Survival fraction, lifetimes, and nebular emission physics of long-lived magnetar remnants in short gamma-ray bursts

The short gamma-ray burst GRB 180618A showed a minute-scale, bright thermal optical component (emerging ~15 minutes post-burst and collapsing chromatically at ~35 minutes) that Jordana-Mitjans et al. (2022) interpreted as merger ejecta reheated by the relativistic wind of a newborn millisecond magnetar — evidence that a binary neutron star merger can leave a neutron-star remnant surviving far longer than the few hundred milliseconds usually assumed. The open problem is to determine, at population level, how many such magnetar remnants survive among short gamma-ray bursts (with and without extended gamma-ray emission) and in joint gravitational-wave/gamma-ray detections, for how long, and through what emission physics: the wind pair multiplicity, the time-dependent pair-dominated opacity, the thermalization-versus-nonthermal-escape efficiency in asymmetric ejecta, and the interplay between residual fallback accretion and spin-down that together set how the rotational energy is radiated as early optical/UV/X-ray counterparts. As of 2026 the question is open on every front: X-ray plateau statistics imply only 18–37% of short GRBs host long-lived magnetar engines (a debated, model-dependent identification), essentially no second minute-scale thermal optical counterpart has been reported, the multidimensional radiative-transfer treatment of pair-rich merger nebulae needed to make such counterparts predictable has not been carried out, and the accretion–spin-down interplay remains degenerate in every event modeled to date.

Background

Short gamma-ray bursts (GRBs) — flashes of MeV gamma rays lasting less than about two seconds — are produced by mergers of compact binaries containing neutron stars, as confirmed by the gravitational-wave event GW170817 and its gamma-ray counterpart GRB 170817A, followed about a day later by a kilonova, a transient powered by radioactive decay of heavy r-process nuclei synthesized in the neutron-rich merger ejecta. A central unresolved issue is the nature and lifespan of the merger remnant: prompt collapse to a black hole, a hypermassive neutron star (supported temporarily by differential rotation and thermal pressure, collapsing within ~1 second), a supramassive neutron star (uniformly rotating and rotationally supported, collapsing on longer timescales), or a permanently stable neutron star. A rapidly rotating, strongly magnetized neutron-star remnant — a millisecond magnetar — stores roughly $10^{52}$ erg of rotational energy and releases it as a magnetized relativistic wind whose magnetic-dipole spin-down luminosity and timescale are set by the surface dipole field $B$ and initial spin period $P_{\rm i}$.

If the magnetar survives, its wind inflates a hot nebula (a pulsar-wind nebula: the bubble of magnetized, particle-filled plasma blown by the wind) behind the expanding merger ejecta. Metzger & Piro (2014) showed that the optical and X-ray output of such a system is governed by the nebula's pair content — quantified by the pair multiplicity, the number of electron–positron pairs per baryon in the wind, which raises the effective opacity far above the ordinary electron-scattering value ($\kappa\approx0.2$ cm² g⁻¹) — and by how efficiently the spin-down luminosity thermalizes in the ejecta at early times versus leaking out non-thermally once the nebula becomes optically thin. Metzger, Beniamini & Giannios (2018) further showed that fallback accretion — gas from the disrupted merger disk that falls back onto the remnant at late times — can compete with spin-down, altering the net energy injection into the ejecta.

Jordana-Mitjans et al. (2022) reported the short GRB 180618A at redshift $z=0.554$, with ultraviolet and optical multicolor observations beginning 1.4 minutes post-burst — the earliest such data for any short GRB. After a fast-fading afterglow, a thermal optical component emerged at ~15 minutes and then dropped abruptly and chromatically ($F_\nu\propto t^{-4.6}$) at 35 minutes, with thermal luminosity declining as $L_{\rm th}\propto t^{-2.2}$; a late X-ray rebrightening at $\sim4\times10^{4}$ s was interpreted as directly tracked spin-down. Modeling the emission as a pair-rich magnetar nebula reheating low-mass polar ejecta ($M_{\rm ej}\lesssim10^{-4}$ M⊙ after correcting for a pair multiplicity $\kappa_\pm\sim10^{3}$) gave engine parameters $P_{\rm i}\approx4$ ms and $B\approx6\times10^{15}$ G, but these depend on assumed beaming factor and radiative efficiency, and the required pair-dominated opacity and thermalization efficiency are themselves uncertain. The fraction of short GRBs with extended gamma-ray emission — a lower-level gamma-ray tail that follows the initial sub-second spike and lasts tens to hundreds of seconds — is 13–50%, sitting loosely near estimates of the fraction of binary neutron star mergers expected to leave supramassive remnants (18–65%), but this consistency of two loose numbers is not a measurement.

The authors' own closing statement poses the question: future early-time studies of short GRBs with extended emission and joint gravitational-wave/gamma-ray detections should 'statistically constrain how long and how many of these cosmological magnetars survive the merger,' which requires both a population sample of minute-scale multicolor counterparts and a predictive emission model that defines what signature a magnetar of given $P_{\rm i}$, $B$, and ejecta geometry actually produces.

Problem Statement

For the population of short gamma-ray bursts (including those followed by extended gamma-ray emission — a lower-level gamma-ray tail that follows the initial sub-second spike and lasts tens to hundreds of seconds) and for joint gravitational-wave/gamma-ray binary-neutron-star detections, determine how many merger remnants are long-lived magnetars and how long they survive, together with the nebular emission physics required to read that answer from observations. Concretely: (a) measure, with quantified uncertainties, the fraction of events whose central engine is a neutron-star remnant surviving on observationally accessible timescales (from seconds, through the minutes-to-hours window probed by early optical/UV and X-ray follow-up, out to the years probed by radio), and the distribution of survival times — the 'how long and how many' of the source paper's closing question; and (b) specify, as a predictive model, the microphysics by which a surviving magnetar's spin-down energy is radiated, with the model required to address all of the following — the wind pair multiplicity (the number of electron–positron pairs per baryon in the magnetar wind), the resulting time-dependent pair-dominated opacity, the efficiency of thermalization in the ejecta versus non-thermal escape as the nebula becomes optically thin, the radiative-transfer effects of asymmetric (latitude-dependent) ejecta, and the role of residual fallback accretion (gas from the disrupted merger disk falling back onto the remnant) in enhancing or shortening the magnetar's spin-down — so that minute-scale multicolor optical/UV and X-ray counterparts of the kind observed in GRB 180618A can be predicted from engine parameters ($P_{\rm i}$, $B$, ejecta mass and geometry) and inverted to constrain them. The two components are inseparable parts of one objective: a survival fraction is only meaningful once the emission model defines what signature a magnetar with given parameters produces, and every population statistic must correct for the selection effects that model implies; conversely the microphysics is only tested by applying it to an observed sample. Answers may come from observational population studies, from theoretical emission modeling, or from a combination, but a complete answer must state both constraints it delivers — the survival fraction ('how many') and the survival-time distribution ('how long') — together with the predictive emission model specified in (b) and the physical assumptions and selection-effect corrections on which they rest.

References

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