lensed by an intermediate-mass black hole (IMBH) hosted by the galaxy. Neither the public data nor our Very Long Baseline Interferometry (VLBI) observations can confirm or rule out the presence of an IMBH acting as a (milli-)lens hosted by the galaxy, a scenario still compatible with the set of radio observations presented in this work.

Kilonovae and Optical Afterglows from Binary Neutron Star Mergers. II. Optimal Search Strategy for Serendipitous Observations and Target-of-opportunity Observations of Gravitational Wave Triggers

In the second work of this series, we explore the optimal search strategy for serendipitous and gravitational-wave-triggered target-of-opportunity (ToO) observations of kilonovae and optical short-duration gamma-ray burst (sGRB) afterglows from binary neutron star (BNS) mergers, assuming that cosmological kilonovae are AT2017gfo-like (but with viewing-angle dependence) and that the properties of afterglows are consistent with those of cosmological sGRB afterglows. A one-day cadence serendipitous search strategy with an exposure time of ∼30 s can always achieve an optimal search strategy of kilonovae and afterglows for various survey projects. We show that the optimal detection rates of the kilonovae (afterglows) are ∼0.3/0.6/1/20 yr−1 (∼50/60/100/800 yr−1) for Zwicky the Transient Facility (ZTF)/Multi-channel Photometric Survey Telescope (Mephisto)/Wide Field Survey Telescope (WFST)/Large Synoptic Survey Telescope (LSST), respectively. A better search strategy for SiTian than the current design is to increase the exposure time. In principle, a fully built SiTian can detect ∼7(2000) yr−1 kilonovae (afterglows). Population properties of electromagnetic (EM) signals detected by serendipitous observations are studied in detail. For ToO observations, we predict that one can detect ∼11 yr−1 BNS gravitational wave (GW) events during the fourth observing run (O4) by considering an exact duty cycle of the third observing run. The median GW sky localization area is expected to be ∼10 deg2 for detectable BNS GW events. For O4, we predict that ZTF/Mephisto/WFST/LSST can detect ∼5/4/3/3 kilonovae (∼1/1/1/1 afterglows) per year, respectively. The GW detection rates, GW population properties, GW sky localizations, and optimistic ToO detection rates of detectable EM counterparts for BNS GW events at the Advanced Plus, LIGO Voyager, and ET&CE eras are detailedly simulated in this paper.

A long-duration gamma-ray burst with a peculiar origin.

It is generally believed that long-duration gamma-ray bursts (GRBs) are associated with massive star core collapse1, whereas short-duration GRBs are associated with mergers of compact star binaries2. However, growing observations3-6 have suggested that oddball GRBs do exist, and several criteria (prompt emission properties, supernova/kilonova associations and host galaxy properties) rather than burst duration only are needed to classify GRBs physically7. A previously reported long-duration burst, GRB 060614 (ref. 3), could be viewed as a short GRB with extended emission if it were observed at a larger distance8 and was associated with a kilonova-like feature9. As a result, it belongs to the type I (compact star merger) GRB category and is probably of binary neutron star (NS) merger origin. Here we report a peculiar long-duration burst, GRB 211211A, whose prompt emission properties in many aspects differ from all known type I GRBs, yet its multiband observations suggest a non-massive-star origin. In particular, substantial excess emission in both optical and near-infrared wavelengths has been discovered (see also ref. 10), which resembles kilonova emission, as observed in some type I GRBs. These observations point towards a new progenitor type of GRBs. A scenario invoking a white dwarf (WD)-NS merger with a post-merger magnetar engine provides a self-consistent interpretation for all the observations, including prompt gamma rays, early X-ray afterglow, as well as the engine-fed11,12 kilonova emission.

A Short Gamma-Ray Burst from a Protomagnetar Remnant

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.