# Deep JWST/NIRCam imaging of Supernova 1987A

Mikako Matsuura, M Boyer, Richard G Arendt, J Larsson, C Fransson, A Rest, A.P Ravi, S Park, P Cigan, T Temim, et al.

► To cite this version:

Mikako Matsuura, M Boyer, Richard G Arendt, J Larsson, C Fransson, et al. Deep JWST/NIRCam imaging of Supernova 1987A. Monthly Notices of the Royal Astronomical Society, 2024, 532 (4), pp.3625-3642. 10.1093/mnras/stae1032. hal-04590785

HAL Id: hal-04590785

https://hal.science/hal-04590785v1

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# Deep JWST/NIRCam imaging of Supernova 1987A

Mikako Matsuura<sup>1,★</sup> M. Boyer,<sup>2</sup> Richard G. Arendt<sup>3,4</sup> J. Larsson<sup>5</sup> C. Fransson,<sup>6</sup> A. Rest,<sup>2,7</sup> A. P. Ravi<sup>8</sup>,<sup>10</sup> S. Park,<sup>8</sup> P. Cigan,<sup>9</sup> T. Temim,<sup>10</sup> E. Dwek,<sup>3</sup> M.J. Barlow<sup>11</sup> P. Bouchet,<sup>12,13</sup> G. Clayton<sup>14</sup>,<sup>15</sup> J. Chevalier,<sup>15</sup> J. Danziger,<sup>16</sup> J. De Buizer<sup>17</sup>, I. De Looze<sup>18</sup> G. De Marchi,<sup>19</sup> O. Fox<sup>2</sup>, C. Gall,<sup>20</sup> R. D. Gehrz,<sup>21</sup> H. L. Gomez,<sup>1</sup> R. Indebetouw,<sup>15,22</sup> T. Kangas<sup>23,24</sup> F. Kirchschlager<sup>11,18</sup> R. Kirshner,<sup>25</sup> P. Lundqvist<sup>5</sup> J.M. Marcaide,<sup>26</sup> I. Martí-Vidal<sup>26,27</sup> M. Meixner,<sup>28</sup> D. Milisavljevic,<sup>29,30</sup> S. Orlando<sup>31</sup>, M. Otsuka<sup>32</sup> F. Priestley,<sup>1</sup> A.M.S. Richards,<sup>33</sup> F. Schmidt,<sup>11</sup> L. Staveley-Smith<sup>34</sup>,<sup>35</sup> J. Spyromilio,<sup>36</sup> J. Vink<sup>37,38,39</sup> Lifan Wang,<sup>40</sup> D. Watson,<sup>41,42</sup> R. Wesson<sup>1,11</sup> J. C. Wheeler<sup>43</sup>, C.E. Woodward,<sup>21</sup> G. Zanardo<sup>33</sup>, D. Alp<sup>5</sup> and D. Burrows<sup>44</sup>

*Affiliations are listed at the end of the paper*

Accepted 2024 April 15. Received 2024 April 15; in original form 2023 September 1

## ABSTRACT

JWST/NIRCam obtained high angular resolution (0.05–0.1 arcsec), deep near-infrared 1–5 µm imaging of Supernova (SN) 1987A taken 35 yr after the explosion. In the NIRCam images, we identify: (1) faint H₂ crescents, which are emissions located between the ejecta and the equatorial ring, (2) a bar, which is a substructure of the ejecta, and (3) the bright 3–5 µm continuum emission exterior to the equatorial ring. The emission of the remnant in the NIRCam 1–2.3 µm images is mostly due to line emission, which is mostly emitted in the ejecta and in the hotspots within the equatorial ring. In contrast, the NIRCam 3–5 µm images are dominated by continuum emission. In the ejecta, the continuum is due to dust, obscuring the centre of the ejecta. In contrast, in the ring and exterior to the ring, synchrotron emission contributes a substantial fraction to the continuum. Dust emission contributes to the continuum at outer spots and diffuse emission exterior to the ring, but little within the ring. This shows that dust cooling and destruction time-scales are shorter than the synchrotron cooling time-scale, and the time-scale of hydrogen recombination in the ring is even longer than the synchrotron cooling time-scale. With the advent of high sensitivity and high angular resolution images provided by JWST/NIRCam, our observations of SN 1987A demonstrate that NIRCam opens up a window to study particle-acceleration and shock physics in unprecedented details, probed by near-infrared synchrotron emission, building a precise picture of how an SN evolves.

**Key words:** circumstellar matter – dust, extinction – ISM: supernova remnants – infrared: ISM.

## 1 INTRODUCTION

The explosion of Supernova (SN) 1987A was detected on 1987 February 23,<sup>1</sup> in a neighbouring galaxy, the Large Magellanic Cloud, at a distance of 49.6 kpc (Pietrzyński et al. 2019). This is the nearest SN explosion detected in 400 yr, since Kepler's SN in 1604 (McCray 1993). Due to its proximity, SN 1987A can be well resolved with modern telescopes, and was observed at almost every wavelength from the γ-ray to the radio (Arnett et al. 1989).

The iconic Hubble Space Telescope (HST) optical image of SN 1987A consists of the inner ejecta, the equatorial ring, and the outer rings. These components are also illustrated by our image (Fig. 1),<sup>2</sup> taken by NIRCam (Rieke et al. 2023) on board the JWST

(Gardner et al. 2023). The equatorial and outer rings are circumstellar material that was ejected from the progenitor (Jakobsen et al. 1991; Crotts, Kunkel & Heathcote 1995). It is not known exactly how these rings were formed, but most likely the process involved a binary system (e.g. McCray 2003). It has been suggested that until about 20 000 yr ago, the progenitor star was a red supergiant (Crotts & Heathcote 1991). The mass-loss wind from the red supergiant was diverted to an equator by a binary system, and this process may eventually have led to the formation of the iconic rings around SN 1987A (Lloyd, O’Brien & Kahn 1995). Morris & Podsiadlowski (2007) further proposed that the merger of a binary companion spun out the rings, and at the same time the merger turned the progenitor into a blue supergiant. However, it still remains unknown when and how the progenitor turned into a blue supergiant, but it could involve dredge-up or binary merger (Saio, Nomoto & Kato 1988; Arnett et al. 1989; Fransson et al. 1989). A few Galactic examples of similar rings are seen around blue supergiants and luminous blue variables (Brandner et al. 1997; Smith 2007; Smith, Bally & Walawender 2007; Smith et al. 2013).

<footnote>*E-mail: matsuuram@cardiff.ac.uk</footnote>

<footnote n="1">¹http://www.cbat.eps.harvard.edu/iauc/04300/04316.html</footnote>

<footnote n="2">²The image is modified from the original image published at https://webbtelescope.org/contents/media/images/2023/136/01H8Q02S452MC9CAF0VSJ3ZTFX</footnote>
<figure><img src="image_2.png" /><figcaption>**Figure 1.** NIRCam five-colour image (blue: F150W, cyan: F164N, cyan: F200W, yellow: F323N, orange: F405N, and red: F444W), capturing the bright equatorial ring, shocked gas beyond the equatorial ring, and the very faint outer rings. The inner ejecta stands out in cyan as it has strong F164N emission. Within the inner ejecta, a bar crosses across approximately from the east to west. Two crescents are found in very faint blue/cyan emission between the inner ejecta and the equatorial ring. The hotspots are bright spots in the NIRCam/SW images within the equatorial ring, while the outer spots are found exterior of the equatorial ring. North is towards the top and east is to the left. The original image was created by Alyssa Pagan (STScI), and the image credits belong to NASA, ESA, CSA, Mikako Matsuura (Cardiff University), Richard Arendt (NASA-GSFC, UMBC), Claes Fransson (Stockholm University), and Josefin Larsson (KTH).</figcaption></figure>

The outer ejecta expand with a speed exceeding 10 000 km s⁻¹ (Phillips et al. 1988; McCray 1993), triggering shocks within the circumstellar material. Shocks brightened the equatorial ring (labelled in Fig. 1) in about 1995 (Sonneborn et al. 1998; Lawrence et al. 2000), and the brightness increased in X-ray, optical, infrared (IR) and radio until about 2010 (Dwek et al. 2010; Fransson et al. 2015; Frank et al. 2016). Approximately in 2010, the shocks passed the equatorial ring, and material outside of the equatorial ring is now detected as faint outer spots (labelled in Fig. 1) in the optical images (Fransson et al. 2015; Larsson, Fransson & Alp 2019). Around this

time, the light curves in different bands started to diverge. The hard X-ray and radio brightness continued to increase (Frank et al. 2016; Cendes et al. 2018), though at a slower rate than before 2003. Meanwhile, the near-infrared (NIR) brightness, as observed using *Spitzer*, peaked around 2010 and started declining thereafter (Arendt et al. 2016, 2020). The cause of diverged light curves at different wavelengths was unknown. The mid-IR emission is known to be from silicate dust in the equatorial ring (Bouchet et al. 2006; Dwek et al. 2010; Arendt et al. 2016; Matsuura et al. 2022). Although it has been suggested to be ‘hot’ (~500 K) dust (Bouchet et al. 2006;
<table><caption>**Table 1.** JWST/NIRCAM observations of SN 1987A.</caption><thead><tr><th>Channel</th><th>Filter</th><th>λ<sub>p</sub></th><th>BW</th><th>λ<sub>B</sub></th><th>λ<sub>R</sub></th><th>PSF</th><th>t<sub>exp</sub></th><th>I<sub>v BK</sub></th><th>σI<sub>v BK</sub></th><th>Error</th></tr></thead><tbody><tr><td rowspan="4">Short</td><td>F150W</td><td>1.501</td><td>0.318</td><td>1.331</td><td>1.668</td><td>0.05</td><td>1347</td><td>0.175</td><td>0.067</td><td>0.134</td></tr><tr><td>F164N*</td><td>1.645</td><td>0.02</td><td>1.635</td><td>1.653</td><td>0.056</td><td>37 716</td><td>0.802</td><td>0.112</td><td>0.059</td></tr><tr><td>F200W</td><td>1.989</td><td>0.457</td><td>1.755</td><td>2.226</td><td>0.066</td><td>1155</td><td>0.235</td><td>0.081</td><td>0.131</td></tr><tr><td>F212N</td><td>2.121</td><td>0.027</td><td>2.109</td><td>2.134</td><td>0.072</td><td>28 287</td><td>0.782</td><td>0.084</td><td>0.058</td></tr><tr><td rowspan="4">Long</td><td>F323N*</td><td>3.237</td><td>0.038</td><td>3.217</td><td>3.255</td><td>0.108</td><td>28 287</td><td>-0.015</td><td>0.007</td><td>0.05</td></tr><tr><td>F356W</td><td>3.568</td><td>0.781</td><td>3.14</td><td>3.98</td><td>0.116</td><td>1347</td><td>-0.418</td><td>0.004</td><td>0.067</td></tr><tr><td>F405N*</td><td>4.052</td><td>0.045</td><td>4.028</td><td>4.074</td><td>0.136</td><td>37 716</td><td>0.485</td><td>0.071</td><td>0.034</td></tr><tr><td>F444W</td><td>4.408</td><td>1.029</td><td>3.88</td><td>4.986</td><td>0.145</td><td>1155</td><td>-0.02</td><td>0.014</td><td>0.066</td></tr></tbody></table>

λ<sub>p</sub>: pivot wavelength in µm (Tokunaga & Vacca 2005). BW: bandwidth in µm. λ<sub>B</sub> and λ<sub>R</sub>: the half-power wavelengths in µm of a passband at blue and red wavelengths, at which the transmission falls to 50 per cent of its peak value.

PSF: FWHM of the empirical PSF. t<sub>exp</sub>: exposure time on source in sec. I<sub>v BK</sub> and σI<sub>v BK</sub>: mean and 1σ of ‘blank sky’ level in a nearby field (MJy sterad<sup>-1</sup>). Error: mean error level estimated from the error map in a nearby field but off from SN 1987A. This error combines Poisson noise and read noise (MJy sterad<sup>-1</sup>). *F164N, and F323N, and F405N filters are used together with the wide-band filters F150W, F322W2, and F444W, respectively.

Dwek et al. 2010), the emitting region and the source of the NIR 3–4 µm flux was not well established, because no instrument had the resolution and sensitivity to clearly spatially resolve SN 1987A at 3–4 µm before JWST. Modelling the total flux of NIR to mid-IR spectra from JWST/NIRSpec and Mid-Infrared Instrument (MIRI) shows that the combination of dust, synchrotron, and atomic emission contributes to overall emission at this wavelengths (Jones et al. 2023; Larsson et al. 2023). NIRCam imaging provides the highest angular resolution and best sensitivity available with JWST, and can pinpoint the emitting regions of different components of dust, synchrotron, and atomic lines (Arendt et al. 2023). This will allow us to understand precisely the physics of dust, synchrotron, and atomic line emission in shocked and post-shocked regions in SN 1987A, enabling us to witness the real-time physics of an evolving SN.

## 2 JWST/NIRCAM OBSERVATIONS OF SN 1987A

We obtained deep images of SN 1987A, using NIRCam on JWST under general observer program 1726. The data were acquired on 2022 September 1 and 2, days 12 974 and 12 975 after the SN explosion was first detected. NIRCam has a short wavelength channel (0.6–2.3 µm, hereafter SW) and a long-wavelength channel (2.4–5.0 µm, hereafter LW). Eight filters were used: four in the SW channel, and four in the LW channel. The properties of the filters are summarized in Table 1.

We chose to use the subarray of NIRCam detectors rather than using the full array, because this observing mode saves total observing time and avoids total saturation by nearby field stars. SN 1987A measures approximately 8 arcmin across its outer rings, and the whole system was completely covered by the 320 × 320 pixel subarray of the detector by the SUB320 mode of the Module B camera. In the SW channel, the total area covered is approximately 25 arcsec × 25 arcsec, with a ~5 arcsec gap between the subarrays of the four detectors. In the LW channel, the field of view is 20.45 arcmin × 20.50 arcmin using a single subarray (with larger pixels). Fig. 2 demonstrates the coverage of SUB320 images of the LW channel. The NIRCam pixel scales are 0.0317 arcmin in the SW channel and 0.0647 arcmin in the LW channel. The full width at half-maxima (FWHM) of the point spread functions (PSF) are 0.066 arcmin at F200W and 0.16 arcmin at F356W.<fnref n="3" /> The RAPID

<figure><img src="image_3.png" /><figcaption>**Figure 2.** NIRCam long wavelength channel images, covering the entire field of view around SN 1987A. North is top, east is to the left, and the area covers 25.4 arcmin × 25.2 arcmin. The colour allocations are – blue: F323N, green: F356W, red: F405N, and brown: F444W.</figcaption></figure>

reading mode was chosen for wide-band filters, while MEDIUM 8 was used for longer exposures with the narrow-band filters. The total exposure times on source are summarized in Table 1, ranging from 1155 to 37716 s per filter band.

The NIRCam sensitivities were approximately 30 per cent better than the pre-flight prediction (Rigby et al. 2023). As a consequence, we achieved 0.004–0.11 MJy sr<sup>-1</sup> at 1σ noise levels, measured in a blank sky area (Table 1), which constitutes very deep imaging of SN 1987A. Fig. 2 shows that this deep image can detect Brα emission in the ambient interstellar medium, evident as the patchy background emission in F405N filter (red).

### 2.1 Data reduction

The NIRCam data were reduced using the JWST Calibration Pipeline (Bushouse et al. 2023) software version 2022–3. The calibration

<footnote n="3">³https://jwst-docs.stsci.edu/jwst-near-infrared-camera/nircam-performance/nircam-point-spread-functions</footnote>
<table><caption>**Table 2.** The strong lines falling into the NIRCam filter bands (with their vacuum wavelength in µm). Lines are listed separately for the ejecta and the equatorial ring, and are taken from Larsson et al. (2023).</caption><thead><tr><th>Channel</th><th>Filter</th><th>Ejecta</th><th>Ring</th><th>Note</th></tr></thead><tbody><tr><td rowspan="4">Short</td><td>F150W</td><td>[Fe I] 1.444; [Si I] 1.607; [Fe II] + [Si I] 1.64 blend</td><td>[Fe II] 1.534; [Fe II] 1.600; [Fe II] 1.644</td><td></td></tr><tr><td>F164</td><td>[Fe II] + [Si I] 1.64 blend</td><td>[Fe II] 1.644</td><td></td></tr><tr><td>F200W</td><td>H I 1.875 (Paα); He I 2.059; H<sub>2</sub> 2.122</td><td>H I 1.875 (Paα); He I 2.059; H I 2.166 (Brγ)</td><td></td></tr><tr><td>F212</td><td>H<sub>2</sub> 2.122 ν = 1–0 S(1)</td><td>He I 2.114; [Fe II] 2.135</td><td></td></tr><tr><td rowspan="4">Long</td><td>F323N</td><td>(H<sub>2</sub>ν = 1–0 O(5))</td><td></td><td>Continuum</td></tr><tr><td>F356W</td><td>H<sub>2</sub> 3.846; Unidentified ~3.88µm</td><td>H I 3.297; H I 3.744</td><td>Lines weak relative to continuum</td></tr><tr><td>F405N</td><td>H I 4.052 (Brα)</td><td>H I 4.052 (Brα)</td><td></td></tr><tr><td>F444W</td><td>H I 4.052 (Brα); H I 4.654; H<sub>2</sub> 4.695</td><td>H I 4.052 (Brα); H I 4.654</td><td></td></tr></tbody></table>

F164N bandpass, and the filter width corresponds to −2203 to 1082 km s⁻¹ in the rest frame. In the ejecta as a whole, the SINFONI and NIRSpec spectra show that the line intensity ratio of [Fe II] to [Si I] is approximately 1/3 to 1/2 (Jerkstrand, Fransson & Kozma 2011; Larsson et al. 2023). The ratio of the contribution of the two lines to the intensity in the F164N band might be slightly different at different locations. In the ring, the 1.644 µm [Fe II] line dominates the line intensity, and little contribution from [Si I] line is present. Similarly, the F212N filter covers the H<sub>2</sub> ν = 1–0 S(1) line from −1432 to 2100 km s⁻¹ and the F405 filter covers Brα from −1319 to 2083 km s⁻¹. The majority of the intensities of these lines are within the filter bands, but the broader (but fainter) components are not covered by these narrow-band filters.

The fluxes of the LW channel have more contribution from the continuum. The NIRSpec spectra of the ejecta and the equatorial ring show that the continuum rises with wavelength both in the equatorial ring and in the ejecta (Larsson et al. 2023).

Although the F323N filter was designed to detect the 3.234 µm H<sub>2</sub>ν = 1–0 O(5) line (Black & Dishoeck 1987), the image of SN 1987A in this filter is similar to those made using the broad-band filters of the LW channel, in particular F356W. We conclude that the F323N flux is dominated by the continuum rather than the H<sub>2</sub> line in all features, except for faint crescent-shaped structures observed between the ejecta and the equatorial ring. This will be explored further in Matsuura et al. (in preparation).

### 2.2.1 Auxiliary data: ALMA

ALMA observed SN 1987A with two tunings, 301.75–305.57 GHz in kinematic local standard of rest (993.51–981.09 µm) and 313.73–317.52 GHz (955.57–944.17 µm), on 2021 November 2 as part of the Project 2021.1.00707.S. The total integration time was 10 813 s. The synthesized beam was 0.081 arcsec × 0.061 arcsec with a position angle of 33.7° for the major axis. The shortest baselines allowed for a maximum recoverable scale of 0.993 arcsec. This implies that the diffuse emission, which extends more than 1 arcsec, is likely to be over-resolved, resulting in some flux loss.

## 3 RESULTS AND DISCUSSIONS

SN 1987A contains three major components which we describe separately, namely, the inner ejecta (Section 3.1), the equatorial ring and associated diffuse emission (Sections 3.2 and 3.3), and the outer rings (Section 3.4). Fig. 1 highlights these features using the NIRCam images.

The main findings of the NIRCam images are (1) the substructure of the ejecta, namely, the bar (Section 3.1.1), (2) crescents, faint emission between the ejecta and the equatorial ring, and (3) the first

identifications of the NIR synchrotron emission from the hotspots, which are bright spots composing of the equatorial ring, the outer spots and diffuse emission, which are exterior to the equatorial ring (Sections 3.2 and 3.3). The analysis of crescents will be reported in a separate paper.

### 3.1 Ejecta

The ejecta are detected in all four filter band images in the SW channel (Fig. 4). The keyhole or hourglass shape of the ejecta extends from the north to the south. The ejecta contain a dip or ‘hole’ near the centre with a slight offset to the north. This ‘hole’ is not a hole in the gas, but probably due to the heavy extinction (self-absorption) by recently formed SN ejecta dust (Matsuura et al. 2011; Indebetouw et al. 2014; Cigan et al. 2019). At the waist of the ejecta, a small bar is seen from the east to the west in blue colour in Fig. 1 and F150W and F164N images (Fig. 4). The southern part of the ejecta breaks into two branches near the equatorial ring.

On the other hand, the LW images do not show a clear keyhole shape in the ejecta (Fig. 5). Instead, there are two faint blobs in the north and the south in the F323N and F356W images. In fact, the northern blob in the F323N and F356W images corresponds to the hole in the SW channel images. In the middle of the two blobs, a ‘gap’ spreads from the east to the west. The exception is the F444W image, which shows a hint of the keyhole shape and the bar, similar to that seen in the SW images.

The ratio of F444W/F356W in Fig. 6 (right bottom) indicates an excess in F444W in the bar. This suggests that the dust self-absorption towards the bar is still strong at F356W, while the dust self-absorption is less in F444W.

While the SW bands have emission line contributions, the ejecta emission in the LW bands is dominated by continuum emission (see Section 3.3 and Fig. 11; also Larsson et al. 2023). The NIRCam spectral energy distribution (SED) decomposition analysis shows that in the LW bands the ejecta fluxes are indeed dominated by the continuum (Arendt et al. 2023). The source of the continuum is presumably from cold SN ejecta dust (Matsuura et al. 2011, 2015; Indebetouw et al. 2014). The ALMA (Atacama Large Millimeter/submillimeter Array) continuum images at 450 and 870 µm show more dust towards the centre of the system (Cigan et al. 2019). This causes dust self-absorption around the waist of the ejecta at NIR wavelengths, which is seen as a dip at the centre of the ejecta in NIRCam images. As dust extinction is higher at the shorter wavelength of the NIR, the SW images of this area trace mainly the foreground of the heavily obscured dusty regions in the ejecta.

The dust emission extends across the ejecta (Cigan et al. 2019), but is fainter towards the north and south edges of the ejecta, causing less extinction at NIR wavelengths. These regions are also heated by the
<figure><img src="image_4.png" /><figcaption>**Figure 4.** Short-wavelength channel images of the equatorial ring and the faint extended emission beyond the ring and ejecta. Within the ejecta, a bar, extending from the east to the west is found. The equatorial ring consists of more than 20 hotspots, while over 10 outer spots exterior to the equatorial ring are detected. Faint diffuse emission extends far beyond the outer hotspots. The coverage of the images is 3 arcmin × 3 arcmin, with the north at the top, and the colour stretch is in the unit of MJy sr⁻¹.</figcaption></figure>

UV radiation from the equatorial ring, hence, the NIR emission is detected as blobs in the LW images. The total flux of the ejecta is about 12 µJy at F356W. Extrapolating the cold (~20 K) dust emission from the millimetre wavelengths to the NIR would result in less than 1 µJy at F356W. Ejecta dust must be heated to temperatures higher than 20 K in the blobs to emit 12 µJy in this band. The obscuration of the hole is seen because cold dust in the centre absorbs the emission from the dust in the far end of the warmer ejecta.

### 3.1.1 Bar

The bar is a small structure, sticking out to the east and to the west at the waist of the ejecta (Figs 1 and 4). The east component of the bar is slightly tilted to the southeast. The west side of the bar has been detected by HST Hα imaging before (e.g. Larsson et al. 2013), but in the optical images, the east side of the bar was not detected clearly. This indicates dust extinction in the line of sight on the east side of the bar.

The apparent structure of the bar slightly differs from one wavelength to another (Fig. 4). In the F164N image, the bar looks like a triangular shape, separated from the main body of the ejecta. The bar is also found in the F150W and F200W images, but less distinct than in F164N. In the F212N image, there is only a faint trace of the bar structure. This shows that the emission causing the bar is mainly from the atomic lines such as [Fe II] + [Si I] (F150W and F164N) and probably also [Fe I] at 1.44 µm in F150W, or H I and He I (F200W), but not H₂ (F212N).

At the location of the bar, there is a dip in surface brightness in F323N and F356W (Fig. 5). Fig. 7 shows the F356W image in colour, with contours of the F164N image. The bar in the F164N image corresponds to the location of the low surface brightness region in the F356W image. The waist of this bar also contains some emission of the dust in the ALMA 315 GHz image (Fig. 8). This region probably contains a large mass of dust, so that it absorbs and scatters near-IR light. If there is substantial IR extinction, the [Fe II], [Si I], H I, and
<figure><img src="image_5.png" /><figcaption>**Figure 5.** NIRCam LW channel images. The outer spots exterior to the equatorial ring are as bright as the hotspots within the ring in the LW images, apart from F405N. The ejecta emission in the LW images is dominated by continuum, mostly due to dust. The dip in the centre of F343N, F356W, and F405N is caused by dust self-absorption. Only two blobs are detected in these images. The F405N image traces both the dust continuum and Brα in the ejecta. The coverage of the images is 3 arcmin×3 arcmin, with the north at the top, and the colour stretch is in the unit of MJy sr⁻¹.</figcaption></figure>

He I emission must come from the surface of the bar structure on the near side to us, and therefore does not reflect the whole structure of the bar.

### 3.1.2 [Si I] and [Fe II] emission

In general, the 1.64 µm [Fe II] line is often used as a tracer of shocks. In SN remnants, there is the complication of blending of two lines ([Si I] and [Fe II]) in the F164N filter. These two atomic stages have quite different ionization potentials. Fe⁺ has an ionization potential of 16.2 eV (Mouri, Kawara & Taniguchi 2000). The ionization energy of Si⁰ is only 8.2 eV.

The top left panel of Fig. 6 shows the ratio of the F164N image to the F150W image ($I_{v\,F164N}/I_{v\,F150W}$). The interpretation of this image requires some caution, because the F150W image, which is typically expected to be a continuum, actually contains atomic lines, such

as [Fe I]. Nevertheless, the ratio image combined with the F164N image should show characteristics of the [Si I] and [Fe II] emitting regions, as the F164N intensity is typically three times brighter than the F150W intensity in the ejecta.

The $I_{v\,F164N}/I_{v\,F150W}$ ratio in Fig. 6 shows two peaks, one at the north-northeast and the other at the northwest edges of the ejecta. The primary peak at the northwest ejecta is found in the original F164N image, so that this is due to an excess of [Si I] and [Fe II] line intensities, and not due to the dip in the [Fe I] line intensity in F150W.

While the 1.64 µm [Fe II] line is often used as a tracer of shocks in supernova remnants (SNRs) or active galactic nuclei, this line in SN 1987A might not be solely due to shock excitation. The excitation mechanisms of lines might be different in the outer and the inner parts of the 'inner ejecta'. (Note that historically, 'outer ejecta' refers to the ejecta with high velocity but low density, extending beyond the keyhole, and some part of this has passed the equatorial ring
<figure><img src="image_6.png" /><figcaption>**Figure 6.** The colours show the intensity ratios of two filter bands. The line contours are plotted to guide the eye; these represent the *F212N* image, smoothed to the angular resolutions of the filter bands used to compute the intensity ratios.</figcaption></figure>

and has been interacting with the ring and its exterior material.) The prominent [Fe II] line at 1.644 µm is due to the $a^4F_{9/2}-a^4D_{7/2}$ transition (Mouri et al. 2000). The critical density for collisional de-excitation is $n_{crit} \sim 10^5 \text{ cm}^{-3}$ at the electron temperature of $T_e = 10^4 \text{ K}$ (Shull & Steenberg 1982; Greenhouse et al. 1991). At lower kinetic temperature and lower density, the excitation of the [Fe II] 1.644 µm line is mainly by UV fluorescence and non-thermal excitation and ionization, followed by radiative de-excitation to the upper $a^4F_{9/2}$ level of the 1.644 µm line (Jerkstrand et al. 2011). Thermal collisional excitation is not important in the inner region: it has a very low temperature, only <200 K even at 8 yr after the explosion (Jerkstrand et al. 2011), compared to the excitation temperature of the $a^4F_{9/2}$ level of 11 445 K.

However, thermal collisional excitation may be important for the [Fe II] line in the region close to the ring, where the X-ray photoionization and heating (Fransson et al. 2013) may result in a temperature >2000 K, enough to excite this line. The secondary peak of the emission at the north-northeast of the ejecta close to

the equatorial ring (the top left panel of Fig. 6) probably traces the interaction of the ejecta with the equatorial ring at that location (Larsson et al. 2023).

Integrated across the entire ejecta, the [Si I] line is predicted to contribute more to the *F164N* intensity than the [Fe II] line (Larsson et al. 2023). At the primary peak, instead of [Fe II], the [Si I] line might contribute more to the intensity, and its excitation could be due to recombination at temperatures below 200 K (Raymond et al. 2018; Larsson et al. 2023).

### 3.1.3 H₂ emission

The *F212N* image in Fig. 4 and the $I_v F212N/I_v F200W$ ratio in Fig. 6 show the H₂ emitting region in the ejecta. The overall shape of the H₂ emitting region within the ejecta is similar to those of
<figure><img src="image_7.png" /><figcaption>**Figure 7.** F356W image in colour contours in the unit of MJy sr⁻¹, with F164N represented by line contours. It looks like that the bar emission in F164N is a dip in intensity in the F356W image.</figcaption></figure>

the Hα (F625W; Larsson et al. 2019) and [Fe II] emitting regions (Fig. 4), but the H₂ peak is located in the south, corresponding to the locations where the Hα and [Fe II] emissions are fainter. The H₂ strength has almost an anticorrelation with the [Fe II] strength: the peak of [Fe II] is in the northwest, and that side of the ejecta has weaker H₂ emission. The anticorrelation can also be found in Hα and H₂ (Larsson et al. 2019). This could be the effect of the strength of the X-ray and 912–1100 Å UV illumination from the equatorial ring onto particular locations within the ejecta. On the west side of the equatorial ring and just outside the ring, there are ongoing strong shocks, while on the southeast side, only weak emission is found from the equatorial ring (Section 3.2; also Frank et al. 2016). The X-ray radiation from the southeast of the ring imprinting onto the southeast part of the ejecta is also weaker, hence, there is more molecular gas found in the southeast than in the rest of the ejecta.

### 3.1.4 Brα emission

The bottom left image of Fig. 5 is the Brα-dominated F405N image, and the I<sub>V</sub>F405N/I<sub>V</sub>F356W ratio in Fig. 6 shows the Brα emitting region in the ejecta. The distinct key-hole shape of the ejecta, found in Fig. 4 and the HST Hα image (Larsson et al. 2019), is not obvious in Brα. This is an illusion of the colour stretch used in Fig. 5, and there is a slight dip in the brightness. As seen in Fig. 5, Brα is very faint in the ejecta, compared with the equatorial ring. Nevertheless, the peak of the Brα emission on the west side of the ejecta is part of the bar. Very faint Brα emission from the north of the ejecta is also identified as a blob. It is not clear whether the additional very faint blob on the east side of the ejecta is also due to the bar, with a possible additional contribution from the outer ring.

In the LW channel images, the F405N image is distinctly different from the rest of the images (Fig. 5). In the F323N and F356W images, there are two separate blobs, one in the north and the other in the south of the ejecta. In the F405N image, the north blob is detected, but the south blob is missing. It seems that the Brα emission from

just inside the equatorial ring is stronger than the Brα emission from the south blob, burying the emission from the south blob.

## 3.2 The equatorial ring and its exterior

The NIRCam images of SN 1987A captured detailed structures found within and exterior to the equatorial ring (Fig. 1). The equatorial ring is composed of at least 22 hotspots along its circumference. Hotspots are bright parts within the equatorial ring in UV and optical, and in the past, associated with the forward shocks colliding into dense clumps (Sonneborn et al. 1998). They have the highest density within the circumstellar material. These hotspots within the equatorial ring are the brightest structures within the SN 1987A system in the SW images (Fig. 4).

Exterior to the equatorial ring, there are rings of faint diffuse emission. The outer diffuse emission is composed of at least three ellipses, possibly four, though these numbers and shapes are subjective (Fig. 9). Three separate arcs are clearly seen in the northeast part within the diffuse emission (Fig. 4). It is unclear whether ellipses 2 and 4 are actually a single ellipse or two separate ellipses that are offset in the north and south directions. The diffuse emission is expected to be due to the reverse shock (Larsson et al. 2023). JWST/NIRSpec observations of the He I 1.083 µm line were used to determine the 3D morphology of the line, which revealed a bipolar shape with bubble-like structures extending outwards on both sides of the equatorial ring (Orlando et al. 2020; Larsson et al. 2023). Projection and limb brightening of the shock surface may create several elliptical structures in the images.

While the faster (> 10 000 km s⁻¹) but low-density outer ejecta expands continuously since the SN explosion (Larsson et al. 2016; Kangas et al. 2022), it interacts with the circumstellar envelope. Until about 2014, the locations of the main interactions of the outer SN ejecta with the circumstellar material were within the equatorial ring (France et al. 2015). This interaction is now most actively occurring at the equatorial ring on the north and west sides and the exterior to the equatorial ring. The interaction causes emission, which contains continuum emission (a combination of synchrotron and dust, discussed in the next section) and line emission, and appears as diffuse emission outside of the equatorial ring. Diffuse emission is detected by HST (Larsson et al. 2019) but it is clearer in JWST NIRCam images, because of the brighter contribution of synchrotron emission in the NIR than in the Hα emission observed with HST.

In these diffuse rings, we found spots, which we refer to as 'outer spots' to distinguish them from the hotspots within the equatorial ring (Larsson et al. 2019). These outer spots and hotspots are spatially unresolved at the NIRCam angular resolutions (FWHM of <0.05 arcsec; Table 1). Having outer spots within the diffuse ring suggests that the mass-loss material from the red supergiant may not have been smooth, but had contained many clumps within. As the thin but faster SN ejecta expands and collides with the circumstellar material, the gas in these clumps is now ionized and shocked, and forms as 'outer spots'.

Fig. 5 shows that the outer spots are as bright as the hotspots, or even brighter on the east side in the LW images, except for in the F405N image. These images are in contrast to the SW images, which show brighter hotspots than the outer spots. The outer spots in the LW images appear to form an outer portion of the equatorial ring, extending all around it (Arendt et al. 2023). In general, both the hotspots and the outer spots are brighter on the western side than on the eastern side.
<figure><img src="image_8.png" /><figcaption>**Figure 8.** F164N image (left) and F356W image (right) with ALMA 315 GHz continuum emission shown as line contours. The ALMA continuum image contains synchrotron radiation in the ring and dust thermal emission in the ejecta. The hotspots and the outer spots detected in the F356W image generally have corresponding spots in ALMA 315 GHz synchrotron emission.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 9.** Illustrating the three or four faint ellipses within the diffuse emission. The ellipses 1 and 2 have well-defined edges, while ellipses 3 and 4 have partial ellipses, and their full structures are not clear. Ellipse 4, tracing on the south part of the diffuse emission, might be tracing a few outer spots, rather than a full ellipse, so that the total number of ellipses could be three or four.</figcaption></figure>

## 3.3 Power-law spectra at 3–4 µm in the equatorial ring and its exterior

In order to understand the nature of emitting sources, we measure the SEDs within several apertures across SN 1987A. The locations of

<figure><img src="image_10.png" /><figcaption>**Figure 10.** Small regions (in colour) used for SEDs in Fig. 11 and the white elliptical annuals used for the ring and outer spots photometry in Fig. 12 (also in Appendix B), overlaid on F323N image.</figcaption></figure>

apertures are marked in Fig. 10, and the extracted SEDs are plotted in Fig. 11. The ‘background’ or ISM continuum level, which is estimated from the nearby field (Section 2), has been subtracted beforehand.

The fluxes at F323N, F356W, and F444W follow a power-law continuum across SN 1987A (black lines in Fig 11). The ejecta have the steepest power-law index $\alpha$ at a value of 3.1, where $\alpha$
<figure><img src="image_11.png" /><figcaption>**Figure 11.** The spectra at represented regions indicated in Fig. 10. The black lines are power-law fits to the *F323N*, *F356W*, and *F444W* fluxes and the grey horizontal lines show the filter widths. The uncertainties of the fluxes are smaller than the plotting symbols.</figcaption></figure>

is defined as $F_v \propto v^{-\alpha}$. Apart from the ejecta, as expected from the $I_v$ F444W/$I_v$ F356W ratio map in Fig. 6, the steepest power-law index is found on the western side of the outer spots. The power-law indices are $\alpha = 2.9$ and 1.9 at the western and eastern outer spots, respectively. The power-law indices are as small as $\alpha = 1.6$ at the hotspots on the ring and $\alpha = 1.5$ within the diffuse emission.

### 3.3.1 NIR continuum: synchrotron and dust

In light of the power-law SEDs in the 3–4 µm wavelength range, we discuss the sources of this emission in the equatorial ring and its exterior. Historically, the emission at 3–4 µm was solely attributed to dust thermal emission from the equatorial ring, but synchrotron emission can be an additional and important source at the current epoch. In shocked gas, fast-moving electrons collide with dust grains and collisionally heated dust grains, which have a temperature of 180–200 K (Dwek et al. 2010; Matsuura et al. 2018), and radiate their energy at the NIR wavelengths (Fig. 12). The modified blackbody emission from the warm dust drops sharply from the SED peak towards NIR wavelengths, in accordance with the Wien's law approximation to the Planck function. Thus, NIR emission has been explained by additional 'hot' (370–460 K; Dwek et al. 2010) dust consisting of smaller grains. Because the heat capacities of small grains are lower, they can reach much higher energy than the larger grains.

On the other hand, our total SED analysis suggests that synchrotron emission can contribute substantially to the NIR flux in the ring and its exterior. Fig. 12 shows the integrated SED of the ring and outer hotspots from the IR to the millimetre wavelengths. The total fluxes in the *JWST F323N*, *F356W*, and *F444W* bands are plotted in open green circles, where the aperture sizes are described in Appendix B. The historical measurements of the radio synchrotron emission before 2014 are plotted in blue circles, and they demonstrate the power law of the synchrotron emission (Zanardo et al. 2010, 2013, 2014). The day 10 942 flux measurement at 8.7 GHz from Cendes et al. (2018) is plotted with the filled purple circle. The total ALMA 315 GHz flux in 2021 is plotted with an orange open circle and the flux excluding the ejecta is plotted in a filled orange circle. The radio fluxes continued to increase since 2014 until at least until 2017 (Cendes et al. 2018), so that pre-2014 fluxes are lower than the latest (day = 10 942).

<figure><img src="image_12.png" /><figcaption>**Figure 12.** The SEDs from NIR to radio in the past 2 yr and the past. The filled circles of ALMA and *JWST/NIRCam* data points show the fluxes of the equatorial ring and the outer spots. For *JWST/NIRCam*, additionally the diffuse emission is included in the fluxes plotted with empty circles. The grey dashed line shows the 'warm' dust model from Matsuura et al. (2022).</figcaption></figure>

flux measurement at 8.7 GHz (Cendes et al. 2018). The power-law index at the millimetre and submillimetre wavelengths has been estimated as $\alpha = 0.74$ by Zanardo et al. (2013), fitting the blue open circles. More recently, Cigan et al. (2019) estimated $\alpha = 0.70$.

In Fig. 12, the grey line shows the power law of $\alpha = 0.70$ crossing the 8.7 GHz flux at day = 10 942. Even though it is not intended to fit the *JWST* fluxes, the grey line can explain the *F323N* and *F356W* fluxes very well, showing that a substantial fraction of the NIR fluxes is synchrotron emission. The ALMA flux at 315 GHz is probably underestimated, because the interferometry had a fully recoverable size of only 0.99 arcsec, and any structure extending beyond that size, such as diffuse emission, is over-resolved and its flux is not properly accounted.

Similar analysis of the *JWST* NIRSpec and MIRI/MRS total SED also found that synchrotron radiation is the most important source of the continuum in the 3–4 µm range (Jones et al. 2023). Hot (~300 K) dust starts to contribute to *F444W*, only.

In other young SNRs, Cassiopeia A and the Crab Nebula, the synchrotron emission contributes to the continuum emission from IR wavelengths at 2 µm to cm wavelengths (Jones et al. 2003; Gomez et al. 2012; Domček et al. 2021). It is not surprising synchrotron emission contributes to the continuum in SN 1987A, too.

Jones et al. showed that the total SED of SN 1987A at 3–4 µm additionally contains minor contributions from free-free and bound-free emission. However, their flux density decreases at longer wavelengths, which is opposite to the increasing continuum with longer wavelengths in the measured spectra. Hence, the contributions of free-free and bound-free emissions into the 3–4 µm spectra are likely very minor.

### 3.3.2 Spatial distributions of dust and synchrotron

Now, we carefully examine the spatial distributions of dust and synchrotron emission.

The *F356W* image is compared with the ALMA continuum image in Fig. 8 (right panel). The ALMA continuum image traces synchrotron in the ring and its exterior, though it does trace dust in the ejecta. In 2021, the emitting regions of the ALMA 315 GHz
(950 µm) flux correspond to hotspots within the equatorial ring and the outer spots exterior to the equatorial ring (Fig. 8). In particular, on the eastern side, the 315 GHz emission is found mostly in the outer spots, and hardly found in the hotspots.

Fig. 13 compares the F356W image and dust continuum image. The left top panel of the figure shows the F164N image, and contours of the F164N image are plotted in the other three panels in order to guide the locations of hotspots and diffuse emission. The lower panels are MIRI/MRS images, replicated from Jones et al. (2023). The left lower panel shows H17.46 µm image, demonstrating that ionized gas is emitted from the hotspots but only weakly from the outer spots, similar to F164N ring image. In contrast, the 7.4436–7.4572 µm dust image (the right lower panel) reveals that the dust emission is from the exterior to the equatorial ring, that is, either outer spots or diffuse component. These outer spots and diffuse emission have the steepest power-law index at F444W/F356W (Fig. 6). The dust thermal emission at ≲ 7 µm corresponds to the Wien's law regime of the blackbody, which can explain a steep (α ~ 3; Fig. 11) power-law index. The brightest hotspots within the western ring in F356W image (right top panel) have little dust emission at the 7 µm continuum (right bottom panel), hence, these hotspots are most likely synchrotron-dominated.

There is a morphological difference amongst dust, synchrotron, and atomic lines (Hα and [Fe II]): dust emission appears at the limited locations of the diffuse emission and outer spots, while synchrotron emission is found in both outer spots and hotspots, as well as diffuse emission. Hα and [Fe II] emission is strongest at the hotspots. The difference in emitting locations is caused by the time-scale of these three physical processes. More narrowly confined dust emitting regions in comparison with synchrotron emitting regions is explained by the shorter cooling time-scale of dust grains than synchrotron electrons in the post-shocked region. The radially expanding shocks gradually engulf new circumstellar material, including dust grains. Fast-moving electrons in hot shocked gas collide with dust grains, heating these grains. The heated grains emit the NIR and MIR and radiation, which also cools down dust grains and gas in post-shocked regions. Further, a high collision of charged particles can sputter atoms from the surface of dust grains, and making dust grain size smaller (dust destruction). The cooling time-scale was predicted to be 12–22 yr at a gas density of $n_H = 10^4$ cm⁻³ for grain sizes of >0.2 µm, and at a post-shock velocity of 600–700 km s⁻¹. Smaller grains cool faster than that. Whereas the dust destruction time-scale was predicted to be 4–15 yr (Dwek et al. 2010). Overall the shocks have passed the equatorial ring after about 2010 (Fransson et al. 2015), although the western side of the ring seems to have interacted until about 2014 (Larsson et al. 2019). Little dust emission from the hotspots within the equatorial ring in 2022 shows that either the cooling time-scale or destruction time-scale of dust grains is less than 8 yr. If the cooling rate is responsible for fading MIR dust emission, the grain size can be smaller than 0.2 µm. Otherwise, dust grains in the hotspots are destroyed, hence, little emission is left at the hotspots.

On the other hand, the radiative loss time-scale of relativistic electrons may be longer than the 10-yr time-scale, as discussed in the next section, hence, synchrotron radiation is still emitted in hotspots, even if the shocks have passed 12 yr ago.

The brightness of the hydrogen recombination lines also changes within the recombination time-scale, which is $\tau = (\alpha_B n_H)^{-1} = 1.22 \times 10^5/n_H$ yr (eq. 15.7 of Draine 2011), where $n_H$ is the hydrogen number density in cm⁻³. With estimated density of $n_H = 10^4$ cm⁻³, the recombination time-scale is 12 yr. Having Hα in the

western hotspots still but gradually fading since 2014 matches with explanation by this recombination time-scale.

### 3.3.3 Broken power-law synchrotron emission at NIR wavelengths

Synchrotron radiation makes a substantial contribution to the NIR continuum. The NIR power-law index of the continuum is about $\alpha=1.5–1.6$ at hotspots. This is higher than the millimetre power-law index, measured to be $\alpha = 0.7–0.74$ (Zanardo et al. 2013; Cigan et al. 2019). Having little MIR dust emission at hotspots, it is very unlikely that the tail of dust emission increases the power law at hotspots. The synchrotron power-law index at NIR wavelengths is likely to be intrinsically higher than that at millimetre wavelengths at hotspots.

Though synchrotron radiation, in general, follows a power law ($F_v \propto v^{-\alpha}$), the radiative lifetime of the more energetic electrons, which are responsible of the emission at higher frequencies, is shorter, leading to a steepening of the spectrum at higher frequencies (Ginzburg & Syrovatskii 1965; Longair 2011; Domček et al. 2021). The breakpoint of the power-law index is in general found at optical and mid-IR wavelengths in SN remnants (Bühler & Blandford 2014; Domček et al. 2021). Having a broken law in the IR wavelengths may explain the steeper NIRCam power-law index than that in the millimetre wavelength.

Moreover, previous millimetre power-law index measurements suggest the synchrotron power-law index itself could have some spatial variations; $\alpha = 0.6–1.0$ with an aperture of 0.8 arcsec (Zanardo et al. 2014).

We test if such a broken IR law of the synchrotron emission is feasible in SN 1987A. If a relativistic electron can lose energy by synchrotron radiation before an electron accelerates to that energy (or frequency), there is no electron emitting synchrotron radiation at that frequency. The typical frequency, $\nu_{br}$, where radiative loss affects the spectrum is given by

$$ \nu_{br} \approx 7.2 \times 10^{16} \left( \frac{B}{100 \mu\text{G}} \right)^{-3} \left( \frac{t_{\text{age}}}{100 \text{ yr}} \right)^{-2} \text{Hz}, \quad (1) $$

where B is the magnetic field, and $t_{\text{age}}$ is the electron's radiative lifetime (Domček et al. 2021, also in Pacholczyk 1970). The collision of the blast wave with the equatorial ring occurred somewhere between 1999 and 2005 (Luo & McCray 1991; Chevalier & Dwarkadas 1995; McCray & Fransson 2016), so that the lifetime should be less than 27 yr. Having the break wavelength/frequency to be at 3 µm requires the magnetic field of $B \ge 2.1$ mG. The polarization measurement of the millimetre synchrotron emission estimated the magnetic field of a few mG at the post-shocked region in 2015–2016 (Zanardo et al. 2018). This magnetic field can cause a broken power-law index of the synchrotron emission.

Theoretical models of SN shock evolution show a broken power law in the presence of a strong magnetic field. Berezhko, Ksenofontov & Völk (2011) predicted the broken law to be present at IR wavelengths in SN 1987A. The model involved a 20 mG magnetic field downstream, as this could explain non-detection of synchrotron emission in X-ray in their model. In this case, the synchrotron broken law already appears at the MIR, and continues with the steep power-law index into the NIR. The predicted spectra had time variation, and the IR power-law index of about 1.2 was predicted in 2020. This predicted index is slightly smaller than the observed one, but much steeper than the millimetre power-law index. On the other hand, Petruk et al. (2023) predicted a much lower magnetic field (<20 µG), and in this case, the power-law break falls at a shorter
<figure><img src="image_13.png" /><figcaption>**Figure 13.** Comparison of the emitting regions of ionized gas (F164N and 7.46 µm H16–5 Pfα lines), F356W continuum and 7 µm (7.43–7.45 µm) continuum. The 7.46 µm H16–5 Pfα lines and 7 µm continuum images are from MIRI/MRS, replicated from Jones et al. (2023). The black contour lines trace F164N, though due to slightly different smoothing of the F164N data to the new pixel scales, there is a subtle difference in contour lines from one image to the other. The dust continuum, represented by the 7 µm continuum, is mainly emitted from outer spots and diffuse emission, while F356W emission is found in both hotspots and outer spots, as well as diffuse emission.</figcaption></figure>

wavelength than the NIR. These contrasting predictions show that the measurement of a magnetic field is the key next step.

### 3.3.4 The NIR continuum as a tracer of shocks and ionized lines as a tracer of recombining regions

Historically, synchrotron emission, detected in radio and X-ray wavelengths, was found only from the equatorial ring, and the outer spots were not considered (e.g. Park et al. 2002; Ng et al. 2013; Zanardo et al. 2014). That was definitely the case at least until about 2014, when the blast waves were about to pass the equatorial ring (Fransson et al. 2015). Although not fully resolved down to the scale of the hotspots and outer spots, the radii of the radio and the X-ray emitting ring sizes have continued to increase since then (Frank et al. 2016; Cendes et al. 2018). This trend is more prominent in the hard (0.5–10 k eV) X-ray emission. The soft (0.3–0.8 keV) X-ray emission radius stagnated around day 6000 in 2004 (Frank et al. 2016), and it is considered to be from hotspots in the equatorial ring (Dewey et al. 2012). In the ALMA image taken in 2015, the synchrotron emission extended just beyond the equatorial ring (Cigan et al. 2019).

Now, the synchrotron emission in X-ray and radio wavelengths must be emitted by both the ring hotspots and the outer spots, as

shown by the JWST F323N, F356W, and F444W images and the ALMA 315 GHz image. That would explain the recent increase in the radii of the radio and hard X-ray emission. Considering that the brightness from the west side of the equatorial ring and outer spots dominates the overall brightness in the F323N, F356W, F444W, and radio flux, as well as X-ray, the current interactions of shocks with the circumstellar material must be dominant on the western side of the equatorial ring and the material beyond the ring. The majority of the blast waves have already passed the equatorial ring on the eastern side.

Shocked gas is typically traced by X-rays, and the electron temperature is estimated to be $1–3 \times 10^7$ K (Ravi et al. 2021). Around that temperature, in case A, hydrogen recombination lines have emissivity decreasing with increasing temperature, as $T^{-0.92}$ (Storey & Hummer 1995). The outer spots, where current interaction is ongoing, are expected to have a higher temperature than the ‘hotspots’ (Note that the name of ‘hotspots’ is based on observations in 1990s and the temperature has changed since, and does not reflect the current temperature.). This explains why hotspots are brighter than the outer spots in hydrogen recombination lines, such as Brα (IvF405N in Fig. 5) and HST Hα images (Larsson et al. 2019). That makes the contrast between the hydrogen recombination
<figure><img src="image_14.png" /><figcaption>**Figure 14.** The detection of the outer rings in four filter bands. The arrows in the *F212N* image show the locations of potential detections of the outer rings. The black shadow in the *F405N* is an artefact, due to incomplete flat-field correction.</figcaption></figure>

line images, which show the hotspots to be brighter than the outer spots, and the continuum images (*JWST F323N*, *F356W*, and *F444W* images), which have equally bright hotspots and outer spots.

The continuum may contain some emission from small dust grains (Dwek et al. 2008; Matsuura et al. 2022; Arendt et al. 2023; Jones et al. 2023). The emission is from collisionally heated dust in shocks. Even if arising from a combination of synchrotron and dust emission, the power-law continuum emission in the NIR wavelength would be a good tracer of the shocked region.

In contrast, the ionized lines originate from hotspots within the equatorial ring, which contain more substantial gas mass but have lower temperatures. The recombination time-scale is much longer than the synchrotron cooling time, and a lower temperature

favours higher recombination line emissivity. The hotspots within the equatorial ring thus remain brighter for longer time-scales in recombination lines.

## 3.4 Outer rings

The outer rings are considered to have been expelled from the progenitor about 20 000 yr before the SN explosion (Panagia et al. 1996; Crotts & Heathcote 2000). The outer rings have been detected mainly in atomic lines, such as [N II], [O III], and Hα in the optical wavelengths (Panagia et al. 1996; Tziamtzis et al. 2011), and Ne and [O I] lines in the MIR wavelengths (Jones et al. 2023).

In our NIRCam images, very faint emission from the outer rings was detected in three filter bands *F164N*, *F200W*, and *F405N*.
(Fig. 14). All three filter bandpasses contain atomic lines: *F164N* contains 1.641 µm and [Fe II] line, *F200W* is mainly from 1.875 µm *Paα*, and 2.059 µm He I lines, and *F405N* is from *Brα*. A hint of part of the outer rings is recognizable in the *F212N* image, too. Non-detections in the continuum-dominated filters may suggest that the emission from the outer rings is dominated by the atomic lines with very faint H₂ in *F212N*. This supports the proposition that the gas in the outer rings was ionized by the initial SN flash (Tziamtzis et al. 2011) and that lines from the ionized gas can be still detected at this late epoch.

The outer rings are known to cross the ejecta and the equatorial ring as seen in projection from Earth (Tziamtzis et al. 2011). We assess if the emission from the outer rings may still contribute to some structures in the *JWST* image in 2022 in Appendix A. From the measurement of the surface brightness, the outer ring emission accounts for a negligible fraction (less than 10 per cent) of the surface brightness of the ejecta in *F164N* and *F200W* images.

## 4 CONCLUSIONS

Very deep and high angular resolution NIRCam imaging of SN 1987A reveals unprecedented details of NIR emission from this young SN remnant. This includes the detection of the NIR synchrotron emission, the discovery of the crescents, and the characterization of the bar, which is a substructure of the ejecta.

Regions currently undergoing shocks are traced by the NIR continuum through synchrotron and thermal dust emission at 3–5 µm. This continuum emission is spatially resolved in NIRCam images, and is found in hotspots within the equatorial ring, in outer spots exterior to the equatorial ring, as well as in the diffuse emission exterior to the equatorial ring. At 3–5 µm, the outer spots are as bright as the hotspots, despite the fact that the hotspots have the highest density within the circumstellar system. This is probably because stronger magnetic fields and enhanced local density at shocks increase the synchrotron radiation in the outer spots. Future monitoring and modelling of synchrotron emission will reveal how the synchrotron emission evolves with time in an SNR, after the passage of the shock front. Such time sequence observations will provide a rare opportunity amongst SNRs that would constrain hydrodynamic models of SN shocks (e.g. Borkowski, Blondin & McCray 1997; Dewey et al. 2012; Kirchschlager et al. 2019; Orlando et al. 2020), and particle acceleration in the shocked regions (e.g. Domček et al. 2021). With the advent of high sensitivity and high angular resolution images provided by *JWST/NIRCam*, our observations of SN 1987A demonstrate that NIRCam opens up a window to study particle-acceleration physics probed by NIR synchrotron emission in SN remnants and other synchrotron emitting objects such as active galactic nuclei and blazars.

The NIRCam 1–2 µm images reveal the clumpy structure of the ejecta, including the bar. The presence of clumpy structure is consistent with SN explosion models that predict the fragmentation of the SN ejecta into clumps due to Rayleigh–Taylor instabilities, which are triggered by the explosions (Wongwathanarat, Muller & Janka 2015).

The NIRCam images show that the centre of the ejecta is still obscured by dust, and seen as a ‘hole’ at the 3–5 µm wavelengths. The presence of dust obscuration in the ejecta means that optical and NIR emission mainly traces the foreground of this heavily self-absorbed dusty region. As this heavily self-absorbed dust region expands further, the optical depth of the dust will decrease in time. Once the dust optical depth has dropped sufficiently, a complete picture of the ejecta will be visible at the NIR wavelengths. A

new feature, the crescents, is discovered between the ejecta and the equatorial ring. The crescents are probably either the ionization front of the inner ejecta, irradiated by X-ray and UV emitted from the outer spots, or the edge of the reverse shocks bounced back from the equatorial ring. Details of our analysis of these crescents will be published elsewhere.

Very deep and high angular resolution NIRCam images revealed detailed structures in the young and close SNR, SN 1987A. They will be used to understand the physical processes that shape complex structures of SNe. The processes to be investigated include mass loss from the progenitor star, the SN explosion, dynamical processes, heating (by X-ray and UV radiation, and $^{44}$Ti decay, and potentially the neutron star) and cooling processes (atomic and dust radiations, as well as adiabatic). All of these processes are imprinted into the complexity of emissions from the ejecta and circumstellar matter in this SNR. The next stage is to disentangle all of these processes to truly understand the cause of the complexity and time evolution of the SNRs.

## ACKNOWLEDGEMENTS

This work is based on observations made with the NASA/ESA/CSA *JWST*. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for *JWST*. These observations are associated with program no. 1726.

This paper makes use of the following ALMA data: ADS/JAO.ALMA#2021.1.00707.S. ALMA is a partnership of ESO (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ.

This work also utilizes *JWST* data from the program no. 1232, obtained from the Mikulski Archive.

This work presents results from the European Space Agency (ESA) space mission *Gaia*. *Gaia* data are being processed by the Gaia Data Processing and Analysis Consortium (DPAC). Funding for the DPAC is provided by national institutions, in particular the institutions participating in the Gaia MultiLateral Agreement (MLA).

MM and RW acknowledge support from the STFC Consolidated grant (ST/W000830/1). MJB and RW acknowledge support from European Research Council (ERC) advanced grant SNDUST 694520. IDL and FK acknowledge funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (no. 851622 DustOrigin). RDG was supported, in part, by the United States Air Force. Work by RGA was supported by NASA under award no. 80GSFC21M0002. APR and SP are supported in part by the STScI grant, JWST-GO-01726.032-A. CG is supported by a Villum FondeN Young Investigator Grant (project no. 25501).

## DATA AVAILABILITY

The pipeline reduced data are available at MIKULSKI archive (http://dx.doi.org/10.17909/dzkq-7c90).

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## APPENDIX A: CONTRIBUTION OF THE OUTER RING ONTO THE EJECTA

The outer rings are known to cross the ejecta and the equatorial ring as seen in projection from Earth (Tziamtzis et al. 2011). We assess if the emission from the outer rings may still contribute to some structures in the JWST image in 2022. The outer rings have been gradually fading since the HST image was taken in 2003 (Tziamtzis et al. 2011), and we use the 2003 HST image as a guide of the outer ring, rather than more recent HST images. The left panel of Fig. A1
<figure><img src="image_15.png" /></figure>

**Figure A1.** Left: HST F625W image in 2003 (Tziamtzis et al. 2011), showing the outer rings in the line contours. The north outer ring crosses the equatorial ring and ejecta, indicated by three black arrows. Since then, the outer ring has faded gradually. Right: F212N image in colour contours, overlayed with line contours identical to the left panel (green and white). The colour bar is in the unit of MJy sr⁻<fnref n="1" />. The local peak on the east crescent could have some gain of the surface brightness from the outer rings.

shows the HST F625W image. The levels of the line contours are adjusted to highlight the outer rings, the equatorial ring and the ejecta. The northern outer ring is encircled with a black line. This ring is highlighted at three locations using black arrows: the ring enters the equatorial ring from the north-east, and proceeds towards the centre of the equatorial ring, then exits from the south-west of the ejecta. Afterwards, it crosses the equatorial ring for the second time. Grey contour lines show some local increase in intensities at these three locations. The right panel of Fig. A1 shows the NIRCam F212N image of the same region in the colour contour. The line contours are taken from the HST F625W image. At the position of the white arrow, the north outer ring and the east crescent meet. This crossing point coincides with the local peak of the crescent. This local peak might have some gain in the intensity from the outer ring. Alternatively, the colour seems to be blue, which might be a sign of a field star at this location. From the measurement of the surface brightness, the outer ring emission accounts for a negligible fraction (less than 10 per cent) of the surface brightness of the ejecta in F164N and F200W images.

## APPENDIX B: MEASUREMENTS OF THE FLUXES IN THE EQUATORIAL RING AND OUTER SPOTS

In order to measure the NIR continuum fluxes in the equatorial ring and outer spots, two different types of elliptical apertures were used. These fluxes were measured with an elliptical aperture of 1.50 and 1.37 arcsec major and minor axes (Fig. 10), with a negligible contribution from field stars within the aperture. The filled circles show the fluxes measured from another elliptical annulus that includes the ring and the outer spots, but excludes the ejecta and diffuse emission outside the outer spots. The major axes of the inner and outer annulus are 0.65 and 1.28 arcsec and minor axes of these are 0.52 and 0.90 arcsec.

<footnote n="1">¹Cardiff Hub for Astrophysical Research and Technology (CHART), School of Physics and Astronomy, Cardiff University, The Parade, Cardiff CF24 3AA, UK</footnote>

<footnote n="2">²Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA</footnote>

<footnote n="3">³Observational Cosmology Lab, Code 665, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA</footnote>

<footnote n="4">⁴Center for Space Sciences and Technology, University of Maryland, Baltimore County, Baltimore, MD 21250, USA</footnote>

<footnote n="5">⁵Department of Physics, KTH Royal Institute of Technology, The Oskar Klein Centre, AlbaNova, Stockholm SE-106 91, Sweden</footnote>

<footnote n="6">⁶Department of Astronomy, Stockholm University, The Oskar Klein Centre, AlbaNova, Stockholm SE-106 91, Sweden</footnote>

<footnote n="7">⁷Department of Physics and Astronomy, The Johns Hopkins University, 366 Bloomberg Center, 3400 N. Charles Street, Baltimore, MD 21218, USA</footnote>

<footnote n="8">⁸Department of Physics, University of Texas at Arlington, Box 19059, Arlington, TX 76019, USA</footnote>

<footnote n="9">⁹U.S. Naval Observatory, 3450 Massachusetts Ave NW, Washington, DC 20392-5420, USA</footnote>

<footnote n="10">¹⁰Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA</footnote>

<footnote n="11">¹¹Department of Physics and Astronomy, University College London (UCL), Gower Street, London WC1E 6BT, UK</footnote>

<footnote n="12">¹²DRF/IRFU/DAp, CEA-Saclay, F-91191 Gif-sur-Yvette, France</footnote>

<footnote n="13">¹³NRS/AIM, Université Paris Diderot, F-9119, Gif-sur-Yvette, France</footnote>

<footnote n="14">¹⁴Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803, USA</footnote>

<footnote n="15">¹⁵Department of Astronomy, University of Virginia, 530 McCormick Road, Charlottesville, VA 22904, USA</footnote>

<footnote n="16">¹⁶Osservatorio Astronomico di Trieste, Via Tiepolo 11, Trieste, Italy</footnote>

<footnote n="17">¹⁷SOFIA-USRA, NASA Ames Research Center, Mail Stop 232-12, Moffett Field, CA 94035, USA</footnote>

<footnote n="18">¹⁸Sterrenkundig Observatorium, University of Ghent, Krijgslaan 281 - S9, B-9000 Ghent, Belgium</footnote>

<footnote n="19">¹⁹European Space Research and Technology Centre, Keplerlaan 1, NL-2200 AG, Noordwijk, The Netherlands</footnote>

<footnote n="20">²⁰DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 128, Copenhagen DK-2200, Denmark</footnote>

<footnote n="21">²¹Minnesota Institute for Astrophysics, University of Minnesota, 116 Church Street S. E., Minneapolis, MN 55455, USA</footnote>

<footnote n="22">²²National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, 22903, VA, USA</footnote>
²³Finnish Centre for Astronomy with ESO (FINCA), University of Turku, FI-20014 Turku, Finland

²⁴Tuorla Observatory, Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland

²⁵TMT International Observatory, 100 West Walnut Street, Pasadena, CA 91124, USA

²⁶Departamento de Astronomía y Astrofísica, Universidad de València, C/Dr Moliner 50, E-46100 Burjassot, Spain

²⁷Observatori Astronòmic, Universitat de València, C. Catedrático José Beltrán 2, E-46980 Paterna, València, Spain

²⁸Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr, Pasadena, CA 91109, USA

²⁹Department of Physics and Astronomy, Purdue University, 525 Northwest-ern Avenue, West Lafayette, IN 47907, USA

³⁰Integrative Data Science Initiative, Purdue University, WestLafayette, IN 47907, USA

³¹Istituto Nazionale di Astrofisica-Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, I-90134 Palermo, Italy

³²Department of Science, Kyoto University, Asakuchi-shi, Okayama Observatory, Okayama, 719-0232, Japan

³³JBCA, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK

³⁴International Centre for Radio Astronomy Research (ICRAR), The University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia

³⁵Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA

³⁶ESO, Karl-Schwarzschild-Str 2, Garching D-85748, Germany

³⁷Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, NL-1098 XH Amsterdam, the Netherlands

³⁸GRAPPA, University of Amsterdam, Science Park 904, NL-1098 XH Amsterdam, the Netherlands

³⁹SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, NL-3584 CA, Utrecht, the Netherlands

⁴⁰Department of Physics and Astronomy, Texas A&M University, College Station, TX 77843, USA

⁴¹Cosmic Dawn Center (DAWN), Københavns Universitet, Rådmandsgade 62, DK-2200 København, Denmark

⁴²Niels Bohr Institute, University of Copenhagen, Jagtvej 128, Copenhagen DK-2200, Denmark

⁴³Department of Astronomy, The University of Texas at Austin, 2515 Speedway, Austin, TX 78712-1205, USA

⁴⁴Department of Astronomy and Astrophysics, Penn State University, University Park, PA 16802, USA

This paper has been typeset from a TEX/LATEX file prepared by the author.