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

N. Jordana-Mitjans Department of Physics, University of Bath, Claverton Down,
Bath, BA2 7AY, UK  C. G. Mundell Department of Physics, University of Bath,
Claverton Down, Bath, BA2 7AY, UK  C. Guidorzi Department of Physics and Earth
Science, University of Ferrara, via Saragat 1, I-44122, Ferrara, Italy  INFN –
Sezione di Ferrara, Via Saragat 1, 44122 Ferrara, Italy  INAF – Osservatorio
di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti 101, 40129
Bologna, Italy  R. J. Smith Astrophysics Research Institute, Liverpool John
Moores University, 146 Brownlow Hill, Liverpool, L3 5RF, UK  E. Ramírez-Ruiz
Department of Astronomy and Astrophysics, University of California, Santa
Cruz, CA 95064, USA  DARK, Niels Bohr Institute, University of Copenhagen,
Jagtvej 128, DK-2200, Copenhagen, Denmark  B. D. Metzger Columbia Astrophysics
Laboratory, Columbia University, New York, New York 10027, USA  Center for
Computational Astrophysics, Flatiron Institute, New York, NY 10010, USA  S.
Kobayashi Astrophysics Research Institute, Liverpool John Moores University,
146 Brownlow Hill, Liverpool, L3 5RF, UK  A. Gomboc Center for Astrophysics
and Cosmology, University of Nova Gorica, Vipavska 13, 5000 Nova Gorica,
Slovenia  I. A. Steele Astrophysics Research Institute, Liverpool John Moores
University, 146 Brownlow Hill, Liverpool, L3 5RF, UK  M. Shrestha Astrophysics
Research Institute, Liverpool John Moores University, 146 Brownlow Hill,
Liverpool, L3 5RF, UK  M. Marongiu INAF – Osservatorio Astronomico di Cagliari
- via della Scienza 5 - I-09047 Selargius, Italy  A. Rossi INAF – Osservatorio
di Astrofisica e Scienza dello Spazio, via Piero Gobetti 93/3, 40129 Bologna,
Italy  B. Rothberg LBT Observatory, University of Arizona, 933 N.Cherry
Ave,Tucson AZ 85721, USA  George Mason University, Department of Physics &
Astronomy, MS 3F3, 4400 University Drive, Fairfax, VA 22030, USA

###### 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 $L_{\rm th}\propto t^{-(2.22\pm 0.14)}$ 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.

High energy astrophysics — Time domain astronomy — Gamma-ray bursts —
Magnetars — Polarimetry

††facilities: Swift (BAT, XRT and UVOT), Fermi (GBM), Liverpool:2m (RINGO3 and
IO:O), LBT (LBC and MODS)††software: Matplotlib (Hunter, 2007), SciPy
(Virtanen et al., 2020), PyFITS (Barrett & Bridgman, 1999), Astropy (Astropy
Collaboration et al., 2013, 2018), Astropy Photutils (Bradley et al., 2016),
Astroalign (Beroiz et al., 2019), Xspec and PyXspec (v12.9.1; Arnaud 1996;
Arnaud et al. 1999), HEAsoft (v6.22.1; Blackburn 1995), RMFit (v4.3.2; Gamma-
ray astronomy Group 2014)

##  1 Introduction

Gamma-ray bursts (GRBs) are bright extragalactic flashes of gamma-ray
radiation and briefly the most energetic explosions in the Universe (Gehrels
et al., 2009). Their catastrophic origin —the merger of compact star binaries
for short-duration GRBs (Paczynski, 1986; Fryer et al., 1999; Tanvir et al.,
2013; Lee & Ramirez-Ruiz, 2007; Abbott et al., 2017b) or the collapse of
massive stars for long GRBs (Woosley, 1993; MacFadyen & Woosley, 1999; Bloom
et al., 1999)— drives the formation of a newborn compact remnant (black hole
or magnetar) that powers two highly relativistic jets. In the framework of the
standard fireball model and after the initial prompt gamma-ray emission (e.g.,
Mészáros & Rees 1997; Piran 1999), the relativistic ejecta are decelerated by
the circumburst medium by a pair of external shocks: a short-lived reverse
shock and a forward shock (e.g., Rees & Meszaros 1992; Sari & Piran 1999;
Kobayashi 2000). This lagging emission called the afterglow radiates via
synchrotron emission and can be detected seconds to years after the burst at
wavelengths across the electromagnetic spectrum (e.g., Costa et al. 1997; van
Paradijs et al. 1997; Gehrels et al. 2009).

Short GRBs represent the $9\%$ of the total detected by the Swift Burst Alert
Telescope (BAT; Lien et al. 2016), resulting in a significantly lower
frequency of real-time multiwavelength studies when compared to long GRBs.
Additionally, the optical counterparts of short GRBs are typically a few
hundred times fainter than those of massive star collapse origin (Kann et al.,
2011). This challenges the early follow-up and the study of short GRBs with
small and medium-sized telescopes, and limits the available data to several
hours post-burst, in the kilonovae time domain (Tanvir et al., 2013; Abbott et
al., 2017a; Troja et al., 2018).

Successful broadband follow-up of short GRBs began with the discovery of the
X-ray and optical afterglow of the GRB 050709 (Villasenor et al., 2005; Fox et
al., 2005; Hjorth et al., 2005), and first radio afterglow of the GRB 050724
(Berger et al., 2005). After these events, there have been numerous detections
of short GRB afterglows (e.g., Fong et al. 2015), including the first
detection of a kilonova (Tanvir et al., 2013; Berger et al., 2013) and the
joint discovery of the GW170817/GRB 170817A/kilonova, which confirmed that
binary neutron stars are progenitors of at least some short GRBs (Abbott et
al., 2017a, b; Tanvir et al., 2017; Goldstein et al., 2017). Still, the
remnant of neutron star binary mergers remains largely debated (Murguia-
Berthier et al., 2014, 2021; Ruiz & Shapiro, 2017; Metzger et al., 2018;
Metzger, 2019; Margalit & Metzger, 2019; Mösta et al., 2020; Beznogov et al.,
2020). More recently, giant flares from extragalactic magnetars have been
associated as sources of low-luminosity short-duration GRBs (Roberts et al.,
2021; Svinkin et al., 2021; Fermi-LAT Collaboration et al., 2021).

Here, we present the early-time multiwavelength observations and polarization
constraint of the GRB 180618A, a short GRB with extended emission (e.g.,
Norris & Bonnell 2006). So far, the only polarization measurement of a short
GRB optical counterpart has been the $P=0.50\%\pm 0.07\%$ detection in the
GW170817 kilonova at $\approx 1.5\,$days after the merger (Covino et al.,
2017) —consistent with polarization from the Galactic dust. Short GRBs with
extended soft gamma-ray emission are rarely studied at lower frequencies
(Perley et al., 2009; Knust et al., 2017), as they are a small fraction of the
total detected by the BAT ($\approx 1\%$; Lien et al. 2016). Such elusive
objects are merger candidates and display the typical short-hard prompt gamma-
ray emission followed by variable soft gamma-ray emission spanning 10$-$100 s
(Norris & Bonnell, 2006; Perley et al., 2009; Hu et al., 2014; Kaneko et al.,
2015). Candidate mechanisms powering such extended gamma-ray emission after
the merger include late-time activity from the central engine (Metzger et al.,
2008; Bucciantini et al., 2012), interaction with a pulsar-wind cavity
(Ramirez-Ruiz et al., 2019), prolonged accretion from the gravitationally-
bound material ejected pre-merger (Rosswog, 2007; Lee & Ramirez-Ruiz, 2007;
Lee et al., 2009), or a two-component outflow viewed slightly off-axis (Barkov
& Pozanenko, 2011).

This work is structured as follows. In Section 2, we present the GRB 180618A
optical observations and data reduction of the UltraViolet and Optical
Telescope, and the 2-m Liverpool Telescope —including the RINGO3
multiwavelength polarimeter/imager and the IO:O camera. As well, we detail the
8.4-m Large Binocular Telescope observations; that is, deep-field imaging with
the Large Binocular Cameras and spectroscopy of the host galaxy candidates
with the Multi-Object Double Spectrographs. In Section 3, the properties of
the optical and gamma-ray emission are presented. In Section 4, the physical
origin of the peculiar multiwavelength emission of the GRB 180618A is
discussed —in particular, the optical emission. In Section 5, the implications
of the GRB 180618A results are discussed in the wide context of neutron star
mergers. In Section 6, we summarize our findings.

We assume flat $\Lambda$CDM cosmology $\Omega_{m}=0.32$,
$\Omega_{\Lambda}=0.68$ and $h=0.67$, as reported by Planck Collaboration et
al. (2020). We adopt the convention $F_{\nu}\propto t^{-\alpha}\nu^{-\beta}$,
where $\alpha$ is the temporal index and $\beta$ is the spectral index. Note
that the spectral index is related to the photon index like $\beta=\beta_{\rm
PI}-1$. Unless stated otherwise, all uncertainties reported in this paper are
given at $1\sigma$ confidence level.

##  2 Observations and Data Reduction

On 18 June 2018 at T0=00:43:13 Universal Time (UT), the BAT from the Neil
Gehrels Swift Observatory triggered an alert for the GRB 180618A (Meegan et
al., 2009; LaPorte et al., 2018). The GRB 180618A was detected by the BAT
(Barthelmy et al., 2005; Sakamoto et al., 2018), the Gamma-ray Burst Monitor
(GBM; Meegan et al. 2009; Hamburg et al. 2018), and the Konus instrument from
the Wind satellite (Aptekar et al., 1995; Svinkin et al., 2018) as a short-
duration and spectrally-hard bright GRB with a long-duration weak emission
tail at low gamma-ray energies (see Figure 1). Further GRB 180618A detections
include the Astrosat Cadmium Zinc Telluride Imager (CZTI; Singh et al. 2014;
Sharma et al. 2018), and the Insight Hard X-ray Modulation Telescope (HXMT;
Liu et al. 2020; Song et al. 2022).

###  2.1 Ultraviolet and Optical Light Curves

At $86\,$s after the detection of the GRB 180618A by the BAT, the UltraViolet
and Optical Telescope (UVOT; Roming et al. 2005) from the Neil Gehrels Swift
Observatory started optical observations in an unfiltered band (white).
Subsequently, the UVOT continued observations with the uvw1,uvm2,uvw2
ultraviolet and the v,b,u optical filters (see Figure 2). At $202.5\,$s after
the BAT alert, the 2-m fully robotic Liverpool Telescope (LT; Steele 2004)
—with site at Roque de Los Muchachos Observatory (ORM, Spain)— automatically
started follow-up observations (Guidorzi et al., 2006) with the RINGO3 three-
band polarimeter and imager (Arnold et al., 2012). The LT observations
consisted of three consecutive observing sequences of $10\,$minutes each with
the RINGO3 in three simultaneous bands (BV,R,I), followed by six single
$10\,$s exposures with the $r$ filter of the IO:O optical widefield
camera111https://telescope.livjm.ac.uk/TelInst/Inst/IOO/, and two extra
observing sets of $10\,$minutes with the RINGO3. Three $300$-s exposures with
the IO:O g,r,i filters were also scheduled via the LT
phase2UI222https://telescope.livjm.ac.uk/PropInst/Phase2/, and autonomously
executed by the LT at $7.4\times 10^{4}\,$s post-burst.

Figure 1: The GRB 180618A light curves at $64\,$ms resolution as detected by
the BAT and the GBM instruments. (a) Count rate per enabled detector of the
15$-$350 keV energy band of the BAT. (b) Count rates of the 8$-$900 keV energy
band of the GBM sodium iodide (NaI) detector, and the 200 keV$-$40 MeV of the
GBM bismuth germanate (BGO) detector. In a different y-axis, we present the
evolution of the peak energy ($E_{\rm peak}$); the values were derived from a
cutoff power-law model fit to the 8 keV$-$40 MeV $\nu F_{\nu}$ spectrum. In
the x-axis, the T0 corresponds to the BAT trigger time. Table 1: The GRB
180618A ultraviolet and optical photometry corresponding to the Swift UVOT
white,v,b,u,uvw1,uvm2, uvw2 bands, LT RINGO3 BV,R,I bands and LT IO:O g,r,i
bands.

Band | t${}_{\rm mid}-$T0 | t${}_{\rm exp}/2$ | mag | mag${}_{\rm\,err}$ | Fν | F${}_{\nu\,\rm{err}}$
---|---|---|---|---|---|---
| (s) | (s) |  |  | (Jy) | (Jy)
white | 91 | 5 | 17.32 | 0.13 | 2.27e-04 | 2.7e-05
white | 101 | 5 | 17.48 | 0.14 | 1.96e-04 | 2.6e-05
white | 111 | 5 | 17.34 | 0.13 | 2.23e-04 | 2.6e-05
white | 121 | 5 | 17.44 | 0.13 | 2.04e-04 | 2.5e-05
white | 131 | 5 | 17.51 | 0.14 | 1.91e-04 | 2.5e-05
white | 141 | 5 | 17.45 | 0.13 | 2.02e-04 | 2.5e-05
white | 151 | 5 | 17.39 | 0.13 | 2.13e-04 | 2.5e-05
white | 161 | 5 | 17.40 | 0.13 | 2.11e-04 | 2.5e-05
white | … | … | … | … | … | …

Note. — The tmid corresponds to the mean observing time, the T0 is the BAT
trigger time, and the texp is the length of the observing time window. Note
that the photometry is not corrected by either Galactic, i.e. with reddening
E(${B-V}$)${}_{\rm MW}=0.065\pm 0.003$ (Schlegel et al., 1998), or host galaxy
extinction. Table 1 is published in its entirety in machine-readable format. A
portion is shown here for guidance regarding its form and content.

Figure 2: The GRB 180618A light curves at gamma-ray, X-ray, ultraviolet and
optical bands. The data corresponds to the Swift BAT, Swift XRT, Swift UVOT
white,v,b,u,uvw1,uvm2,uvw2 bands, LT RINGO3 BV,R,I bands and LT IO:O g,r,i
bands. The Swift BAT and XRT observations were obtained from the web interface
provided by the Leicester University (Evans et al., 2009); the BAT data were
binned to a signal-to-noise of 7 and the absorbed 0.3$-$10 keV XRT light curve
was converted to observed flux density at 1 keV. For completeness, we include
the optical observations and upper limits reported in the Gamma-ray
Coordination Network (GCN) from the MASTER II (Tyurina et al., 2018), Tien
Shan Astronomical Observatory (Mazaeva et al., 2018) and Xinglong-2.16m (Zhu
et al., 2018). Note that the GCN observations do not include filter
corrections. In the x-axis, the T0 corresponds to the BAT trigger time. In the
y-axis, the flux density is converted to the RINGO3 R magnitude. Detections
have $1\sigma$ error bars, and non-detections are presented as $3\sigma$ upper
limits —note that the MASTER data are $5\sigma$ upper limits. In the bottom
panel, we present the temporal evolution of the photon index in the GBM 8
keV$-$40 MeV gamma-ray band, the BAT 15$-$150 keV gamma-ray band, the XRT
0.3$-$10 keV X-ray band, and the RINGO3 1.2$-$3.5 eV optical band. Figure 3:
Observing fields of the GRB 180618A sky region. The GRB 180618A location is
labelled in red, the stars (S) in light brown, and the galaxies (G) in black.
(a) The 2-m LT $I$-band RINGO3 image of the GRB 180618A optical transient. (b)
The $r$-band LBC image from the 8.4-m LBT. (c) The LBT $z$-band image. The
field of view of the magnified LBT images corresponds to the white rectangle
of panel a. The LBT images reveal three galaxies at a similar redshift near
the UVOT sub-arcsec localization of the GRB 180618A (shown in red at $90\%$
confidence level). The G1 is the host galaxy of the short GRB 180618A, with
spectroscopic redshift $z=0.554\pm 0.001$.

Figure 4: The RINGO3 photometric analysis of Figure 3 field stars (S) using
the temporal binning of the GRB 180618A light curves. Note that the stars S1
and S2 are not included in the analysis because the LT repointed to the GRB
180618A coordinates after the IO:O observations, and they fall outside of the
revised RINGO3 field of view. From left to right, panels correspond to the
RINGO3 BV, R, and I bands. Figure 5: From top to bottom, the two-dimensional
MODS-2 spectra of the galaxy G2 and the GRB 180618A host galaxy (G1), the
corresponding night-sky emission lines, and the one-dimensional spectrum of
the G1 galaxy. In the G1 spectrum, we identify two oxygen emission lines
(corresponding to [OII] and [OIII]) redshifted at $z=0.554\pm 0.001$. The
night-sky spectrum displayed here demonstrates that both oxygen emission lines
(marked in solid red arrows) are not unsubtracted night-sky emission lines;
the [OII] line is not coincident with any night-sky line, and the [OIII] line
is found between two (dotted red lines).

The UVOT photometry (Vega; see Table 1) was derived from Level 1/2 products,
which are already pre-processed by the telescope pipeline. We used the Level 1
event products to get higher temporal resolution. They were converted into
sky-coordinated data with the coordinator tool from the HEASoft v6.22.1
package (Blackburn, 1995), and the hot pixels were removed with uvotscreen.
The photometry was background-subtracted and measured with uvotevtlc using the
default aperture radius of 5 arcsec from the instrument calibration (Poole et
al., 2008; Breeveld et al., 2011). For the Level 2 images, we used the
equivalent uvotsource tool. Furthermore, the images were aligned with
uvotskycorr and co-added with uvotisum, requiring a minimum significance of
$3\sigma$ for the detection of the optical transient. Those stacked frames in
which the optical transient did not reach the signal-to-noise threshold are
reported as $3\sigma$ flux upper limits in Table 1.

The RINGO3 photometry (Vega; see Table 1) was derived by integrating the
source photon counts across the eight polaroid positions (e.g. Jordana-Mitjans
et al. 2020) —thus cancelling any polarization signal. Each $10\,$minutes
integration consists of ten single 1-minute exposures from which we
individually derived the photometry using the Astropy Photutils package
(Bradley et al., 2016). If the signal-to-noise did not reach a minimum
$3\sigma$ significance, we co-added consecutive frames. The RINGO3 magnitudes
and flux density were absolute-calibrated in Vega system following a standard
procedure (e.g., see Jordana-Mitjans et al. 2020) with observations of five
dereddened A0 type stars (BD +30 2355, BD +67 675, BD +25 2478, HD 96781, HD
208368; Høg et al. 2000). The standard stars and the GRB 180618A field
observations were scheduled via the LT phase2UI using the same instrumental
setup of the night of the burst, and they were executed on 13 and 16 June
2019. This spectral calibration added $\approx 0.07\,$mag uncertainty to the
photometry. To test for the instrument stability during observations, we
checked the flux variability of five stars in the GRB 180618A field of view
(see Figure 4). Using the temporal binning of the GRB 180618A light curves,
the stars displayed on average a $\approx 0.04\,$mag deviation from the mean.

The IO:O camera photometry (AB; see Table 1) was derived for each of the
$10$-s individual frames. For the observing sequence at $7.4\times 10^{4}\,$s
post-burst, we integrated the observations into a single $900\,$s exposure per
band. The optical transient was not detected, and the flux upper limits are
presented in Table 1 at $3\sigma$ confidence level. We calibrated the IO:O
bands by cross-matching ten 12$-$17 mag stars from the Sloan Digital Sky
Survey (SDSS) Data Release 12 catalogue (Alam et al., 2015).

###  2.2 Optical Polarimetry

In the RINGO3 configuration, we measure the polarization of a source by
extracting the flux at each of the eight polaroid positions (e.g. Jordana-
Mitjans et al. 2020), which we then convert to Stokes parameters ($q-u$)
following Clarke & Neumayer 2002. The polarization uncertainties are derived
from a Monte Carlo error propagation, starting from $10^{6}$ flux values.

We measured the instrumental $q-u$ using $\approx 75$ measurements of seven
unpolarized standards per band (BD +32 3739, BD +33 2642, BD +28 4211, HD
212311, HD 14069, HD 109055, G191B2B; Turnshek et al. 1990; Schmidt et al.
1992), which were observed during 200 days before and 10 days after the date
of the burst. Note that we used an asymmetric time window of data given a
small shift of $\Delta u\approx 0.005$ in the instrumental $u$ parameter after
10 days post-burst. For the chosen time window, there was no significant drift
of the instrumental $q-u$ and the Pearson’s correlation coefficients were low
$|r|<0.1$ with $p$-values$>0.3$.

To derive the most constraining polarization measurement for the GRB 180618A,
we used the entire $10\,$minutes epoch corresponding to t${}_{1}=203-800\,$s
post-burst of the BV band, which is the RINGO3 band with the highest signal-
to-noise. We detected the optical transient at a signal-to-noise $\approx 27$
in each of the eight images of the polaroid. At this signal-to-noise level,
the observed polarization ($P\approx 1\%$) was within the instrument
sensitivity, and we estimate a $2\sigma$ upper limit of $P_{BV}<6.1\%$. Due to
the slowly fading emission during the second and third observing epoch, at
t${}_{2}=822-1417\,$s and t${}_{3}=1438-2035\,$s post-burst respectively, the
polarization upper limits could still be derived but were less well
constrained, with $P_{BV,\,{\rm\\{t_{2},t_{3}\\}}}<10.7\%,17.0\%$ ($2\sigma$).
For the R and I bands, the $2\sigma$ upper limits for the three epochs are
$P_{R,\,{\rm\\{t_{1},t_{2},t_{3}\\}}}<14.5\%,30.2\%,37.0\%$ and
$P_{I,\,{\rm\\{t_{1},t_{2},t_{3}\\}}}<23.5\%,36.8\%,38.6\%$. We note that the
RINGO3 depolarization factor (Jordana-Mitjans et al., 2021) is negligible in
the BV band (D${}_{BV}=1$) and small in the R and I bands
(D${}_{\\{R,I\\}}=0.98,0.94$).

###  2.3 LBT Photometry

To search for the GRB 180618A host galaxy, and thus determine the GRB 180618A
redshift, we used the Large Binocular Telescope (LBT; Hill et al. 2006) —an
optical/infrared telescope with twin 8.4-m mirrors located on the Mt. Graham
International Observatory, Arizona, USA.

On 5 April 2022, deep-field $r$\- and $z$-band imaging of the GRB 180618A
location were acquired with the Large Binocular Cameras (LBC; Ragazzoni et al.
2000; Giallongo et al. 2008). The total exposure time for each filter was 36
minutes and the data were reduced with the data reduction pipeline developed
by the Istituto Nazionale di Astrofisica (INAF-Osservatorio Astronomico di
Roma; Fontana et al. 2014), which includes bias subtraction and flat-fielding,
bad pixel and cosmic ray masking, astrometric calibration, and coaddition. The
average seeing was $\approx 1.4\,$arcsec and the mean airmass of the
observations was $\approx 1.4$. The LBC photometry (AB) achieved limiting
magnitudes of $r_{\rm lim}=26.3\,$mag and $z_{\rm lim}=25.6\,$mag ($3\sigma$
limits). In both images, we found three galaxies (G1, G2, and G3; see Figure
3) at a projected angular distance of $d_{\rm G1}=1.6\,$arcsec, $d_{\rm
G2}=3.7\,$arcsec, and $d_{\rm G3}=7.7\,$arcsec from the UVOT sub-arcsec
localization of the GRB 180618A (Siegel et al., 2018). The G3 galaxy was
already catalogued by the Sloan Digital Sky Survey (SDSS; Alam et al. 2015),
with a photometric redshift of $z_{\rm G3}=0.54\pm 0.03$, and brightness
$r_{\rm band,G3}=22.4\pm 0.3\,$mag and $z_{\rm band,G3}=21.2\pm 0.6\,$mag. The
G1 and G2 galaxies were uncatalogued in SDSS, but identified in the O’Connor
et al. (2022) and Fong et al. (2022) surveys of short GRBs host galaxies. From
the LBC images, we measured $r_{\rm band,G1}=22.98\pm 0.06\,$mag, $r_{\rm
band,G2}=23.58\pm 0.11\,$mag in the $r$ band, and $z_{\rm band,G1}=22.62\pm
0.10\,$mag, $z_{\rm band,G2}=22.48\pm 0.09\,$mag in the $z$ band.

###  2.4 LBT Spectroscopy

On 8 April 2022, optical spectroscopy of the G1 and G2 galaxies was obtained
with the Multi-Object Double Spectrographs (MODS; Pogge et al. 2010), i.e.
MODS-1 and MODS-2. Each MODS contains a red and blue channel for spectroscopy.
Both MODS were configured to use the Dual Grating mode
($0.32\,\mu\textnormal{m}-1.05\,\mu$m coverage) and a 1.2 arcsec wide slit
($R\approx 630-1350$ resolution). A position angle $179.3^{\circ}$ was used
for observations. The position angle was selected so that the target galaxies
and the foreground star ($\approx$47 arcsec away) were vertically aligned. The
foreground star was aligned in the centre of the slit as the target galaxies
were too faint to be detected in the acquisition images without incurring too
large of an overhead. Two exposures of $20\,$minutes each were obtained in
each channel, with each MODS. The mean airmass of the observations was
$\approx 1.85$. The mean seeing (as measured from the off-axis wavefront
sensor and guiders) was 1.92$\pm$0.05 arcsec and 1.84$\pm$0.04 arcsec for
MODS-1 and MODS-2, respectively. Observations of the spectrophotometric star
BD+33 2642 were used to flux calibrate the data and remove the instrumental
signatures from the data.

The MODS data were reduced first with the modsCCDRed v2.04 package developed
by the MODS team (Pogge, 2019) to remove the bias and flat-field the data
using a slitless pixel flat. Next, custom IRAF scripts (Tody, 1986) were used
to extract along the central slit using a stellar trace. The observations of
the spectrophotometric standard were combined to measure the trace of the
dispersion along the entire slit. This trace was used along with the
wavelength calibration from arc-lamp lines to rectify the tilt in both x and y
directions for the full frame (8192 pixels $\times$ 3072 pixels). This step
made the x-axis parallel to the dispersion direction and the y-axis purely
parallel with the spatial extent along the slit. Final wavelength calibration
was cross-checked with known strong auroral skylines in the blue ([OI]
$\lambda=5577.3\,$Å) and red ([OI] $\lambda=6300.3\,$Å) channels. One-
dimensional spectra were then extracted from each channel using a
$1.85\,$arcsec wide aperture. This value was chosen to match the mean seeing
of the observations, maximizing the signal-to-noise. Next, the spectra were
flux-calibrated using the spectro-photometric standard star. Telluric features
were removed from the red channels using a normalized spectro-photometric
standard spectrum.

Inspection of the data showed no flux from the galaxies in the blue channels
of both MODS. A faint but significant continuum was detected for the galaxies
in MODS-2 (see Figure 5), but not in MODS-1. The absence of flux in MODS-1 is
consistent with a known technical
issue333https://scienceops.lbto.org/mods/preparing-to-observe/sensitivity/ in
which the sensitivity of the instrument has decreased by a factor of 1.6 since
the 2011 commissioning. As a sanity check, the acquisition images were re-
checked to confirm no differences in the alignment of the foreground star. The
continua of the target galaxies are clearly visible in the individual
background-subtracted exposures for the red channel of MODS-2 (see Figure 5).
The spatial position (y-direction) of the continua from the target galaxies
detected in the MODS-2 red channel exactly matched the angular separation
between them and the foreground alignment star —as measured from the LBC
images (see Figure 3). The final calibrated MODS-2 red channel data have a
fixed instrumental resolution of 8.19 Å per resolution element covering
$5600-10100\,$Å, which corresponds to a resolution of $R\approx 630-1350$ and
0.84 Å/pixel. These values were confirmed using the arc lamps.

##  3 Results

In Section 3.1, we present the temporal and spectral analysis of the optical
emission. In Section 3.2, we estimate the redshift of the GRB 180618A using
the ultraviolet and optical photometry of the transient emission, and we then
associate the GRB 180618A with its host galaxy. In Sections 3.3-3.5, we study
the gamma-ray properties of the short GRB 180618A and its extended gamma-ray
emission.

###  3.1 Optical Emission

We simultaneously fitted the UVOT white, RINGO3 BV,R,I and IO:O r optical
light curves with smoothly-connected broken power laws (Beuermann et al.,
1999; Molinari et al., 2007), i.e. $F=F_{0}[(t/t_{\rm
break})^{n\alpha_{1}}+(t/t_{\rm break})^{n\alpha_{2}}]^{-1/n}$, fixing the
time break across bands and the smoothness parameter to $n=1$ for convergence
(Piranomonte et al., 2008). This serves to help us understand the overall
decay rate of the emission, as well as to check for colour evolution in the
residuals of the best-fitting model. The emission initially decays with
$\alpha_{\rm opt,1}=0.46\pm 0.02$ and suffers a sharp break at $t_{\rm
break}=2120\pm 60\,$s post-burst, with $\alpha_{\rm opt,2}=4.6\pm 0.3$ (see
Figure 6).

Figure 6: The RINGO3 BV,R,I bands and the UVOT white, u bands data modelled
with smoothly-connected broken power laws that have a common time break across
bands. The results of the fit are a break at $t_{\rm break}=2120\pm 60\,$s
post-burst, power-law indexes $\alpha_{1}=0.46\pm 0.02$ pre-break and
$\alpha_{2}=4.6\pm 0.3$ post-break with $\chi^{2}/$dof$=128/116$. Also, we
show with a dotted line the best-fitting model normalized to the near-
ultraviolet UVOT uvw1 band, which has a power-law decay $\alpha_{uvw1}>1.7$,
pre-break. Detections have $1\sigma$ error bars, and non-detections are
presented as $3\sigma$ upper limits. Figure 7: Modelling of the GRB 180618A
RINGO3 optical emission with power-law models. Note that the model accounts
for Galactic dust extinction, but does not include the host galaxy dust
contribution.

This extreme flat-to-steep decay evolution is rare in GRB light curves. To our
knowledge, the only other GRB that has shown a similar flat-to-steep decay
rate transition ($\alpha_{\rm opt,1}=0.44^{+0.08}_{-0.21}$ to $\alpha_{\rm
opt,2}=5.3\pm 0.2$) was found in the fainter optical emission of the short GRB
070707 (Piranomonte et al., 2008) and at a much later time at ${\approx
1.8\,}$days post-burst. In addition, in GRB 180618A the spectral evolution
across the break is chromatic; the RINGO3 I band emission is significantly
underestimated by $>3\sigma$ after the break, and the normalized best-fitting
model overestimates the $3\sigma$ photometric upper limit of the near-
ultraviolet UVOT uvw1 band. Furthermore, we individually fitted the flux decay
rate of the RINGO3 BV, R, I and IO:O r bands after the break with a power law,
finding moderate slopes for redder bands, i.e. $\alpha_{{\rm
opt},\\{BV,r,R,I\\}}=4.6\pm 0.5,3.5\pm 0.5,3.2\pm 0.1,1.4\pm 0.2$.

Using Xspec v12.9.1 (Arnaud, 1996) and $\chi^{2}$ statistics, we modelled the
RINGO3 data with a power law that accounts for the Milky Way dust extinction,
i.e. E(${B-V}$)${}_{\rm MW}=0.065\pm 0.003$ (Schlegel et al., 1998). We find
that the optical photon index significantly evolves during observations, from
$\beta_{\rm opt,PI}=0.7\pm 0.4$ at $t=202-263\,$s post-burst to $\beta_{\rm
opt,PI}=4.0\pm 0.8$ at $t=2990-3586\,$s post-burst; see results in Figure
2-bottom panel, Figure 7 and Table 2. Note that if we include the UVOT data,
we need to add the host galaxy dust extinction to the model, which shifts all
the optical photon indexes by the same amount towards harder values (i.e. the
relative evolution of the $\beta_{\rm opt,PI}$ remains the same).

###  3.2 Redshift and Host Galaxy

We rule out a high redshift origin given the detection of the GRB 180618A
optical counterpart with all the UVOT filters (Krühler et al., 2011). We used
the uvot2pha tool to convert the UVOT data to Xspec spectral files, and a
dust-absorbed power law that includes both Milky Way and host galaxy
contributions, i.e. E($B-V$)HG. Taking into account the spectral coverage up
to the far-ultraviolet of the UVOT uvw2 filter and using the redshifted Lyman-
limit break at $\lambda_{\rm obs}=912(1+z)\,$Å, we fitted the co-temporal UVOT
and RINGO3 data corresponding to $550-1600\,$s post-burst using $\chi^{2}$
statistics. We estimate a redshift $z<1.5$ at $2\sigma$ confidence level from
the E($B-V$)${}_{\rm HG}-\beta_{\rm PI}-z$ parameter space (see Figure 8-a).
Note that not knowing the GRB 180618A intrinsic spectral slope does not affect
the redshift constraint but adds large uncertainty in determining the host
galaxy dust contribution. See also the best-fitting models for the redshifts
$z=0,1,2$ in Figure 8-b.

Figure 8: Estimation of the GRB 180618A redshift from the cotemporal GRB
180618A UVOT and RINGO3 data modelled with a dust-absorbed power law that
includes the redshifted Lyman-limit break. (a) The $\chi^{2}$ distribution of
the best-fitting redshifts for different values of the host galaxy extinction
E($B-V$)HG and photon index $\beta_{\rm PI}$. The confidence level contours at
$2\sigma$ level indicate a redshift $z<1.5$ for the GRB 180618A. (b) Best-
fitting models for redshifts $z=0,1,2$.

Among all three candidate galaxies detected in the LBT images, G1 is the most
likely host galaxy of GRB 180618A given its proximity (see Figure 3). In G1
galaxy spectrum (see Figure 5), we detect two emission lines at $\lambda_{\rm
obs,OII}=5794.8\,$Å and $\lambda_{\rm obs,OIII}=7784.0\,$Å, corresponding to
the unresolved [OII] $\lambda=3726-3729\,$Å doublet and the [OIII]
$\lambda=5007\,$Å line, respectively. That implies a spectroscopic redshift
$z_{\rm G1}=0.554\pm 0.001$ for the G1 galaxy. This value is consistent with
O’Connor et al. (2022) and Nugent et al. (2022) estimates of photometric
redshifts $z=0.4^{+0.2}_{-0.1}$ and $z=0.52^{+0.09}_{-0.11}$, respectively. We
find that the probability of an accidental alignment (Bloom et al., 2002;
Berger, 2010; Fong & Berger, 2013) of the GRB 180618A and G1 galaxy is low,
with $p_{\rm d}\approx 0.02$. Therefore, we associate the G1 redshift
($z=0.554$) with the GRB 180618A. For the G2 galaxy spectrum, we do not detect
emission lines. However, there is a clear drop of the continuum flux bluewards
of $\lambda_{\rm obs}\approx 6200\,$Å that we identify as the
$\lambda=4000\,$Å break, corresponding to a redshift $z_{\rm G2}\approx 0.55$.
Therefore, the LBT spectroscopic analysis suggests that all three galaxies
(G1, G2, and G3) are at a similar redshift.

The GRB 180618A lies in the outskirts of its host, at $10\,$kpc from the
centre of the galaxy (see also O’Connor et al. 2022; Fong et al. 2022). This
is consistent with the large offsets found in short GRBs, and in disagreement
with that of long GRBs (Berger, 2010; Fong & Berger, 2013; Behroozi et al.,
2014; Fong et al., 2022). Note that short GRBs are usually found with offsets
to star-forming disk galaxies or even further away from their elliptical host
(Fong & Berger, 2013; Behroozi et al., 2014). Similar to the environment of
the GRB 180168A, Fong & Berger (2013) found that about $30\%-45\%$ of short
GRBs happen where there is no optical light, i.e. negligible stellar mass.
Like GRB 180618A, most short GRBs display signs of migration from their birth
sites, likely due to natal kicks in binaries (Rosswog et al., 2003; Kelley et
al., 2010; Fong & Berger, 2013). This is consistent with short GRBs exploding
in low ambient density, thus producing fainter afterglows (Berger, 2010; Kann
et al., 2011; Fong et al., 2015).

###  3.3 Duration of the Prompt Gamma-ray Emission

We calculated the duration ($T_{90}$) of GRB 180618A (Lien et al., 2016; Moss
et al., 2022), corresponding to the time interval in which $90\%$ of the burst
fluence is released, using the 64-ms binned and background-subtracted GRB
180618A light curves of the BAT. Using the $battblocks$ tool (Scargle, 1998),
a FTOOLS released as part of HEASoft (Blackburn, 1995), we find $T_{90}=45\pm
10\,$s for the low-energy spectral range of the BAT (i.e. 15$-$100 keV), and
$T_{90}=0.26\pm 0.14\,$s at 100$-$350 keV. The duration of GRB 180618A at the
low-energy bands is two orders of magnitude higher than at high-energy bands,
confirming two spectral components: a short-hard GRB and extended soft gamma-
ray emission below the 100 keV energy band (Hamburg et al., 2018; Sakamoto et
al., 2018; Svinkin et al., 2018). We find that the 0.3-s short-duration GRB
180618A belongs to the hardness-duration cluster of short GRBs, and that it is
one of the hardest detected by the BAT (within the top $\approx 0.5\%$; see
Figure 9-a). Note also that Sakamoto et al. (2018) found negligible spectral
lag for the short-duration gamma-ray pulse of GRB 180618A —a spectral property
typical of short-hard GRBs.

Furthermore, GRB 180618A is a classically short GRB in terms of the duration
reported in the GBM GRB catalogue (von Kienlin et al., 2020), with $T_{\rm
90}(50-300\,{\rm keV})=3.7\pm 0.6\,$s. For the GBM, von Kienlin et al. (2020)
found that the threshold separating short and long GRBs is $T_{\rm 90}=4.2\,$s
—instead of the $T_{\rm 90}\approx 2\,$s of the Burst And Transient Source
Experiment (BATSE; Kouveliotou et al. 1993).

Figure 9: High-energy properties of the GRB 180618A and comparison with
catalogue GRBs; those with duration $T_{\rm 90}>2\,$s are displayed in grey,
and those with $T_{\rm 90}<2\,$s in blue (Kouveliotou et al., 1993). (a) The
BAT band hardness ratio and duration of the GRB 180618A ($t_{1}$), the
$\approx 0.3\,$s short gamma-ray pulse ($t_{2}$), and its $\approx 45\,$s
extended gamma-ray emission ($t_{3}$). The background data corresponds to the
short-hard and long-soft bimodal clustering of the BAT GRB catalogue
(Kouveliotou et al., 1993; Lien et al., 2016). (b) Comparison of the $\approx
0.3\,$s short GRB 180618A flux F and the fluence S (marked in orange) with the
GRB sample from the GBM catalogue (von Kienlin et al., 2020; Poolakkil et al.,
2021). Note that the GRB 180618A flux and fluence have been recalculated to
match the 10$-$103 keV energy range of the GBM catalogue. (c) The photon index
$\beta_{\rm PI}$ and the peak energy $E_{\rm peak}$ of the GRB 180618A (marked
in orange).

###  3.4 Spectral Properties of the Prompt Gamma-ray Emission

We performed a Swift-BAT and Fermi-GBM joint spectral fit in
RMFit444https://fermi.gsfc.nasa.gov/ssc/data/analysis/rmfit/ v4.3.2. We used
the GBM time-tagged events (TTE) data from the NaI 3/4 and BGO 0/1 detectors,
from which we respectively selected the 8$-$900 keV and 200 keV$-$40 MeV
spectral regimes. The 15$-$150 keV BAT spectra were extracted with the
$batbinevt$ tool. Using C-statistics, we modelled the $\nu F_{\nu}$ spectrum
of the $\approx 0.3\,$s short GRB with a simple power law. A power law model
overestimates the data over $\approx 1\,$MeV; consequently, we modelled the
high-energy break with a cutoff power-law model (e.g., see Poolakkil et al.
2021). The best-fitting model suggests that the $\nu F_{\nu}$ spectrum has a
hard slope with mean photon index $\beta_{\rm PI}=-\alpha_{\rm CL}=0.63\pm
0.05$, peak energy $E_{\rm peak}=2290\pm 238\,$keV, and fluence
$S(10-10^{4}\,{\rm keV})=(5.6\pm 0.4)\times 10^{-6}\,$erg cm-2. The photon
index is average compared to other short-hard GRBs detected by the GBM (von
Kienlin et al., 2020; Poolakkil et al., 2021). Yet, the GRB 180618A is one of
the hardest and most energetic gamma-ray pulses among short GRBs; it is within
the $\approx 1\%$ percentile in terms of high-frequency peak energy and within
the top $\approx 5\%$ in total energy released (see Figs. 9-b,c).

For the spectrum corresponding to the $\approx 0.3-45\,$s extended gamma-ray
emission, the best-fitting photon index of a power-law model is $\beta_{\rm
PI}=1.51\pm 0.09$. This intermediate photon index suggests that the spectrum
has a cutoff (e.g., Poolakkil et al. 2021). In order to constrain the peak
energy, we fixed the low-energy index of the cutoff power-law model to the
average of the GBM catalogue (Poolakkil et al., 2021) of short ($\alpha_{\rm
CL}=-0.6$) and long GRBs ($\alpha_{\rm CL}=-1$). We find $E_{\rm peak}=87\pm
18\,{\rm keV}$ and $S(10-10^{4}\,{\rm keV})=(6\pm 1)\times 10^{-7}\,$erg cm-2
for $\alpha_{\rm CL}=-0.6$, and $E_{\rm peak}=125\pm 45\,{\rm keV}$ and
$S(10-10^{4}\,{\rm keV})=(8\pm 2)\times 10^{-7}\,$erg cm-2 for $\alpha_{\rm
CL}=-1$, consistent with Svinkin et al. (2018) findings.

To determine the temporal evolution of the photon index and the peak energy,
we fitted a cutoff power-law model to the 8 keV$-$40 MeV GBM spectrum of each
light curve bin. To avoid merging peaks and valleys, we used a constant
binning as opposed to a fixed signal-to-noise (e.g., Guiriec et al. 2010).
Using the default $64$-ms binned light curves, we find that in a time span of
less than $260\,$ms, the peak energy increases from $E_{\rm peak}=427\pm
138\,$keV to $E_{\rm peak}=2593\pm 473\,$keV with constant photon index. As
seen in other spectroscopically-resolved short GRBs (Guiriec et al., 2010),
the peak energy tracks the light curve with a strong soft-hard-soft spectral
evolution in a short time period; see the peak energy evolution in Figure 1-b
and the photon index in Figure 2-bottom panel.

###  3.5 Intrinsic Energetics

Short GRBs have typically lower fluences and thus follow a different peak
energy ($E_{\rm peak}$) and isotropic energy ($E_{\rm iso}$) relation than
long GRBs (Amati, 2006; Ghirlanda et al., 2009). If we introduce the
k-correction (Bloom et al., 2001) to the 1$-$104 keV rest-frame energy band
for redshift $z=0.554$, we obtain an isotropic-equivalent energy $E_{\rm
iso}=(4.6\pm 0.4)\times 10^{51}\,$erg and luminosity $L_{\rm iso}=(1.9\pm
0.2)\times 10^{52}\,$erg s-1 for the short GRB 180618A, and $E_{\rm iso}=(7\pm
2)\times 10^{50}\,$erg and $L_{\rm iso}=(2.4\pm 0.7)\times 10^{49}\,$erg s-1
for the extended gamma-ray emission. See also the high-energy properties for
redshifts $z=0.01-1.5$ in Figure 10. For redshifts $z\gtrsim 0.1$, the GRB
180618A lays within the cluster of short GRBs —it is one of the hardest and
not compatible with the long GRB population. Overall, the gamma-ray properties
and the GRB-host galaxy offset confirm the merger nature of the GRB 180618A
(e.g., see Zhang et al. 2009; Rastinejad et al. 2022).

Figure 10: Rest-frame peak energy ($E_{\rm peak}$) and isotropic-equivalent
energy ($E_{\rm iso}$) of the short GRB 180618A and its extended gamma-ray
emission at redshift $z=0.554$. The GRB 180618A high-energy parameters are
also displayed for the redshift range $z=0.01-1.5$. For reference, we include
those GRBs with known redshifts classified as giant flares from magnetars
(GFs; Zhang et al. 2020), short GRBs (sGRBs; D’Avanzo et al. 2014; Calderone
et al. 2015; Tsvetkova et al. 2017, 2021), short GRBs with extended gamma-ray
emission (sGRBEEs; D’Avanzo et al. 2014), long GRBs (lGRBs; Tsvetkova et al.
2017, 2021), and X-ray flashes (XRFs; Tsvetkova et al. 2021). For the
population of long GRBs, we represent the best-fitting line from Tsvetkova et
al. (2021), and the corresponding $1\sigma$, $2\sigma$, $3\sigma$ confidence
level. We also add the GRB counterpart of the gravitational wave event
GW170817 (Zhang et al., 2018), and three GRBs with early afterglows
interpreted as black body emission from either a thermalized jet or cocoon:
XRF 060218 (Campana et al., 2006; Thöne et al., 2011), GRB 101225A (Thöne et
al., 2011), and GRB 171205A (D’Elia et al., 2018; Izzo et al., 2019).

##  4 Interpretation and Modelling

The overall temporal and spectral evolution of the optical emission
($\Delta\alpha_{\rm opt}=4.2$, $\Delta\beta_{\rm opt}=3.3$; see Figure 2) are
hard to explain just in terms of flaring activity (i.e. $\Delta t/t>1$; Zhang
et al. 2006) or the external shock from the decelerating relativistic ejecta
(i.e. the afterglow; Piran 1999; Kobayashi 2000). The observed decay rates of
the emission do not agree with $\alpha\approx 1$ expected from forward shock
emission with a typical electron index of the synchrotron energy spectrum, and
the break cannot be reconciled with the passage of the synchrotron cooling
frequency (Sari et al., 1998). We also discard the fast-fading emission from
the reverse shock (Kobayashi, 2000), which typically decays with index
$\alpha\approx 2$ —still slower than in GRB 180618A. A reverse shock scenario
further disagrees with the rapid spectral evolution of the optical emission at
the time of the light curve break, and the non-detection of high values of
polarization during the broad optical peak (Mundell et al., 2013). Finally, we
rule out that the optical break is an effect of the relativistic collimation
of the outflow given that there is no simultaneous steepening of the emission
across the spectrum at that time (Sari, 1999; Racusin et al., 2009).

We suggest that the spectral evolution and rapid decline of the optical
emission of GRB 180618A is produced by thermal emission (e.g., Thöne et al.
2011; Izzo et al. 2019). In Section 4.1, we model the ultraviolet to optical
emission with a black body model. Following, we discuss if the radioactive
decay of the r-processed ejecta from the merger or a thermalized jet are
credible interpretations to explain the origin of the bright thermal emission.
In Section 4.2, we find evidence of a magnetar wind nebula powering thermal
optical emission during $15-60\,$minutes after GRB 180618A —suggesting that
emission before that time is due to the jet afterglow. In Section 4.3, we
detail the properties of the magnetar powering the distinct non-thermal
emission components, from the gamma rays to optical wavelengths. In Section
4.4, we model the overall (X-ray to optical) spectra with physical synchrotron
models. This analysis proves Section 4.2 findings; the emission during
$3-15\,$minutes post-burst is due to the jet afterglow and that corresponding
to $15-60\,$minutes post-burst requires an extra spectral component (i.e.
thermal emission). In Section 4.5, we test closure relations, and we suggest
that the jet geometry and observer viewing angle are key to detecting the
thermal emission.

###  4.1 Thermal Emission

Figure 11: The GRB 180618A best-fitting black body models to the joint
RINGO3/UVOT data at the observer rest-frame. Note that the model accounts for
Galactic dust extinction, but does not include the host galaxy dust
contribution.

In Xspec, we modelled the co-temporal UVOT and RINGO3 data with a black body
profile that includes the Milky Way dust absorption (see Figure 11). At the
observer rest-frame, the best-fitting effective temperatures and luminosities
from the black body models evolve from $T_{\rm eff}=(1.1\pm 0.1)\times
10^{4}\,$K and $L_{\rm th}/D_{\rm L}^{2}=(4.3\pm 0.5)\times 10^{44}\,$erg s-1
Gpc-2 at $t=202-262\,$s post-burst to $T_{\rm eff}=(3.8\pm 0.6)\times
10^{3}\,$K and $L_{\rm th}/D_{\rm L}^{2}=(1.8\pm 0.6)\times 10^{43}\,$erg s-1
Gpc-2 at $t=2990-3586\,$s post-burst (see results in Table 3), where $D_{\rm
L}$ is the luminosity distance. Note that the data is well fitted by the model
without the need of the host galaxy dust extinction component, which is
consistent with the large offset of the short GRB 180618A with the host galaxy
core (i.e. $\approx 10\,$kpc).

Faint thermal emission at optical and infrared wavelengths has been detected
emerging hours to days after short GRBs (Tanvir et al., 2013; Abbott et al.,
2017a) —when the relatively brighter afterglow has subsided. This emission has
been attributed to the kilonova (Metzger et al., 2010; Roberts et al., 2011)
—a type of emission powered by the radioactive decay of the heavy elements
formed after the dynamically-ejected and wind-driven neutron-rich material of
the merger expands from nuclear densities and neutrons are captured via the
r-process. Predictions are that an ultraviolet kilonova precursor could also
be detected from the decay of free neutrons in the fast material (Metzger et
al., 2015). However, the short timescales of the GRB 180618A optical emission
do not support r-process models, for which we also expect lower peak
luminosities ($L_{\rm th}=10^{40}-10^{42}\,$erg s-1; Metzger et al. 2010,
2015) than those estimated in GRB 180618A, i.e. $L_{\rm th}(z=0.554)\approx
2\times 10^{45}\,$erg s-1.

We also discard thermal emission powered by the GRB jet. The measured
luminosities and effective temperatures do not follow the scalings expected
from thermal emission of the relativistic and non-relativistic material of the
jet cocoon (Nakar & Piran, 2017; De Colle et al., 2018). Similarly, maximizing
the signal from the jet cocoon at $z=0.554$ corresponds to a very energetic
long GRB and large energy stored in the jet cocoon, on the order of $E_{\rm
c}\approx 10^{52}\,$erg (Nakar & Piran, 2017). This would be expected to
produce at most a $\approx 22\,$mag near-ultraviolet and $\approx 23.6\,$mag
optical thermal rebrightening. These values are not consistent with the
observed peak luminosities of the thermal emission (e.g., $\approx 18\,$mag in
the UVOT uvw1 band, and $\approx 18.7\,$mag in the RINGO3 BV band), and they
are far below the limiting magnitude of our UVOT and RINGO3 observations.
Furthermore, successful jets in energetic engines like the GRB 180618A are
expected to have an early breakout from the stellar envelope, which cuts
thermalization and leads to most of the jet energy leaving the ejecta
(Ramirez-Ruiz et al., 2002; Duffell et al., 2018; Izzo et al., 2019).

###  4.2 Thermal Emission from a Relativistically-expanding Magnetar Wind
Nebula

Here, we explore the possibility that the observed thermal emission from the
GRB 180618A could be explained by a magnetar wind nebula (Yu et al., 2013;
Metzger & Piro, 2014; Gao et al., 2015; Wollaeger et al., 2019), and the
implications of such a model for this system.

In a magnetar wind nebula, the expanding ejecta shell is continually being
heated from behind by the magnetar winds —a scenario that differs from
fireball models with energy injection at a single point in time (e.g. Mészáros
& Rees 1997; Nakar & Piran 2017). As such, the thermal luminosity is expected
to track the spin-down luminosity of the magnetar as $L_{\rm th}\propto L_{\rm
sd}\propto t^{-2}$ (Yu et al., 2013; Metzger & Piro, 2014). Assuming that the
ejecta radially expands as $R_{\rm ej}\propto t$, the effective temperatures
are expected to follow $T_{\rm eff}=[L_{\rm th}/(4\pi\sigma_{\rm B}R_{\rm
ej}^{2})]^{1/4}\propto L_{\rm th}^{1/4}t^{-1/2}\propto t^{-1}$, where
$\sigma_{\rm B}$ is the Stefan-Boltzmann constant.

Figure 12: Temporal evolution of the effective temperatures ($T_{\rm eff}$),
luminosities ($L_{\rm th}$) and photospheric radii ($R_{\rm ej}$) of the best-
fitting black body models at the host galaxy rest-frame (i.e. $z=0.554$). The
shaded grey area corresponds to the thermal contribution from the magnetar
wind nebula, and the dotted grey line is the expected evolution of the
temperature, luminosity and radius for an optically thin regime.

In Figure 12, we model the temporal evolution of the best-fitting effective
temperatures, luminosities and photospheric radii ($R_{\rm ej}$) with power
laws. For the data after $\approx 1400\,$s post-burst, we find that the
observed scalings $L_{\rm th}\propto t^{-(2.22\pm 0.14)}$, $T_{\rm eff}\propto
t^{-(0.92\pm 0.13)}$ and $R_{\rm ej}\propto t^{+(0.7\pm 0.3)}$ are compatible
with the expected in a magnetar nebula (Metzger & Piro, 2014). The peak
luminosity $L_{\rm th}(z=0.554)\approx 2\times 10^{45}\,$erg s-1 of the GRB
180618A is also within the values expected (Yu et al., 2013; Metzger & Piro,
2014). Note that the data at $900-1400\,$s post-burst are still in agreement
with the model, and that the deviation we detect is likely due to the
contribution from the external shock, which dominates the emission before
$900\,$s post-burst (see Section 4.4).

A new-born millisecond magnetar from a binary neutron star merger is expected
to be close to the centrifugal breakup limit (Giacomazzo & Perna, 2013; Fryer
et al., 2015), with an initial spin period of $P_{\rm i}\approx 1\,$ms.
Consequently, ejecta below a critical mass $M_{\rm ej}\approx
10^{-2}M_{\odot}P_{\rm i}^{-2}$ can be accelerated to trans-relativistic
speeds ($\Gamma\geq 1$) by the magnetar wind (Yu et al., 2013; Gao et al.,
2013), where $\Gamma=(1-\beta^{2})^{-1/2}$ is the Lorentz factor, and
$\beta=v/c$ is the ratio of the velocity between inertial reference frames and
the speed of light in vacuum. From the photospheric radius of the best-fitting
black body model at the engine rest-frame, $R_{\rm
ej}^{\prime}=ct^{\prime}\approx ct_{\rm obs}(1+z)^{-1}(1-\beta)^{-1}\approx
2\Gamma^{2}ct_{\rm obs}(1+z)^{-1}$, we find that the ejecta in the GRB 180618A
system is expanding at mildly relativistic speeds, i.e.
$\Gamma(z=0.554)\approx 9$.

For such low ejecta masses giving rise to high Lorentz factors, the ejecta can
become optically thin before the diffusion time (Yu et al., 2013), i.e. when
the optical depth $\tau^{\prime}=(3M_{\rm ej}\kappa)/(4\pi
R^{\prime\,2})\approx 1$, where $\kappa$ is the opacity and $M_{\rm ej}$ is
the isotropic-equivalent mass of the ejecta. The optical depth can be
significantly raised (for a total mass in the shell) given electron-positron
pair creation in the region behind the ejecta shell (Metzger & Piro, 2014). We
estimate the maximum pair multiplicity by assuming that a fraction $\approx
0.1$ of the magnetar rotational energy $E_{\rm rot}\approx 10^{52}\,$erg is
converted through a pair cascade process into electron-positron pairs in the
nebula (Metzger & Piro, 2014), such that $\kappa_{\pm}\approx(0.1E_{\rm
rot}/m_{\rm e}c^{2})(m_{\rm p}/M_{\rm ej})\approx 10^{3}(M_{\rm
ej}/10^{-3}M_{\odot})^{-1}$. That is, pairs contribute $\approx 10^{3}$ times
to the opacity for their mass than an electron-ion outflow, with typical
electron-scattering opacity $\kappa\approx 0.2\,$cm${}^{2}\,$g-1 (Yu et al.,
2013; Metzger et al., 2015). Given an electron-positron annihilation rate
slower than the outflow expansion speed at late times (Metzger & Piro, 2014),
the pair opacity will still dominate the total opacity by the time of
observations. Therefore, assuming an optically thin regime
($\tau^{\prime}\lesssim 1$) when we start to notice the thermal emission
($t_{\rm obs}\lesssim 10^{3}\,$s) and opacity
$\kappa\approx(0.2\,$cm${}^{2}\,$g${}^{-1})\,\kappa_{\pm}$, we estimate ejecta
masses $M_{\rm ej}\lesssim 10^{-4}M_{\odot}(\kappa_{\pm}/10^{3})^{-1}$ at the
polar regions of the system. This is consistent with the mass-loss expected
from merger remnants (e.g. Oechslin & Janka, 2006; Lee & Ramirez-Ruiz, 2007).

###  4.3 Non-thermal Emission from a Magnetar Wind Nebula

We used the GRB 180618A pipeline-processed products of the Swift X-Ray
Telescope (XRT; Burrows et al. 2005; Evans et al. 2009), and we modelled the
0.3$-$10 keV light curve with a broken power-law function (see Figure 13-a).
We find a significant emission excess of 4.6$\sigma$ at $\approx 4\times
10^{4}\,$s post-burst, which we modelled with a pulse function (Piranomonte et
al., 2008), i.e. $F=F_{0}(t-t_{0})e^{-(t-t_{0})/\tau_{\sigma}}$. The best-
fitting parameters of this model are an initial emission decay rate
$\alpha_{\rm X,1}=0.99\pm 0.07$, and a break at $t_{\rm break}=206\pm 14\,$s
post-burst followed by a steeper decay with $\alpha_{\rm X,2}=1.89\pm 0.06$
(see Figure 13-b). For the late-time rebrightening, we find $t_{0}=(9.3\pm
3.8)\times 10^{3}\,$s and $\tau_{\sigma}=(1.8\pm 0.3)\times 10^{4}\,$s.

If the magnetar winds are powerful enough and the ejecta mass is low, the
nebula can be fully ionized at late times and X-ray emission will be able to
leak out the nebula (Metzger & Piro, 2014; Gao et al., 2015). This is
consistent with the X-ray late-time rebrightening of the GRB 180618A, and
implies that we are directly detecting the magnetar spin-down luminosity. This
emission is expected to follow $L_{\rm sd}=L_{0}(1+t/t_{\rm sd})^{-2}$ with
typical values $L_{0}=1.7\times 10^{50}\,B_{15}^{2}P_{\rm i,-3}^{-4}\,\,$erg
s-1 and $t_{\rm sd}=307\,B_{15}^{-2}P_{\rm i,-3}^{2}\,\,$s (e.g., Metzger
2019), which correspond to a cooled neutron star of $12\,$km fiducial radius
(Beznogov et al., 2020), magnetic field $B_{15}\approx 10^{15}\,$G and initial
spin $P_{\rm i,-3}\approx 1\,$ms (see e.g. Rosswog et al., 2003).

At early times ($t\ll t_{\rm sd}$), the luminosity of the magnetar from the
GRB 180618A is estimated to be $L_{0}\approx L_{\rm iso}f_{\rm b}/\eta$, where
$f_{\rm b}\approx\theta_{\rm j,EE}^{2}/2$ is the beaming factor, $\eta$ is the
radiative efficiency, and $\theta_{\rm j,EE}$ is the jet opening angle. For a
redshift $z=0.554$ and assuming a large opening angle for the magnetar wind
(i.e. $f_{\rm b}/\eta\approx 1$; Bucciantini et al. 2012), the characteristic
luminosity is $L_{0}(z=0.554)\approx 2\times 10^{49}\,$erg s-1. Furthermore,
for a rebrightening of $L_{\rm X}(z=0.554)\approx 4\times 10^{44}\,$erg s-1 at
$t\approx 5\times 10^{4}\,(1+z)^{-1}\,$s post-burst, the characteristic spin-
down time of the magnetar needs to be $t_{\rm sd}\approx 200\,(1+z)^{-1}\,$s.
Note that this is consistent with the early optical plateau of the UVOT white
band that we detect with decay rate $\alpha_{\rm opt,white}=0.12\pm 0.08$ at
$\lesssim 200\,$s post-burst (see also Rowlinson et al. 2013; Knust et al.
2017). Given all these constraints, we estimate the initial spin of the
magnetar remnant of the GRB 180618A to be $P_{\rm i}\approx 4\,$ms, with
magnetic field $B\approx 6\times 10^{15}\,$G.

In the magnetar scenario, a multiwavelength rebrightening could also be
detectable hours to days post-burst given the deceleration of the mildly
relativistic ejecta by the circumburst medium (Gao et al., 2013). At redshift
$z=0.554$, testing this prediction would have required sensitive late-time
observations (e.g. Perley et al. 2009). That is, given typical values of
circumburst medium for a $\approx 10\,$kpc GRB-host galaxy offset ($n\lesssim
10^{-3}\,$cm-3; Fong et al. 2015), the best-case scenario for the GRB 180618A
corresponds to an emission excess with peak luminosity $\lesssim 10^{-9}\,$Jy
at X-ray bands, $\lesssim 25\,$mag at optical bands, and $\lesssim 100\,\mu$Jy
at radio bands (Gao et al., 2013).

Figure 13: Best-fitting models of the XRT X-ray light curve of the GRB
180618A. (a) Broken power-law model. The best-fitting parameters are an
initial decay rate $\alpha_{\rm X,1}=0.79\pm 0.12$, steeping to $\alpha_{\rm
X,2}=1.77\pm 0.04$ at $t_{\rm break}=162\pm 11\,$s ($\chi^{2}/$dof$=131/102$).
(b) Broken power-law model plus a pulse function, with $\alpha_{\rm
X,1}=0.99\pm 0.07$, $\alpha_{\rm X,2}=1.89\pm 0.06$, $t_{\rm break}=206\pm
14\,$s ($\chi^{2}/$dof$=104/99$). The bottom panels are the residuals of the
best-fitting model.

Figure 14: Spectral energy distributions of the cotemporal X-ray, optical and
ultraviolet observations of the GRB 180618A. The XRT, UVOT and RINGO3 data are
modelled with the following synchrotron models. (a) Power laws
($\chi^{2}/$dof$=292/159$). (b) Broken power laws with a joint optical and
X-ray spectral indexes ($\chi^{2}/$dof$=170/158$). The best-fitting spectral
indexes are $\beta_{\rm opt}=0.27\pm 0.02$ and $\beta_{\rm X}=0.84\pm 0.05$.
Note that we have fixed the break of $t_{11}$ epoch (grey color) to the
$t_{10}$ epoch value given the optical upper limits. In dotted lines, we show
the best-fitting synchrotron model and, in solid lines, the model that
includes the dust and hydrogen absorption from the Milky Way and host galaxy
for a redshift $z=0.554$. Detections have $1\sigma$ error bars, and non-
detections are presented as $3\sigma$ upper limits.

Figure 15: Spectral energy distributions of the cotemporal X-ray, optical and
ultraviolet observations of the GRB 180618A. The data after the white-band
UVOT plateau (i.e., $\gtrsim 200\,$s post-burst) are modelled with the
following physical models. (a) Fast cooling of the electrons
($\chi^{2}/$dof$=309/90$). That is, we fix the optical spectral index to
$\beta_{\rm opt}=\beta_{\rm opt,PI}-1=0.5$ and we let the synchrotron
frequency evolve as $\nu_{\rm m}\propto t^{-1.5}$. (b) Slow cooling of the
electrons ($\chi^{2}/$dof$=330/90$), i.e. we fix $\beta_{\rm X}=\beta_{\rm
opt}+0.5$ and the cooling frequency $\nu_{\rm c}\propto t^{-0.5}$. (c) Fast
cooling and a black body profile ($\chi^{2}/$dof$=231/82$). (d) Slow cooling
and a black body profile ($\chi^{2}/$dof$=106/82$). The best-fitting optical
spectral index is $\beta_{\rm opt}=0.30\pm 0.08$ ($\beta_{\rm X}=\beta_{\rm
opt}+0.5$), which corresponds to an electron index $p=1.6\pm 0.1$. The dust
contribution from the host galaxy is estimated to be E($B-V$)${}_{\rm
HG}<0.02$.

###  4.4 Non-thermal Emission from the Afterglow

The deceleration of the relativistic ejecta by the circumburst medium should
also have left an early imprint on the overall emission. To reproduce the
synchrotron spectrum of this external shock (Piran, 1999), we modelled the
joint optical and X-ray spectral energy distributions with absorbed power laws
and broken power laws in Xspec (see Figure 14-a,b). Given the spectral slopes
and the progression of the break to lower frequencies, we find that there must
be at least one break frequency in between the bands and that, it must be
either the synchrotron or cooling frequency in an interstellar medium
(Chevalier & Li, 2000).

For the data after the X-ray light curve break and the optical plateau, we
tried two physical models for the GRB synchrotron spectrum: the fast and slow
cooling of the electrons (e.g., Piran 1999). The best-fitting synchrotron
models display trends in the residuals, and suggest an additional spectral
component at optical bands (see Figure 15-a,b). Therefore, having in mind
Section 4.2 findings, we introduced a black body profile in the model (see
Figure 15-c,d). The best-fitting synchrotron plus black body model suggests a
synchrotron spectrum with a rather hard electron index ($p=1.6\pm 0.1$ with
spectral slopes $\beta_{\rm opt}=\beta_{\rm opt,PI}-1=0.30\pm 0.06$ and
$\beta_{\rm X}=\beta_{\rm opt}+0.5$), an interstellar medium profile, slow
cooling regime, the cooling frequency in between the optical and X-ray bands,
a black body profile contributing from $\approx 900\,$s post-burst, and low
host galaxy dust extinction, i.e. E($B-V$)${}_{\rm HG}<0.02$ —consistent with
the GRB 180618A-host galaxy large offset. Additionally, we note that the
constraint on polarization of $P_{\rm BV}<6.1\%$ at early times supports the
scenario of unpolarized forward shocks in short GRBs (see e.g. Jordana-Mitjans
et al. (2021) for long GRBs).

This physical model suggests that when the thermal emission subsides, we
should detect the underlying afterglow again. At $\approx 3300\,$s post-burst,
the fast-fading afterglow emission should be $F_{\nu}\approx 7\times
10^{-6}\,$Jy, which we speculate that it is consistent with the subtle light
curve flattening of the RINGO3 BV band, with $F_{\nu}=(9\pm 3)\times
10^{-6}\,$Jy. Furthermore, the emission across optical and X-ray bands is
decaying faster than the expected for a spherical expansion (Racusin et al.,
2009), and suggests a collimated relativistic outflow.

###  4.5 A Collimated Outflow

The temporal and spectral properties of the early X-ray emission of the GRB
180618A satisfy closure relations for a jetted outflow with a hard electron
index. That is, the first light curve segment corresponds to the normal
spherical decay rate of the afterglow (with average $\alpha_{\rm X}\approx
1.2$), and the second segment to the post-jet-break decay ($\alpha_{\rm
X}\approx 2$; Racusin et al. 2009). For an interstellar medium profile, slow
cooling regime and with the X-ray band above the cooling frequency, a mean
spectral index $\beta_{\rm X}=0.80\pm 0.06$ implies an electron index
$p=1.6\pm 0.1$; see also the temporal evolution of $\beta_{\rm X}$ in Figure
2-bottom panel. We find a temporal slope $\alpha_{\rm X,1}=(3\beta_{\rm
X}+5)/8=0.93\pm 0.02$ for the normal spherical decay of the afterglow, and
$\alpha_{\rm X,2}=(\beta_{\rm X}+3)/2=1.90\pm 0.03$ after the light curve jet
break, for an uniform jet scenario with lateral spreading (Zhang & Mészáros,
2004; Racusin et al., 2009) —consistent with the observed decay rates of the
X-ray light curve. Taking into account the early jet break at $t\approx
200\,$s post-burst (Sari et al., 1999) and that we are detecting the bright
gamma-ray prompt emission (Yamazaki et al., 2002), we suggest that the
observer faces the jet with a line of sight that likely runs near the jet
edge. Overall, the GRB 180618A suggests that the degree of collimation (Fong
et al., 2015) and the observer viewing angle are key in detecting the short-
lived optical thermal emission (and the late-time X-ray rebrightening) in
short GRBs.

Figure 16: Artist impression of the different energy sources powering the GRB
180618A multiwavelength emission. (a) The material is equatorially ejected by
tidal forces during the neutron star binary merger (Cucchiara et al., 2011),
and radially ejected by hydrodynamic interactions at the neutron stars contact
region (e.g., Metzger 2019). (b) The accretion of the torus onto a
rotationally supported supramassive neutron star remnant (i.e. a millisecond
magnetar) powers two relativistic jetted outflows (Bucciantini et al., 2012;
Metzger et al., 2018) that, via internal dissipation mechanisms, produce the
initial $\approx 0.3\,$s hard prompt gamma-ray emission. At this stage, the
accretion disk releases winds that largely dominate the total mass ejected
(Margalit & Metzger, 2019). (c) The winds from the rotationally-powered
magnetar are collimated by the surrounding ejecta, which give rise to the
$\approx 45$-s duration soft gamma-ray emission (Bucciantini et al., 2012).
(d) As the spin-down luminosity of the magnetar decreases, the jetted winds
become stifled behind the ejecta, which is reheated at larger radii. When the
opacity of the ejecta decreases sufficiently, bright optical thermal emission
is emitted (Yu et al., 2013; Metzger & Piro, 2014; Metzger, 2019). (e) Hours
after the merger, the ejecta is fully ionized by the winds of the long-lived
magnetar, and the magnetar spin-down luminosity is detected (Metzger & Piro,
2014).

##  5 Discussion

The early-time multiwavelength observations of the short GRB 180618A propose a
scenario in which only a long-lived magnetar remnant can account for all the
observed emission components (see Figure 16): the extended soft gamma-ray
emission following the short GRB (Metzger et al., 2008; Bucciantini et al.,
2012), the unusual optical light curve (Yu et al., 2013; Metzger & Piro,
2014), and the additional X-ray component (Metzger & Piro, 2014; Gao et al.,
2015).

Tens of magnetars have been identified in our Galaxy so far (Kaspi &
Beloborodov, 2017), and some of them are regular X-ray bursters that, less
frequently, emit at soft gamma-ray bands (Ridnaia et al., 2021). More
recently, giant flares from extragalactic magnetars have also been associated
with low-luminosity short-duration GRBs (Svinkin et al., 2021). However, the
remnant of a neutron star binary merger is expected to be the more energetic
version of a magnetar, a millisecond proto-magnetar (Metzger & Piro, 2014),
which is rotationally powered with typical energies $E_{\rm rot}\approx
10^{51}-10^{53}\,$erg, and will spin down until its collapse into a black hole
(Margalit & Metzger, 2019).

After a neutron star binary merger, if a newborn rapidly-spinning magnetar has
sufficient spin-down luminosity, the winds will pierce through the ejecta and
be collimated into bipolar jetted outflows that will dissipate Poynting-flux
energy —powering the extended gamma-ray emission of the GRB 180618A (Metzger
et al., 2008; Bucciantini et al., 2012). As the spin-down luminosity of the
magnetar decreases with $L_{\rm sd}\propto t^{-2}$, these winds are trapped
behind the ejecta forming a hot nebula of electron-positron pairs that will
radiate via synchrotron and inverse Compton emission (Metzger & Piro, 2014). A
fraction of the X-ray emission is then absorbed by the neutral ejecta walls,
and reprocessed into optical and infrared photons that are able to escape when
the optical depth of the expanding ejecta decreases enough. This allows the
magnetar-powered kilonova of the GRB 180618A to be a hundred times brighter
than a radioactively-powered kilonova (Yu et al., 2013; Metzger & Piro, 2014).
Hours to days after the burst, if the strong magnetar winds can completely
ionize the ejecta, non-thermal X-ray emission will leak from the nebula
producing an X-ray excess (Metzger & Piro, 2014), similar to that observed at
$\approx 0.5\,$days after the GRB 180618A.

Our multiwavelength data also gives information about the geometry of the
system. Given that we are detecting prompt gamma-ray emission that is bright
and spectrally hard, we are likely facing the GRB jet (Yamazaki et al., 2002).
If the magnetar is releasing energy and accelerating ejecta along the polar
regions of the system, material can easily reach trans-relativistic speeds
(Metzger et al., 2008; Bucciantini et al., 2012). This extra kinetic energy is
consistent with what we observe at optical bands; there is an early and rapid
evolution of the thermal luminosity given the relativistically-expanding
photospheric radius and the fast-fading spin-down luminosity of the magnetar,
which we measure as $L_{\rm th}\propto t^{-(2.22\pm 0.14)}$. For the optical
to be reprocessed within the observed timescales, we require an ejecta mass
$M_{\rm ej}\lesssim 10^{-4}M_{\odot}(\kappa_{\pm}/10^{3})^{-1}$ at the polar
regions of the merger, which is reasonable given that a total ejected mass
$\approx(0.01-0.3)M_{\odot}$ is expected in all directions (Oechslin & Janka,
2006; Murguia-Berthier et al., 2017). This suggests that the merger ejecta
distribution is considerably asymmetric, likely due to long-lasting cavities
drilled by the early relativistic outflows or the disk winds ejecting more
material in equatorial directions (Bucciantini et al., 2012).

Current magnetohydrodynamic simulations cannot form jetted outflows just from
the neutron star binary merger itself (Ruiz & Shapiro, 2017); successful jets
require the formation and delayed collapse within a hundred milliseconds of an
intermediate hypermassive neutron star (Murguia-Berthier et al., 2014).
However, constraints on the nuclear equation of state suggest that $18\%$ to
$65\%$ of the neutron stars binary mergers will result in a less-massive and
rotationally-supported supramassive neutron star remnant with longer lifetime
(Fryer et al., 2015; Margalit & Metzger, 2019). Without the need for the
neutron star remnant to collapse into a black hole, a viable short GRB from a
merger could be powered by direct accretion onto the magnetar (Bucciantini et
al., 2012), or by the enhancement of the spin-down luminosity given the
temporary presence of the accretion disk (Metzger et al., 2018). Yet, baryon
pollution remains a concern in these environments (Lee & Ramirez-Ruiz, 2007;
Murguia-Berthier et al., 2014).

The multiwavelength dataset of the GRB 180618A confirms GW170817/GRB 170817A
findings (Abbott et al., 2017b) —i.e. neutron star binaries as progenitors of
short GRBs. While the remnant of the gravitational wave event GW170817 is
likely a hypermassive neutron star that collapsed into a black hole within the
first few hundreds of milliseconds after the merger (Abbott et al., 2017b;
Metzger, 2019), observations of the short GRB 180618A suggest a different
outcome. We observe that a vast energy reservoir is injected into the system
on timescales much larger than the duration of the accretion disk outflows
—powering several emission components across the spectrum that can only be
explained by a long-lived magnetar remnant. Furthermore, it suggests that
supramassive neutron stars with delayed collapse into a black hole are
remnants of neutron star binary mergers (Fryer et al., 2015; Margalit &
Metzger, 2019), and can power short-hard GRBs and extended soft gamma-ray
emission through accretion and spin-down luminosity (Metzger et al., 2008;
Bucciantini et al., 2012). These findings preserve a good agreement between
the percentage of short GRBs that have extended emission ($13\%-50\%$; Norris
et al. 2010; Lien et al. 2016), and the expected number of remnants from
neutron star binary mergers that can power such emission ($18\%-65\%$;
Margalit & Metzger 2019).

Future early-time studies of short GRBs with extended gamma-ray emission and
joint GW/GRB detections will be able to statistically constrain how long and
how many of these cosmological magnetars survive the merger, characterize the
asymmetries in the distribution of the ejected mass, and probe jet
acceleration in millisecond magnetars.

##  6 Conclusions

We report the multiwavelength observations of the short GRB 180618A; a GRB
with unique gamma-ray, X-ray and optical properties result of a compact object
binary merger at the outskirts of a galaxy at redshift $z=0.554\pm 0.001$.

The bright prompt gamma-ray emission of the GRB 180618A consists of a multi-
peaked structure with total duration $\approx 0.3\,$s and maximum energy
radiated in the MeV domain, making the GRB 180618A one of the most energetic
gamma-ray pulses ever detected among short-duration GRBs (i.e. flux, fluence,
$E_{\rm peak}$). After the typically short and spectrally-hard gamma-ray
pulse, we also detect a period of weak extended gamma-ray emission below
$\approx 100\,$keV, lasting $\approx 45\,$s.

We find no detectable polarization at optical bands, and a rate of change of
the light that initially follows a power-law $F_{\nu}\propto t^{-\alpha}$,
with index $\alpha=0.46\pm 0.02$. The optical emission is surprisingly short-
lived, and the slow decline is replaced by a sudden drop in brightness at
$35\,$minutes post-burst, steepening to $\alpha=4.6\pm 0.3$. The light curve
break progressively passes from the ultraviolet to near-infrared bands.
Afterwards, there is no further detection of the optical transient at the GRB
180618A coordinates.

The GRB 180618A optical counterpart presents temporal and spectral properties
that do not satisfy the characteristic scalings of the synchrotron spectrum of
the GRB afterglow (Sari et al., 1998) —powered by the shock of the
relativistic collimated ejecta with the circumburst medium. In contrast, the
fast-fading X-ray emission is consistent with a decelerating jetted outflow,
and with an extra emission component at $\approx 0.5\,$days post-burst. This
leads us to consider two distinct mechanisms powering the X-ray and unusual
optical emission.

The modelling of the overall emission suggests thermal-like emission from a
relativistically-expanding source dominating the optical emission at $\approx
15-60\,$minutes post-burst, which naturally accounts for the sharp chromatic
drop of the optical emission at high-frequency wavelengths. Furthermore, the
X-ray to optical emission before $15\,$minutes post-burst is consistent with
the fast-fading jet afterglow.

We interpret the unusual spectral and temporal properties of the GRB 180618A
as evidence of a highly magnetized, spinning neutron star that survives for
longer than $\approx 10^{5}\,$s after the merger, and spins down at rate
$L_{\rm th}\propto t^{-(2.22\pm 0.14)}$ powering a relativistically-expanding
hot thermal nebula in the process. Here, we confirm that newborn millisecond
magnetars can power bright emission components across the electromagnetic
spectrum that remain detectable at cosmological distances: i.e. the extended
soft gamma-ray emission following some short GRBs (Metzger et al., 2008;
Bucciantini et al., 2012), optical plateaus at early times (Knust et al.,
2017), the fast-evolving bright thermal optical emission (Yu et al., 2013;
Metzger & Piro, 2014) and the late-time flattening of the X-ray light curve
(Metzger & Piro, 2014; Gao et al., 2015). The early afterglow emission drop,
and the short-lived thermal optical emission may explain why such thermal
emission has not been detected yet in other short GRBs with extended emission;
this discovery opens a new era for searches of gravitational wave counterparts
with fast-cadence surveys.

We thank the anonymous referee for their constructive comments that improved
the clarity and accuracy of the paper. The research leading to these results
has received funding from the European Union’s Horizon 2020 Programme under
the AHEAD project (grant agreement 654215). N.J.-M. and C.G.M. acknowledge
financial support from Mr Jim Sherwin and Mrs Hiroko Sherwin. C.M.
acknowledges support from the Science and Technology Facilities Council and
the UK Research and Innovation (ST/N001265/1). E.R-R. is supported in part by
NASA grant NNG17PX03C, NSF grant AST-1911206, AST-1852393, and AST-1615881,
and the Heising-Simons Foundation. A.G. acknowledges the financial support
from the Slovenian Research Agency (grants P1-0031, I0-0033, J1-8136,
J1-2460). M.M. acknowledges financial support from the Italian Ministry of
University and Research - Project Proposal CIR01_00010. We acknowledge A.
Becker for taking the data, R.T. Gatto for useful discussions, and D. Paris
for the help in the data reduction of the LBC. We thank E. Burns and D.
Burrows for useful discussions.

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Table 2: The GRB 180618A optical photon indexes ($\beta_{\rm opt,PI}$) derived
from the best-fitting power-law models to the RINGO3 data.

tmid-T0 | terr | $\beta_{\rm opt,PI}$ | $\beta_{\rm opt,PI,\,err}$
---|---|---|---
(s) | (s) |  |
231 | 30 | 0.7 | 0.4
340 | 42 | 1.2 | 0.4
473 | 89 | 1.2 | 0.4
749 | 179 | 1.3 | 0.4
995 | 67 | 1.5 | 0.4
1212 | 150 | 1.4 | 0.4
1528 | 90 | 1.2 | 0.4
1825 | 208 | 1.8 | 0.5
2670 | 298 | 2.6 | 0.6
3288 | 298 | 4.0 | 0.8

Note. — The tmid is the mean observing time, the T0 is the BAT trigger time,
and the terr is half the length of the observing time window. Note that the
model does not account for host galaxy extinction.

Table 3: The GRB 180618A effective temperatures ($T_{\rm eff}$) and
luminosities ($L_{\rm th}/D_{\rm L}^{2}$) derived from the best-fitting black
body models to the joint RINGO3/UVOT data.

tmid-T0 | terr | $T_{\rm eff}$ | $T_{\rm eff,\,err}$ | $L_{\rm th}/D_{\rm L}^{2}$ | $L_{\rm th,err}/D_{\rm L}^{2}$
---|---|---|---|---|---
(s) | (s) | ($10^{3}\,$K) | ($10^{3}\,$K) | (1043 erg s-1 Gpc-2) | (1043 erg s-1 Gpc-2)
231 | 30 | 10.9 | 1.0 | 42.6 | 5.3
340 | 42 | 9.8 | 0.7 | 27.9 | 2.2
473 | 89 | 10.0 | 0.5 | 27.9 | 0.7
749 | 179 | 9.8 | 0.6 | 20.7 | 1.7
995 | 67 | 10.2 | 0.6 | 19.8 | 1.0
1212 | 150 | 8.6 | 0.9 | 13.0 | 1.5
1528 | 90 | 7.8 | 1.1 | 9.4 | 1.2
1825 | 208 | 7.2 | 1.2 | 7.1 | 0.9
2670 | 298 | 5.3 | 0.9 | 2.6 | 0.3
3288 | 298 | 3.8 | 0.6 | 1.8 | 0.6

Note. — The tmid is the mean observing time, the T0 is the BAT trigger time,
the terr is half the length of the observing time window, and $D_{\rm L}$ is
the luminosity distance. Note that the model does not account for host galaxy
extinction.