Supernova (SN) 1987A, the nearest core-collapse supernova of the modern era, left behind a compact object whose birth was registered by a burst of neutrinos in February 1987. Michel, Kennel and Fowler (Science, 1987) asked when this pulsar would be seen, proposing escaped MeV gamma rays from a pulsar-inflated nebula, a flattening of the optical light curve as pulsar spin-down overtakes radioactive decay, and pulsed radio or optical emission as the observable channels, and listing the unknowns that govern the answer: birth spin period, magnetic field, early dust formation, beaming geometry, dispersion measure, and the possibility that the neutron star collapsed to a black hole. More than 39 years of searches have not directly detected the compact object in any band, although every 1987-era channel has been tested: the escaped gamma rays proved to be radioactive 56^{56}Co decay lines, no pulsar-driven light-curve flattening occurred, a massive dust reservoir formed in the ejecta and hides the center, and the radio remnant is powered by the circumstellar shock rather than a pulsar. Since 2024, JWST infrared spectroscopy provides indirect evidence for an unresolved ionizing source, most plausibly a cooling neutron star or a pulsar-wind nebula, offset and blueshifted in a way that could imply a natal kick of about 510 km s−1510\,\mathrm{km\,s^{-1}} — but no point source, no pulsations, and no measurement of spin period or magnetic field exist. The open problem is to determine the nature (for example a cooling neutron star, a pulsar with a nebula, or a black hole) and birth properties (spin period and magnetic field) of the SN 1987A compact remnant, and to establish when and through which channels it becomes directly observable.

Background

SN 1987A exploded in the Large Magellanic Cloud (distance about 50 kpc) in February 1987 and is the nearest and best-observed core-collapse supernova of the modern era. In a core-collapse supernova, the iron core of a massive star collapses and radiates its gravitational binding energy as a burst of neutrinos; the neutrino burst detected from SN 1987A (Hirata et al. 1987) established that a compact object — a neutron star or, if that object subsequently collapsed, a black hole — was born on a known date. A neutron star is a roughly 1.4 M⊙1.4\,M_\odot object of radius about 10 km; if it is rapidly rotating and strongly magnetized it acts as a pulsar, a lighthouse-like source whose rotating beams sweep past the observer, and whose spin-down — the loss of rotational energy to a magnetized particle wind, with luminosity scaling roughly as B2/P4B^2/P^4 for spin period PP and dipole surface field BB — can exceed 1038 erg s−110^{38}\,\mathrm{erg\,s^{-1}} for a newborn millisecond pulsar.

The optical display of a type II supernova (the core-collapse explosion of a massive star that retains a hydrogen envelope) is powered mainly by radioactivity: freshly synthesized nickel decays through cobalt to iron (56^{56}Ni→56\to^{56}Co→56\to^{56}Fe), and the gamma rays from that decay deposit their energy in the expanding ejecta. Because the ejecta are initially opaque to gamma rays, the observed light curve declines more slowly than the radioactive decay rate at early times, with the difference escaping later as gamma rays. Michel, Kennel and Fowler (1987) asked when the pulsar expected in SN 1987A would become observable. Adopting an illustrative baseline model with birth spin period P0=10P_0=10 ms and dipole field B0=4×1012B_0=4\times10^{12} G, they estimated that the expanding shells become transparent to MeV gamma rays within the first one to two years (about 410 days at 1 MeV for the outer hydrogen shell, about 620 days for the inner iron shell), and proposed three observable signatures: (i) steady, unpulsed gamma rays from the pulsar-inflated nebula (of order 0.015 counts s−1^{-1} cm−2^{-2} at 1 MeV for a Crab-like spectrum); (ii) a flattening ('leveling') of the optical light curve once pulsar spin-down overtakes the declining radioactive power — potentially within about 10 months for the baseline model, or after about 4 years (a 20-magnitude decline) even for an ordinary 0.24 s pulsar; and (iii) pulsed radio or optical emission. They also listed the unknowns governing the answer: the unknown birth spin and field; possible dust formation in the cooling ejecta before about day 280, which would raise the optical depth and delay transparency; the unknown beam geometry, since pulsed detection requires the beam to sweep past Earth and even efficient pulsars such as the Crab emit only about 10−510^{-5} of their energy at radio wavelengths; the dispersion measure (the free-electron column that delays radio pulses, plausibly exceeding 10410^4 pc cm−3^{-3} for decades and so requiring exhaustive de-dispersion searches); alternative explanations of the slow type II decline such as interaction with dense circumstellar material; and the possibility that the neutron star collapsed to a black hole, testable only through the absence of a signal.

Every one of these channels has since been tested, and the 1987 prediction framework was resolved in the negative: the gamma rays that escaped were the 56^{56}Co decay lines (Matz et al. 1988), not pulsar emission; the light curve never showed a pulsar-driven flattening; the radio emission of the young remnant is synchrotron radiation from the outer (forward) shock of the explosion hitting the circumstellar rings, not pulsar power (Staveley-Smith et al. 2013; Zanardo et al. 2014); and a massive dust reservoir did form in the inner ejecta (Matsuura et al. 2011; Cigan et al. 2019), obscuring the center to this day. The direct searches have grown deep and multi-band: combining ALMA, VLT/HST and Chandra, and absorption models from three-dimensional explosion simulations, the compact object's bolometric luminosity is bounded to roughly 22–138 L⊙L_\odot depending on the dust geometry along the line of sight (Alp et al. 2018); the MeerKAT TRAPUM survey limits pulsed radio emission to 6.3 μ6.3\,\muJy at 1400 MHz (Prayag et al. 2024); and XRISM caps a putative pulsar-wind nebula — the nebula of relativistic particles inflated by a pulsar's wind — at 4.3×10−134.3\times10^{-13} erg cm−2^{-2} s−1^{-1} in 2–10 keV. Two indirect hints exist: NuSTAR's hard X-ray emission has been modeled as a heavily absorbed pulsar-wind nebula embedded in the ejecta (Greco et al. 2021; Greco et al. 2022), and an ALMA dust clump near the expected position of a kicked remnant has luminosity matching the expected thermal power of a cooling neutron star — a newly born neutron star radiating its stored heat, whose temperature evolution tests the dense-matter equation of state — within standard cooling theory (Cigan et al. 2019; Page et al. 2020).

The situation changed qualitatively in 2024. JWST infrared spectroscopy reveals narrow, spatially unresolved, blueshifted argon and sulphur lines from the innermost ejecta, which photoionization models attribute to a compact source of ionizing photons — a cooling neutron star or a pulsar-wind nebula — displaced from the explosion center in a way that could imply a natal kick (the recoil velocity imparted at birth) of about 510±55 km s−1510\pm55\,\mathrm{km\,s^{-1}} (Fransson et al. 2024; Larsson et al. 2025). This is the first signature of the compact object since the 1987 neutrino burst, but it is indirect: both papers state that no direct observation — no point source and no pulsations — exists, and the object's spin period, magnetic field, and even its identity as a neutron star rather than a black hole remain unmeasured. The thirty-year history of the remnant, including the compact-object searches, is reviewed by McCray and Fransson (2016). The unresolved target is: what is the compact remnant of SN 1987A, when and through which channel will it be directly seen, and what are its birth properties?

Problem Statement

Determine the nature and observability of the compact remnant of Supernova 1987A — the object whose formation on a known date in February 1987 was fixed by the neutrino burst. In the form of Michel, Kennel and Fowler's (1987) question 'When will a pulsar in supernova 1987a be seen?', the problem has three linked parts. (i) Establish what the remnant is: a neutron star in some state — for example a cooling embedded neutron star radiating its birth heat, an active pulsar with a pulsar-wind nebula, a radio-quiet young neutron star with a weak dipole field, or a fallback-accreting object — a black hole, or another compact-object state consistent with the observations; a black hole is testable only through detection or persisting non-detection. (ii) If it is a neutron star, measure or quantitatively bound its birth spin period and dipole magnetic field. (iii) Either achieve a direct detection of the object — a resolved point source, pulsations at any wavelength, or a compact nebular signature attributable to it — or establish when such a detection will first become possible, for an explicitly stated instrumental sensitivity and detection threshold, and in which band (pulsed radio or optical emission, thermal ultraviolet/X-ray emission, mid-infrared emission, or nebular gamma-ray emission).

The remnant's nature as determined, and any detectability prediction, must be consistent with the published non-detections and upper limits summarized above and with the JWST narrow-line evidence from the innermost ejecta, whose interpretation — a cooling neutron star, a pulsar-wind nebula, or an alternative such as shock-heated dust or 44^{44}Ti decay — remains open.

References

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