Emission lines due to ionizing radiation from a compact object in the remnant of Supernova 1987A

The nearby Supernova 1987A was accompanied by a burst of neutrino emission, which indicates that a compact object (a neutron star or black hole) was formed in the explosion. There has been no direct observation of this compact object. In this work, we observe the supernova remnant with JWST spectroscopy, finding narrow infrared emission lines of argon and sulfur. The line emission is spatially unresolved and blueshifted in velocity relative to the supernova rest frame. We interpret the lines as gas illuminated by a source of ionizing photons located close to the center of the expanding ejecta. Photoionization models show that the line ratios are consistent with ionization by a cooling neutron star or a pulsar wind nebula. The velocity shift could be evidence for a neutron star natal kick.

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Authors version of manuscript published in Science on 22 Feb 2024

References(5)

Figures(23)

1.84 × 10-6 Mg 6.46 × 10-4 3.12 × 10-2 Al 1.85 × 10-4

2.83 × 10-3 Si 2.26 × 10-1 6.79 × 10-3 S 1.56 × 10-1 1.17 × 10-3 Ar 2.74 × 10-2 1.47 × 10-4 Ca 8.29 × 10-3

3.51 × 10-5 Fe 1.40 × 10-3

4.34 × 10-4 Observations of the [O I] λλ6300, 6364 lines during the first 500 days after explosion were modeled with a maximum velocity ∼ 1700 km s-1 (34). Similarly, observations of these and other lines inwere modeled with a constant emissivity profile to ∼ 1700 km s-1, steeply decreasing at higher velocities. A fit with a parabolic line profile to the [O I] λ6300 line from 1999 (86) gives a good fit up to ∼ 2200 km s-1, but shows an extension above the parabolic profile at higher velocity, as expected from the 3D observations above. From this model we find VFWHM ≈ 3500 km s-1, implying Vcore ≈ 2500 km s-1. A similar fit to the [Fe I] λ 1.443 µm, [Fe II] λ 1.534 µm, and [Fe II] λ 25.98 µm lines (58) give Vcore ≈ 2700 km s-1. As an average we adopt Vcore = 2200 ± 500 km s-1. The ejecta mass inside the oxygen core of a 19 M⊙ progenitor is ∼ 3.0 M⊙, excluding the NS (16). For a uniform oxygen core density with the above mass and Vcore = 2200 km s-1, this corresponds to a density of ∼ 9.66 × 10-20 g cm-3 at 35 years, or a number density of ions of nion = 2.63 × 103 (A/22)-1

5.340 µm line, which is strong especially the south-west region [(13), figures 15 and 16]. However, there is little emission in this line from the central region, indicating that X-rays from the ER do not penetrate to the centre of the ejecta. It is therefore unlikely that external X-ray flux could excite the narrow high-ionisation emission lines we observe in the center, although it could be responsible for some [Ar II] emission from the region close to the ER. A reverse shock is known to be present in SNA, but only close to the ER, with a velocity of ≳ 4000 km s-1 (2). The inner ejecta must have been traversed by a reverse shock, but only within a few days of the explosion. Simulations have predicted the formation of an additional reflected shock, but it is expected to dissipate within a year (37). We therefore exclude the possibility of reverse shock excitation. A surviving companion star is constrained to ≲ 3.7 M⊙ if on the main sequence (15), which corresponds to an A to F star. These produce few ionising photons and have soft spectra, insufficient to explain the [S III], [S IV] and [Ar VI] emission. 2.3 Scenarios with a compact object Assuming no pulsar activity, the minimum bolometric luminosity of a central NS is that of a young CNS, which has a temperature of ≳ 106 K and luminosity ≳ 1034 erg s-1 (26), sufficient to ionize a substantial region of the inner core. We discussed this possibility in the main text and give more details below. For the scenario with a PWN, discussed in the main text, we assume the PWN produces X-ray emission with a power law spectrum with L(ν) ∝ ν-α with power law index α = 1.1