A radio-detected type Ia supernova with helium-rich circumstellar material

Abstract

Type Ia supernovae (SNe Ia) are thermonuclear explosions of degenerate white dwarf stars destabilized by mass accretion from a companion star¹, but the nature of their progenitors remains poorly understood. A way to discriminate between progenitor systems is through radio observations; a non-degenerate companion star is expected to lose material through winds² or binary interaction³ before explosion, and the supernova ejecta crashing into this nearby circumstellar material should result in radio synchrotron emission. However, despite extensive efforts, no type Ia supernova (SN Ia) has ever been detected at radio wavelengths, which suggests a clean environment and a companion star that is itself a degenerate white dwarf star^4,5. Here we report on the study of SN 2020eyj, a SN Ia showing helium-rich circumstellar material, as demonstrated by its spectral features, infrared emission and, for the first time in a SN Ia to our knowledge, a radio counterpart. On the basis of our modelling, we conclude that the circumstellar material probably originates from a single-degenerate binary system in which a white dwarf accretes material from a helium donor star, an often proposed formation channel for SNe Ia (refs. ^6,7). We describe how comprehensive radio follow-up of SN 2020eyj-like SNe Ia can improve the constraints on their progenitor systems.

Main

SN 2020eyj was first detected on 7 March 2020 UT (modified Julian date (MJD) = 58,915.12; see ‘Observations’ section in Methods), at α = 11 h 11 min 47.19 s, δ = 29° 23′ 06.5″ (J2000). The SN was classified as a SN Ia (ref. ⁸) based on a low-resolution spectrum obtained on 2 April 2020, 25 days after the first detection. Comparisons with type Ia and Ibc spectra from the literature support the SN Ia classification (see ‘SN Ia classification’ section in Methods and Fig. 1). Unusual evolution of the later light curve prompted us to obtain a second spectrum on 20 July 2020, 131 days after first detection. The second spectrum was very similar to those of type Ibn SNe (SNe Ibn), which are SNe that interact with helium-rich circumstellar material (CSM) and have spectra characterized by narrow (roughly a few 10³ km s⁻¹) He i emission lines while showing little to no H i (refs. ^9,10).

Fig. 1: The first spectrum of SN 2020eyj is consistent with a type Ia(–CSM).

The SEDM classification spectrum of SN 2020eyj, obtained about 12 days after peak and shown in black, is compared with type Ia-91T SN 2001V, type Ia–CSM PTF11kx, type Ia SN 2004eo and type Ic SN 1994I. Phases are relative to peak, which—in the case of SN 2020eyj—has an uncertainty of a couple of days. Several important absorption features are indicated at the expected wavelengths. Notably, the spectrum of SN 2020eyj lacks any sign of O i 7,774 Å absorption. Spectra have been corrected for MW reddening. Telluric features are indicated by crossed circles.

On the basis of the late-time (tail-phase) CSM-interaction-dominated spectra (Fig. 2), SN 2020eyj falls in the category of the rare subclass of SNe Ia that show evidence of CSM interaction in their optical spectra (SNe Ia–CSM; ref. ¹¹). The narrow emission lines in the spectra of such interacting SNe arise from shock interaction between the fast-moving SN ejecta and the slow-moving CSM¹². SNe Ia–CSM are strong contenders for the single-degenerate (SD) SN Ia formation channel on account of the CSM, which is commonly assumed to originate from a non-degenerate donor star through stellar or accretion winds. Before SN 2020eyj, all of the discovered SNe Ia–CSM exhibited prominent Balmer emission lines and only weak He emission features¹¹.

Fig. 2: The spectra of SN 2020eyj in the tail phase are dominated by CSM interaction.

The spectra of SN 2020eyj at late phases (in black) are compared with the prototypical type Ibn SN 2006jc and the type Ia–CSM SN PTF11kx. The spectra show features common to SNe Ia–CSM, such as the quasi-continuum blueward of 5,700 Å and broad Ca ii emission. The main SN emission features are identified in the top spectrum. The emission lines in SN 2020eyj show strong asymmetry, with attenuated red wings (Extended Data Fig. 3). The bottom spectrum is of the host of SN 2020eyj, obtained at 679 days, some 300 days after the SN had faded below the detection limit of the ZTF. Some unresolved galaxy lines are marked. Phases are relative to first detection, which—in the case of SN 2006jc—was at or after the peak. Spectra have been corrected for MW reddening. Telluric features are indicated by crossed circles.

Typically, CSM interaction contributes substantially to or even dominates the spectral and light-curve evolution of SNe Ia–CSM from the beginning, hindering unambiguous classification as SNe Ia (ref. ¹³). However, in some rare cases, SNe Ia–CSM have shown a delay in CSM interaction^14,15,16, suggesting that the CSM was located far (>10¹⁵ cm) from the binary system at the time of explosion. Notably, PTF11kx cemented SNe Ia–CSM as a bona fide SN Ia subclass by virtue of a delay of about 60 days, allowing for an indisputable SN Ia classification before CSM interaction¹⁵. SN 2020eyj follows a similar evolution as PTF11kx, initially showing a typical SN Ia bell-shaped light curve (Fig. 3) and a spectrum consistent with a SN Ia of the 91T subgroup¹⁷ without clear evidence for CSM interaction (Fig. 1). Then, at 50 days after first detection, the g-band light curve of SN 2020eyj diverges from a steady decline into a plateau that lasts for roughly 200 days. Such an evolution and colour change is not expected for a normal SN Ia (Fig. 3) but is driven by the emergence of spectral features associated with CSM interaction (see ‘Light-curve analysis’ section in Methods). We interpret the start of the plateau at 50 days as the epoch when CSM interaction starts to contribute substantially or even dominate the light curve of SN 2020eyj. Assuming a SN ejecta velocity of 10⁴ km s⁻¹ (ref. ¹⁸), the delay corresponds to an inner boundary to the CSM of about 4 × 10¹⁵ cm. Except for the presence of He emission lines, the late-time spectra of SN 2020eyj are typical for the SN Ia–CSM class, with prominent broad Ca ii emission from the near-infrared (NIR) triplet and without any sign of O iλ7774 emission (Fig. 2). The compact and star-forming host galaxy of SN 2020eyj (see ‘Host galaxy’ section in Methods) is also consistent with those of other SNe Ia–CSM¹¹.

Fig. 3: The multiband light curve of SN 2020eyj can be divided into a diffusion-peak phase and a long-lived interaction-powered tail phase.

The light curves of SN 2020eyj are shown with overplotted SN Ia template fits to the initial peak (see ‘Light-curve analysis’ section in Methods). The most recent mid-infrared epoch (W1 and W2) is outside the date range plotted here and is shown in Extended Data Fig. 2 . Open circles indicate synthetic photometry derived from the spectra. Phase is in rest-frame days since first detection. Apparent magnitudes on the left y axis, absolute magnitudes on the right y axis, in which μ is the distance modulus. Non-detections with 5σ upper limits are indicated by triangles. The photometry has been binned into one-night bins and has been corrected for MW reddening. The diamond markers at the top indicate the epochs of spectroscopy. The bottom panel shows the g − r colour for the nights in which both g and r photometry was obtained, overplotted with the colour evolution of a typical SN Ia. The error bars represent 1σ uncertainties.

Despite the similarities between SN 2020eyj and other SNe Ia–CSM, the presence of He i lines and absence of prominent H i lines remains a striking difference with profound implications for the progenitor system. As H i is easier to ionize than He i, the absence of the lines indicates that the CSM around SN 2020eyj, and thus the companion star, is He-rich and H-poor. Although the late-time spectra of SN 2020eyj are similar to those of SNe Ibn, these SNe are presumed to arise from the core collapse of massive (>10 M_⊙) stars^9,19,20, which are unlikely to be in a binary system with a white dwarf (WD), as they would undergo core collapse long before the WD formed. A merger involving a degenerate He WD donor star is also disfavoured, because in such merger models, only a small amount of unburned He (about 0.03 M_⊙ (ref. ²¹)) is present close to (≲10¹² cm) the WD (ref. ²²), whereas the CSM around SN 2020eyj resides at >10¹⁵ cm. Instead, a strong candidate for the donor star in the SN 2020eyj progenitor system is a non-degenerate He star (initial mass 1–2 M_⊙, for example, ref. ²³). WD + He star systems can be formed by means of binary evolution²⁴ and this SD channel for SNe Ia has garnered recent interest because the very restrictive limits placed by radio non-detections and deep optical imaging²⁵ that exclude most H-rich donor star models still allow for low-CSM-density WD + He star systems^25,26. The possible detection in pre-explosion Hubble Space Telescope imaging of the progenitor system of the type Iax (SNe Ia similar to SN 2002cx (ref. ²⁷)) SN 2012Z, a blue compact source interpreted as a He-star donor²⁸, has further strengthened this hypothesis, although the thermonuclear nature of type Iax SNe is debated²⁹.

The CSM interaction in SN 2020eyj is also confirmed, for the first time in a SN Ia, through the detection of a radio counterpart, at a frequency of 5.1 GHz at 605 and 741 days after the first detection (see ‘Radio’ section in Methods). Follow-up in the X-rays did not yield a detection (see ‘X-ray’ section in Methods). We model the radio synchrotron emission, which results from the shock interaction between the ejecta and the CSM, assuming two basic CSM configurations expected in a SD progenitor system; a constant density shell and a wind-like density profile with density ρ ∝ r⁻² (Fig. 4). A constant density shell could result from a mass ejection event such as a nova, whereas a wind-like CSM profile would be expected from an optically thick wind, in which the mass-transfer rate from the donor star to the WD exceeds the maximum accretion rate of He-rich material that the WD can burn on its surface^26,30. As well as CSM material resulting from a SD scenario, we consider synchrotron emission resulting from the interaction of a SN Ia from a double-degenerate (DD) WD merger interacting with the local interstellar medium (ISM)³¹. For the SD shell model, the radio detections are best explained with a CSM mass of M_csm = 0.36 M_⊙ (see ‘CSM shells’ section in Methods), with the expectation that the radio light curve will start to drop off rapidly at around 900 days. For the SD optically thick wind model, the bolometric light-curve tail (see ‘Bolometric light curve’ section in Methods) and radio detections of SN 2020eyj are well fitted with a mass-transfer rate of 10⁻³–10⁻² M_⊙ yr⁻¹, microphysics parameter ϵ_B = 10⁻⁵–10⁻³ and a CSM mass within 10¹⁷ cm of M_csm = 0.3–1.0 M_⊙. The DD ISM model (the dashed lines in Fig. 4) requires unusually high ISM densities and does not recover the observed decline in flux, ruling out the DD formation channel for SN 2020eyj (see ‘ISM’ section in Methods). The best fit radio light curves of the shell and wind models differ in particular at early phases (Fig. 4), but no radio data were obtained at these epochs. Instead, multifrequency monitoring of the radio counterpart of SN 2020eyj until late phases (>1,000 days) will allow to discriminate between the rapid drop-off of the shell model and a shallower decline expected in the case of a wind-like CSM.

Fig. 4: The radio detections of SN 2020eyj at 5.1 GHz can be reconciled with CSM interaction.

For the wind model, in which the CSM follows a density profile of ρ ∝ r⁻², we assume a pre-SN wind velocity of 1,000 km s⁻¹ and adopt a mass-transfer rate as inferred from fitting the bolometric light curve of SN 2020eyj. Depending on the level of line-of-sight extinction affecting the bolometric light curve (see ‘Bolometric light curve’ section in Methods), the wind model fits the observations (in black, with 1σ uncertainties) well for the microphysics parameter ϵ_B = 1.7 × 10⁻³ (1.5 × 10⁻⁵) and a CSM mass of M_csm = 0.3 M_⊙ (1 M_⊙) within 10¹⁷ cm (see ‘Optically thick wind’ section in Methods) when E(B − V) = 0 mag (0.5 mag). For the shell model, in which the CSM is concentrated in a constant-density CSM shell, we assume ϵ_B = 0.1 and obtain a best estimate for the CSM mass of M_csm = 0.36 M_⊙ and a CSM interaction end time of t_end = 665 days (a width of 8.6 × 10¹⁶ cm; see ‘CSM shells’ section in Methods). In both the wind and shell model fits, ϵ_e = 0.1 is assumed. We also show radio light curves from a model involving a DD SN Ia interacting with the ISM (see ‘ISM’ section in Methods). To fit the individual radio detections, this model requires unusually high ISM densities and neither fit reproduces the observed decline in flux, ruling out the DD scenario.

A viable progenitor scenario for SN 2020eyj needs to explain not only the presence and properties of a He-rich CSM but also its detached configuration. For the delayed type Ia–CSM SN 2002ic, the CSM-free cavity was attributed to a possible drop-off in mass-transfer rate or the emergence of a low-density fast wind evacuating the CSM³². In the case of PTF11kx, the delayed CSM interaction was explained by a scenario involving a symbiotic nova progenitor, in which recurrent novae on the surface of the WD sweep up the wind-deposited CSM into shells¹⁵. SN 2020eyj shows strong similarities to PTF11kx, which may hint at a common progenitor scenario. Their light curves are virtually identical up until day 50 (Extended Data Fig. 1) with risetimes of about 14 days in the g band, which is fast for a SN Ia (ref. ³³). And, except for the nature of the narrow emission lines, both SNe have similar spectra throughout their evolution (Figs. 1 and 2). For SN 2020eyj, a nova progenitor could look like V445 Puppis (V445 Pup; see ‘V445 Puppis’ section in Methods), the only known nova system that showed He-rich, but H-free, ejecta^34,35. Notably, the V445 Pup system is considered a prime candidate progenitor system for the He star + WD SN Ia channel, as it is claimed to be host to a WD with a mass close to the Chandrasekhar limit³⁶. Furthermore, a prominent carbon-rich equatorial dusty disk such as that in V445 Pup (refs. ^34,35) could explain (see ‘V445 Puppis’ section in Methods) the luminous infrared counterpart of SN 2020eyj (Extended Data Fig. 2), which we attribute to an infrared echo from radiatively heated pre-existing dust with a dust mass of order 10⁻² M_⊙ (see ‘Dust properties’ section in Methods). The initial models invoking recurrent novae for the origin of PTF11kx (ref. ¹⁵) were challenged by the CSM masses involved³⁷, which were too large by orders of magnitude for symbiotic nova mass build-up models³⁸. Similarly, the mass resulting from a V445 Pup-like nova outburst (≲10⁻³ M_⊙; see ‘V445 Puppis’ section in Methods) is insufficient to explain the CSM mass observed in SN 2020eyj. However, a recent study of the radio evolution of V445 Pup suggests that the equatorial disk could have pre-dated (and survived) the nova outburst³⁹, which would allow for mass build-up in the disk between nova eruptions. This scenario would require the SN to occur soon after the nova outburst and before the resumption of mass transfer between the donor and WD reforms the disk at small radii. We note that a nova similar to the year 2000 event of V445 Pup would not have been detectable at the distance of SN 2020eyj (see ‘Precursor search’ section in Methods).

SN 2020eyj represents the first observational example of the previously speculated class of SNe Ia–He CSM (ref. ⁴⁰). The presence of a dense CSM, supported by a radio detection, offers strong evidence for the SD scenario for SN 2020eyj, in particular for the He star + WD formation channel. It is estimated that about 10% of all SD SNe Ia arise from this channel⁷, which is probably the dominant source of SNe Ia with short delay times⁴¹. Understanding the timescale of SN Ia activity is important for the chemical evolution of galaxies. The confirmed presence of a He-rich CSM in a SN Ia system also affects SN Ia explosion modelling, as He plays a vital role in double detonation models in which the WD explosion is triggered by the ignition of a massive (≲0.2 M_⊙) He shell on its surface³⁰. Constraining the rate of SNe Ia similar to SN 2020eyj would require systematic spectroscopic follow-up of SNe Ia with long-lived light curves, as monitoring at present often stops after a seemingly normal SN Ia has been classified. Observational properties that SN 2020eyj share with its H-analogue PTF11kx, such as a fast rise and a 91T-like peak spectrum, can potentially guide such follow-up efforts and allow for the discovery and study of more SN 2020eyj-like SNe Ia, including at radio wavelengths.

Methods

Observations

Discovery

SN 2020eyj was discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS)^42,43 on 23 March 2020 UT (ref. ⁴⁴) and subsequently detected as part of the Zwicky Transient Facility (ZTF) survey^45,46, at α = 11 h 11 min 47.19 s, δ = 29° 23′ 06.5″ (J2000). Pre-discovery detections were recovered in the ZTF data on 7 March 2020 UT (MJD = 58,915.12) in both g and r filters. For reference, we list some key characteristics of SN 2020eyj in Extended Data Table 1.

Optical photometry

Follow-up photometry was obtained as part of public and partnership ZTF survey observations⁴⁷ with the ZTF camera⁴⁸ on the Palomar 48-in. telescope (P48) telescope in the g and r bands, and later phases were also covered in the i band. The P48 data were reduced and host subtracted using the ZTF reduction and image-subtraction pipeline⁴⁹, which makes use of the ZOGY algorithm⁵⁰ for reference-image subtraction. Following the rationale illustrated in ref. ⁵¹, we apply the difference image zero point magnitude to convert fluxes from units in detector data number (DN) to μJy and translate fluxes to AB magnitudes. We apply a detection threshold of signal-to-noise ratio (S/N) ≳ 3 and for non-detections we compute 5σ upper limits. The Supplementary Information lists the ZTF magnitudes and upper limits.

Further photometric epochs were obtained with the Liverpool Telescope⁵², the Spectral Energy Distribution Machine (SEDM) on the Palomar 60-in. telescope (P60), the Las Cumbres Observatory telescopes (programme ID NOAO2020B-012) and the Alhambra Faint Object Spectrograph and Camera (ALFOSC) on the Nordic Optical Telescope (NOT), with data reduced and host subtracted using the pipelines described in refs. ^53,54 or standard methods. In this work, we also make use of the forced photometry service from the ATLAS survey^42,55, which contained valuable photometry in the o and c bands. One i-band epoch was obtained from the Pan-STARRS1 data archive⁵⁶.

ZTF and ATLAS also obtained observations of the location of SN 2020eyj in the nights immediately preceding the first detection, with limiting magnitudes in the ZTF g band on 5 and 6 March 2020 UT of 20.8 and 19.7, respectively, and the (binned) observations in the o band from ATLAS on 5 March 2020 UT correspond to a limiting magnitude of 20.2. Phases in this study are relative to the first ZTF detection (MJD = 58,915.212, 7 March 2020 UT) in rest-frame days, unless stated otherwise. Given the excellent constraints on the nights before, this epoch is also close to the time of first light.

All magnitudes are reported in the AB system. The extinction in the Milky Way (MW) was obtained from ref. ⁵⁷ as E(B − V) = 0.024 mag. MW reddening corrections are applied using the extinction law with R_V = 3.1 (ref. ⁵⁸), whereas SN reddening corrections are applied using R_V = 2. The photometric magnitudes of SN 2020eyj are listed in the Supplementary Information. The ATLAS and P48 light curves are shown in Fig. 3, binned into one-night bins to enhance the S/N.

Optical spectroscopy

The first optical spectrum of SN 2020eyj was obtained with the SEDM (ref. ⁵⁹) mounted on the P60 (ref. ⁶⁰), 25 days after first detection. All SEDM spectra are automatically reduced and calibrated with pysedm (ref. ⁶¹) and the quality of the SEDM spectrum of SN 2020eyj was further improved using hypergal (ref. ⁶²). Follow-up spectroscopy was obtained from 131 days onward with the Low-Resolution Imaging Spectrometer (LRIS)⁶³ on the Keck I telescope and the ALFOSC on the NOT (ref. ⁶⁴). A host spectrum was obtained at 678 days, after the SN had fully faded from view. The spectra were reduced in a standard manner, using LPipe (ref. ⁶⁵) and PypeIt^66,67 for Keck/LRIS and NOT/ALFOSC, respectively.

A log of the obtained spectra is provided in Extended Data Table 2 and the epochs of spectroscopy are indicated by the diamond markers on top of the light curves in Fig. 3. The spectra were absolute-flux-calibrated against the r-band magnitudes using the Gaussian process interpolated magnitudes and then corrected for MW extinction. All spectral data and corresponding information will be made available through the WISeREP public database⁶⁸. We present the peak SEDM spectrum in Fig. 1 and the later sequence of spectra in Fig. 2.

The initial spectrum obtained with SEDM is characterized by broad absorption features (see ‘SN Ia classification’ section). The later spectra are shaped by broad Fe ii lines, in particular the quasi-continuum blueward of 5,700 Å (refs. ^{69,70,71,72,73}) and a prominent Ca ii NIR triplet. Superimposed on the continuum are narrow He i emission lines, as well as Hα. We measure full width at half maximum (FWHM) velocities of the He i emission lines and Hα in the spectra obtained with Keck at 131 and 251 days by fitting a Lorentzian profile to the complete lines, as well as to just the blue wings. The red wings in the He and Hα lines are markedly attenuated (see Extended Data Fig. 3 and ‘Dust properties’ section), so the intrinsic FWHM velocities are better represented by (double) the blue-wing FWHM. We report these FWHM velocities in Extended Data Table 2. The FWHM velocities of the He i emission lines range from 1,100 to 2,700 km s⁻¹ (corrected for the red wings), with no sign of a narrow (<1,000 km⁻¹) component detected in some SNe Ibn and interpreted as coming from the unshocked CSM^71,74. There is also no sign of material stripped from the donor star by the SN ejecta^75,76, which is predicted to show up as narrow emission (<1,000 km⁻¹ (ref. ⁷⁷)).

The asymmetric line profile we associate with the SN also applies to the Hα emission line, suggesting the presence of H in the CSM. In the spectrum obtained at 131 days, Hα has an equivalent width of 14 Å, not corrected for contribution by the host. By comparison, the He i emission lines at 5,876 Å, 6,678 Å and 7,065 Å in the same spectrum have equivalent widths of 47 Å, 43 Å and 61 Å, respectively. As H is easier to ionize than He, the more prominent He lines means that the CSM must predominantly consist of He. By epoch 329 days, the Hα luminosity has dropped to the luminosity of the Hα narrow emission line in the host spectrum obtained at 678 days (see ‘Host galaxy’ section).

Infrared photometry

Following a report⁷⁸ of a mid-infrared detection of SN 2020eyj in the 2021 data release of the NEOWISE Reactivation (NEOWISE-R)⁷⁹ survey, we queried the IPAC Infrared Science Archive for any NEOWISE-R detections at the position of SN 2020eyj. After filtering poor-quality data and binning individual exposures following the method described in ref. ⁸⁰, the SN was recovered in both W1 and W2 filters (3.4 and 4.6 μm, respectively) in all four 2020 and 2021 epochs, with the earliest detection at 59 days after first detection (Fig. 3 and Extended Data Table 3). The host is not detected in (stacked) WISE data before the SN explosion (Extended Data Fig. 2, top panels), so we assume that the contribution from the host is negligible and all observed flux is because of the SN.

Radio

We observed SN 2020eyj with the electronic Multi-Element Radio Linked Interferometer Network (e-MERLIN) in two epochs. The first epoch, with a duration on target and phase calibrator of about 16 h, was conducted on 19 November 2021 (centred on MJD 59,538.29), 605 days after first detection and included six e-MERLIN telescopes (Mk2, Kn, De, Cm, Da and Pi). The second epoch was conducted during six consecutive days between 6 and 12 April 2022 (mean MJD 59,678.59, 741 days after first detection). Between fix and six telescopes (including the Lovell) participated, with some antennae missing part of the runs owing to technical problems. Owing to the much smaller field of view of the Lovell telescope, the pointing centre of the second epoch was shifted by 1 arcmin to include an in-beam calibrator in the primary beam of this telescope. 3C 286 and OQ 208 were used as amplitude and band-pass calibrators, respectively. The phase calibrator, J1106+2812, was correlated at position α_J2000.0 = 11 h 06 min 07.2617 s and δ_J2000.0 = 28° 12′ 47.065″, at a separation of 1.7° from the target, and was detected with a flux density of 150 mJy. We centred our observations at a frequency of 5.1 GHz, using a bandwidth of 512 MHz. The data were correlated with the e-MERLIN correlator at Jodrell Bank Observatory, using four spectral windows, each of 512 channels, with 1-s integrations and four polarizations.

We calibrated and processed the data using the e-MERLIN CASA pipeline⁸¹ version v1.1.19 running on CASA version 5.6.2. We used the 10-mJy in-beam source to self-calibrate the residual phases and amplitudes of the target source. Cleaning was done with the software package wsclean⁸². Final images of the target were produced with a synthesized beam of 80 mas × 35 mas at a position angle of 28° and 94 mas × 71 mas at a position angle of −71°, in the first and second epochs, respectively. The 1σ root mean squares of the images is 17 and 8 μJy beam⁻¹, respectively. The target is detected in both epochs as an unresolved source as characterized with task IMFIT. We estimate the uncertainty of the peak flux density to be a quadratic sum of the image root mean square and a conservative 10% amplitude scale calibration error. The final flux density of the source is 80 ± 20 and 60 ± 10 μJy beam⁻¹ in the first and second epochs, respectively. The radio source is located at an average position of α_J2000.0 = 11 h 11 min 47.1763 s and δ_J2000.0 = 29° 23′ 06.45″, with an estimated uncertainty of 10 mas.

The average position of the e-MERLIN detections relative to the optical positions of SN 2020eyj is shown in Extended Data Fig. 4. The radio detection is consistent with the position of the SN in the ALFOSC epoch at 382 days (r band) and the position reported in GaiaAlerts of the detection of SN 2020eyj in the G band at 42 days.

X-ray

On 27 April 2022, 758 days after first detection, we observed SN 2020eyj for 3.8 ks with the X-ray telescope XRT between 0.3 and 10 keV aboard the Neil Gehrels Swift Observatory^83,84. We analysed the data with the online tools of the UK Swift team (https://www.swift.ac.uk/user_objects/) that use the methods described in refs. ^85,86 and the software package HEASoft version 6.26.1. SN 2020eyj evaded detection down to a count rate of 0.003 count s⁻¹ (3σ limit). To convert the count-rate limit into a flux limit, we assumed a power-law spectrum with a photon index Γ of 2 and a galactic neutral hydrogen column density of 1.9 × 10²⁰ cm⁻² (ref. ⁸⁷). Here the photon index Γ is defined as the power-law index of the photon flux density (N(E) ∝ E^−Γ). Between 0.3 and 10 keV, the count-rate limit corresponds to an unabsorbed flux of 1.1 × 10⁻¹³ erg cm⁻² s⁻¹ and a luminosity <2.4 × 10⁴¹ erg s⁻¹. It is possible that a deeper observation would have yielded a detection, as the type Ia–CSM SN 2012ca was detected in X-rays at a similar epoch, with a luminosity on the order 10⁴⁰ erg s⁻¹ (ref. ⁸⁸).

SN Ia classification

During the peak phase of SN 2020eyj, an optical spectrum was obtained with the low-resolution (R ≈ 100) SEDM on the P60, 25 days after first detection. This high S/N spectrum was characterized by broad absorption features (Fig. 1), based on which SN 2020eyj was classified as a SN Ia at redshift z = 0.03 (ref. ⁸). Using SNIascore, a deep-learning-based classifier of SNe Ia based on low-resolution spectra⁸⁹, it was noted that the SN could be a type Ibc SN erroneously classified as SN Ia because of the degeneracy between peak spectra of SNe Ibc with those of SNe Ia at post-peak phases, but their classifier favoured a SN Ia classification anyway. In general, based on the comparison study in ref. ¹³, type Ibc SNe erroneously classified as type Ia(–CSM/91T) are much less common than the inverse. Here we scrutinize the SEDM spectrum using comparisons with SNe from the literature, based on spectral matching with the SuperNova IDentification (SNID)⁹⁰ and Superfit⁹¹ classification tools, for which the SNID template library has been supplemented with the type Ibc templates from ref. ⁹². We adopt a g-band peak epoch of MJD = 58,929 ± 2, based on the light-curve fitting described in the ‘Light-curve fit’ section, with the uncertainty driven by the poor sampling of our photometry around peak.

The top 10 SNID (rlap > 10) and Superfit matches are all of type Ia (Fig. 1) and include normal SNe Ia such as SN 2004eo (ref. ⁹³) and 91T-like SNe such as SN 2001V (ref. ⁹⁴). The best matching SN of type Ibc (rlap = 8) is the type Ic SN 1994I (refs. ^95,96,97,98). The phases corresponding to the matched SNe Ia are all post-peak, ranging from 12 days to about 50 days post-maximum, whereas the matching SN Ibc spectra are all within a few days from peak. The phase of the SEDM spectrum of SN 2020eyj is 12 days post-maximum, which corroborates the SN Ia classification.

In terms of spectral features, the SEDM spectrum shows broad absorption lines that, based on the spectral comparisons, can be unambiguously identified as Si ii, Fe ii and Ca ii (Fig. 1). Compared with normal SNe Ia as exemplified by SN 2004eo, the Si ii features in SN 2020eyj are shallow. Diluted Si ii absorption is common for 91T-like SNe Ia, as in the spectrum of SN 2001V. Type Ia–CSM are known to show 91T-like spectra around peak¹³. As a SN strongly interacting with a CSM, the presence of diluted Si ii in the SEDM spectrum of SN 2020eyj is consistent with a type Ia(–CSM) classification. In terms of expansion velocity, the velocity of the Si iiλ6355 absorption feature in the SEDM spectrum is 8,900 ± 600 km s⁻¹. This velocity is on the slow side for the SN Ia sample described in ref. ⁹⁹ but consistent with the SN Ia sample in ref. ¹⁰⁰ and comparable with, for example, SN 2004eo (Fig. 1).

Another notable feature in the SEDM spectrum is the complete lack of O i 7,774 Å absorption (Fig. 1), even though O i absorption in SNe Ia is common, in particular 91T-like SNe Ia can have shallow or non-existent O i (ref. ¹⁰¹). This is clearly visible in the matched spectrum of SN 2001V. By contrast, SNe Ibc that lack O i absorption are extremely uncommon, especially at about 12 days post-peak^102,103, as exemplified by type Ic SN 1994I in Extended Data Fig. 5. In this figure, we have also included the type Ibn SN 2006jc at a phase similar to that of the SEDM spectrum, to highlight that SN 2020eyj does not show any sign of He i emission lines or the quasi-continuum expected for a type Ibn at this phase.

An absence of oxygen lines is typical for type Ia–CSM spectra, both as an absorption feature around peak and as emission in later epochs^11,104, as seen in the early and late spectra of PTF11kx in Figs. 1 and 2, respectively. Similarly, the late spectra of SN 2020eyj lack any sign of O iλ7774 emission (Fig. 2). Other features in the late-time spectra of SN 2020eyj that are typical for type Ia–CSM include prominent broad Ca ii emission and a high Hα/Hβ Balmer ratio, which indicates that the emission lines are probably produced through collisional excitation rather than recombination¹¹. The high S/N spectrum at 251 days shows both Hα and Hβ emission, but after correcting for contribution by the host, only Hα shows some residual flux related to the transient. We note that, at this late phase, SN 2020eyj is about four magnitudes brighter than expected from a normal SN Ia, such as SN 2004eo (ref. ⁹³), and the spectrum is dominated by CSM-interaction features.

In conclusion, based on its spectral features, we classify SN 2020eyj as a type Ia(–CSM) SN. Furthermore, as we discuss in the ‘Light-curve fits’ section, the light curves of SN 2020eyj show strong similarities to those of PTF11kx, the SN that cemented SNe Ia–CSM as a subclass.

Light-curve analysis

Light-curve fits

The light curve of SN 2020eyj (Fig. 3) can be divided into two phases, similar to its spectral evolution. In the first phase, lasting around 50 days, the light curve follows a fairly typical bell-like shape, peaking at m ≈ 17.2 in both the r band and the ATLAS bands, which—at a luminosity distance of 131.4 Mpc (see ‘Host galaxy’ section)—corresponds to M ≈ −18.4, not accounting for host extinction. During the first phase, the light curve has a red g − r colour, consistent with the classification spectrum. The second phase, the tail phase from 50 days onward, is characterized by a slowly evolving light curve with spectra that are dominated by CSM interaction. Although the r-band light curve continues to fade, albeit at a slower rate of about 0.6 mag per 100 days between days 50 and 251, the g-band light curve plateaus. This results in a g − r colour change to blue (see bottom panel of Fig. 3), which—based on the spectra—is driven by the pseudo-continuum blueward of 5,700 Å. This Fe ii feature, typical for CSM-interaction-powered spectra, is well traced by the ZTF g band (4,100–5,500 Å). From 251 days onward, the light curve fades in all bands at a rate of about 1 mag per 100 days.

The transition between the two phases is well captured by the photometry at 50 days, when the decline in the g band is abruptly halted and changes to a plateau lasting around 200 days. This divergence of the g-band light curve from a smooth decline is probably the epoch in which CSM interaction starts contributing (substantially) to the light curve and in which the spectra start to look like those of SNe Ibn. But even though the late spectra may be similar to SNe Ibn, the light curve is unlike those of documented SNe Ibn. SNe Ibn are characterized by uniform, rapidly evolving blue light curves¹⁰⁵, peaking at M_r ≈ −19.5. There is a handful of long-lived, slowly evolving SNe Ibn reported in the literature, but they are either much brighter than SN 2020eyj (refs. ^106,107) or have a much longer risetime¹⁰⁸. None of the literature SNe Ibn show a long-duration (>300 days), slowly evolving light-curve tail such as that observed in SN 2020eyj. It is worth noting that there have been suggestions in the literature that some SNe Ibn may come from thermonuclear explosions, hidden by a dense CSM¹⁰⁹. The discovery of SN 2020eyj seemingly supports that notion.

The post-peak decline rates and peak magnitudes of SNe Ia are strongly correlated (the Phillips relation¹¹⁰), with brighter (fainter) SNe Ia evolving slower (faster). We fit the first phase of the multiband light curves with SNooPy¹¹¹, to determine whether the width (stretch) of SN 2020eyj is consistent with the expected peak luminosity. The light curve of SN 2020eyj up to 50 days is well described by a SN Ia light curve with an adopted stretch of s_BV = 1.2 ± 0.1 and an extinction of E(B − V) = 0.5 ± 0.1 mag (adopting a total-to-selective extinction ratio R_V = 2.0), resulting in a peak magnitude approximately 0.06 mag fainter than expected from the Phillips relation. The required line-of-sight extinction is considerable but is consistent with the host extinction of $E(B-V)=0.5{4}_{-0.12}^{+0.14}\,{\rm{mag}}$ derived from host galaxy Balmer lines (see ‘Host galaxy’ section). We apply the same fitting method to the light curve of PTF11kx, consisting of published and previously unpublished photometry. For PTF11kx, we adopt the same stretch factor of 1.2 and obtain an extinction of E(B − V) = 0.27 ± 0.02 mag, consistent with the extinction A_V ≈ 0.5 mag derived in ref. ¹⁵. After correcting for the fitted host extinction, the resulting absolute-magnitude light curves of SN 2020eyj and PTF11kx are practically identical in the g and r bands for the first approximately 45 days, even though the fits are independent (Extended Data Fig. 1). The r-band light curves peak at M_r ≈ −19.3 for both SNe, consistent with both SNe Ia and SNe Ia–CSM, although both SNe are on the fainter end of the sample of SNe Ia–CSM described in ref. ¹¹. From the light-curve fits, we obtain for SN 2020eyj risetimes in the g and r bands of 14 ± 2 and 16 ± 2 days since first detection, respectively. This is fast for a SN Ia (ref. ³³) but similar to PTF11kx (Extended Data Fig. 1).

An important caveat about the light-curve fit is that the intrinsic decline rate of SN 2020eyj could seem slower because of the contribution by CSM interaction. On the basis of the colour evolution of the light curve, we know from day 50 onward that the CSM contribution is notable, but it is reasonable to assume that some CSM interaction already contributes to the light curve at earlier epochs. This means that the stretch parameter we measure should be regarded as an upper limit and, as a result, so is the peak luminosity of the fit. SN 2020eyj, but also PTF11kx, are not typical SNe Ia, so the colours and peak magnitude could (to some extent) also be a property intrinsic to the class.

Bolometric light curve

The light curve of SN 2020eyj around peak has limited photometric coverage, in both wavelength and cadence, which hinders the construction of a precise, full bolometric light curve. Instead, we compute the bolometric light curve based on the SN Ia light-curve template fit obtained in the ‘Light-curve fits’ section, for epochs when the photometry (notably the g band) still matches well with the fitted light curve (up to 38 days after first detection; Extended Data Fig. 1). From the fitted optical light curves, we flux calibrate, correct for host extinction and integrate the spectral time series from ref. ¹¹² from the ultraviolet (UV) to the NIR (1,000–25,000 Å). For the tail phase, we integrate the Keck spectra at 131 and 251 days from 3,000 to 10,000 Å and apply a linear extrapolation in the UV to zero flux at 2,000 Å. There is little spectroscopic (Fig. 2) and colour (Fig. 3) evolution between the Keck spectrum at 251 days and the NOT spectrum at 328 days, so we extend the pseudo-bolometric light curve to the final photometric epoch at 383 days obtained with ALFOSC on the NOT assuming a constant bolometric correction applied to the g-band magnitude. Extended Data Fig. 6 shows the bolometric luminosity inferred from the template fit, the Keck spectra and the final photometric epoch. The template fit to the initial peak included considerable line-of-sight extinction of E(B − V) = 0.5 mag (see ‘Light-curve fits’ section). To account for the possibility that SN 2020eyj may be intrinsically faint rather than a normal SN Ia substantially dust-extincted, we also include a bolometric light curve for E(B − V) = 0. Depending on dust extinction, the total integrated energy radiated across the bolometric light curve amounts to 0.6–1.2 × 10⁵⁰ erg.

Host galaxy

The host of SN 2020eyj is a faint and compact galaxy with designation SDSS J111147.15+292305.9 (Extended Data Fig. 4). We retrieved science-ready co-added images from the Galaxy Evolution Explorer (GALEX) general release 6/7 (ref. ¹¹³), the Sloan Digital Sky Survey data release 9 (SDSS DR 9; ref. ¹¹⁴) and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS, PS1) DR1 (ref. ⁵⁶) and measured the brightness of the host using LAMBDAR (Lambda Adaptive Multi-Band Deblending Algorithm in R; ref. ¹¹⁵) and the methods described in ref. ¹¹⁶. We augment this dataset with an optical r-band image obtained with ALFOSC on the NOT on 2 May 2022 and UV observations from Swift/UVOT from 27 April 2022. The photometry on the UVOT images was done with uvotsource in HEASoft and an aperture encircling the entire galaxy (aperture radius 8″). Extended Data Table 4 lists all measurements. We fit the host galaxy spectral energy distribution (SED) with the software package Prospector version 0.3 (ref. ¹¹⁷) to determine the host galaxy properties. We assumed a Chabrier initial mass function¹¹⁸ and approximated the star formation history (SFH) by a linearly increasing SFH at early times followed by an exponential decline at late times (functional form t × exp(−t/τ), in which t is the age of the SFH episode and τ is the e-folding timescale). The model was attenuated with the model in ref. ¹¹⁹. The priors were set identical to those in ref. ¹¹⁶. The fit resulted in a low host-galaxy mass of $\log (M/{M}_{\odot })=7.7{9}_{-0.34}^{+0.15}$.

We obtained a host galaxy spectrum with LRIS/Keck after SN 2020eyj had faded from view, at 678 days. We identify unresolved (≲150 km s⁻¹) host-galaxy lines in the spectrum, such as numerous Balmer lines in emission and absorption, [N ii] λλ6548,6583, [O ii] λλ3726,3729, [O iii] λλ4959,5007 and [S ii] λλ6716,6731, based on which we measure a redshift of z = 0.0297 ± 0.0001. Adopting a flat cosmology with H₀ = 70 km s⁻¹ Mpc⁻¹ and Ω_M = 0.3, this redshift corresponds to a luminosity distance to SN 2020eyj of 131.4 Mpc, which we use throughout this paper. Correcting for MW extinction, the adopted distance results in a host galaxy absolute magnitude of M_r = −15.8.

On the basis of the Balmer decrement measured in the host spectrum, we estimate a host extinction with $E(B-V)=0.5{4}_{-0.12}^{+0.14}\,{\rm{mag}}$, in agreement with the extinction obtained by fitting the light curves of SN 2020eyj with a SN Ia template (see ‘Light-curve fits’ section). The line ratios of log₁₀([O iii] λ5007/Hβ) = 0.39 and log₁₀([N ii] λ6583/Hα) = −1.26 put the host galaxy well into the regime of star-forming galaxies on the Baldwin–Phillips–Terlevich diagram¹²⁰. Adopting the parameterization of the empirical oxygen calibration O3N2 by ref. ¹²¹, we obtain an oxygen abundance of 12 + log(O/H) = 8.14 ± 0.03. Such a low oxygen abundance is expected for a low-mass galaxy¹²².

The host properties of 16 SNe Ia–CSM were reported in refs. ^11,123. The authors concluded that all objects in their samples exploded in star-forming late-type galaxies (spiral and dwarf galaxies) with absolute magnitudes between M_r = −20.6 and −18.1 mag. The hosts of three SNe in this sample evaded detection in archival SDSS images, implying an absolute magnitude M_r > −18 mag. SN 2020eyj exploded in a markedly low-luminosity star-forming dwarf galaxy with an absolute r-band magnitude of only M_r = −15.8 mag (not corrected for host attenuation). However, the modelling of the host-galaxy SED and the Balmer decrement show non-negligible attenuation of 0 < E(B − V) < 0.55 mag (3σ confidence interval from host SED modelling) or 0.2 < E(B − V) < 1 mag (3σ confidence interval from the Balmer decrement), which would alleviate the apparent extremeness of the host galaxy.

Dust properties

Infrared emission is commonly observed in interacting SNe and can be attributed to the condensation of dust in the SN ejecta or in the shocked CSM, or to pre-existing dust in the CSM that is heated radiatively by the SN emission or by the ejecta–CSM shock interaction^{124,125,126,127}. The mid-infrared luminosity of SN 2020eyj is at a similar level as for the most infrared-luminous interacting SNe, such as type IIn– and Ia–CSM SNe, and at 4.5 μm is 6–10 magnitudes brighter than normal type Ia SNe and ≳4 magnitudes brighter than the type Ibn SN 2006jc (Extended Data Fig. 2, bottom panel).

Assuming optically thin dust, the flux F_ν can be written as¹²⁸:

$${F}_{\nu }=\frac{{M}_{{\rm{d}}}\,{B}_{\nu }({T}_{{\rm{d}}}){\kappa }_{\nu }(a)}{{d}^{2}},$$

(1)

in which M_d is the mass of the dust, B_ν the Planck blackbody function, T_d the temperature of the dust, κ_ν(a) the dust absorption coefficient as a function of dust particle radius a and d is the distance to the observer. For simplicity, we assume a simple dust population of a single size composed entirely of amorphous carbon with a grain size of 0.1 μm with the corresponding absorption coefficient κ as in refs. ^129,130 and fit the WISE data to obtain an estimate of the dust temperature and mass. We note that the dust mass depends on assumed grain size, which we cannot constrain on the available data. Varying the grain size from 0.01 to 1.0 μm changes the derived dust mass by an order of magnitude¹²⁹. Over the first three epochs, up to 412 days, we derive a constant dust temperature of around 800 K (Extended Data Table 3), consistent with a lack of colour evolution in the WISE photometry (Fig. 3). Only at the fourth WISE epoch (614 days) do we see a marked decrease in the dust temperature, to 608 ± 23 K. These dust temperatures are well below the expected evaporation temperature of dust (1,500 K for silicates and 1,900 K for graphite grains¹²⁷). As well as the dust temperatures, we obtain dust mass estimates of (1.8 ± 0.3) × 10⁻³ M_⊙ to (9.9 ± 2.1) × 10⁻³ M_⊙ for the first and final WISE epochs, respectively (Extended Data Table 3). The dust mass estimated for the final epoch corresponds to a CSM mass of 1 × (0.01/r_dg) M_⊙, in which r_dg is the dust-to-gas ratio. The total integrated energy emitted in the infrared is 9 × 10⁴⁹ erg (Extended Data Table 3), which is similar to the integrated energy emitted in the optical (see ‘Bolometric light curve’ section).

In the case of optically thin dust that we consider here, the blackbody radius can be interpreted as a lower limit to the radius at which the dust resides. In the case of SN 2020eyj, the blackbody radius is (2.5 ± 0.2) × 10¹⁶ cm in the first epoch and increases thereafter to (6.4 ± 0.6) × 10¹⁶ cm at 614 days (Extended Data Table 3). Assuming a SN ejecta velocity of 10⁴ km s⁻¹, by 59 days, the ejecta would only have travelled about 20% of the distance inferred from the blackbody fit at that epoch. Combined with the constant dust temperature, this suggests that the infrared emission of SN 2020eyj is dominated by pre-existing dust being radiatively heated by CSM interaction emission, as was the case in type Ia–CSM SN 2005gj (ref. ¹³⁰). Furthermore, because the dust reached a temperature of 800 K as early as 59 days and showed no notable evolution afterwards, it is unlikely that any surrounding dust was evaporated owing to the SN, because such hot dust would have dominated the infrared flux. For a peak SN luminosity of roughly 10⁴³ erg s⁻¹ (Extended Data Fig. 6), the dust evaporation radius is R_evap = (0.34–2.6) × 10¹⁷ cm, depending on dust grain size and composition¹²⁷. The lack of dust at the sublimation temperature implies that the immediate region surrounding the SN was devoid of dust, much like the CSM-free cavity inferred from the SN light curve.

The He i and Hα emission-line profiles show the red wing being attenuated with time (Extended Data Fig. 3). Such an evolution in the line profiles has been interpreted to result from condensation of dust in the ejecta or the shocked CSM, obscuring the red wing of the emission line^126,131,132. Similar line profiles have been observed in many SNe Ia–CSM¹¹ and in the prototypical type Ibn SN 2006jc, in which the evolution of the line profiles was attributed to dust condensing in a cool dense shell produced by the interaction of the ejecta with CSM also producing a substantial infrared excess¹²⁵. Notably, such line-profile evolution has also been observed in the He nova V445 Pup, which was attributed to dust obscuration within the shell³⁵. In particular, for type Ia–CSM SN 2005gj, dust formation was inferred from line profiles¹¹, whereas the bulk of the infrared emission was also attributed to pre-existing dust¹³⁰. Although the line asymmetry in the spectra of SN 2020eyj is consistent with dust formation, it must be noted that asymmetric line profiles can also arise from optical depth effects, for example, in SNe Ibn (ref. ²⁰). A substantial contribution to the infrared flux by newly formed dust is also not consistent with the lack of colour evolution in the light curve of SN 2020eyj from day 100 onward. The dust formation in SN 2006jc was accompanied by a reddening of the optical light curve¹²⁵, which we do not observe in SN 2020eyj past 100 days. So, we attribute the bright infrared counterpart of SN 2020eyj predominantly to pre-existing dust, similar to the conclusion drawn for the infrared counterpart of the prototypical type Ia–CSM SN 2002ic (ref. ¹²⁴).

CSM origin

Optically thick wind

Using progenitor models for the He star donor SN Ia channel from ref. ⁶, Moriya et al.²⁶ investigated the CSM properties resulting from this channel, in which accretion from a non-degenerate He star allows the accompanying WD to reach the Chandrasekhar limit. The study by Moriya et al.²⁶ focused on the low-circumstellar-density regime, in which the CSM properties in the WD + He star systems still adhere to the stringent CSM constraints imposed by radio non-detections of SNe Ia (refs. ^4,5,133,134). Here we explore if the models with sufficiently dense CSM, with a wind-like density profile (ρ ∝ r⁻²), can explain the interaction-powered light-curve tail of SN 2020eyj and the detections at radio wavelengths. To quantify the properties of the CSM, we fit the CSM-interaction-powered tail of the bolometric light curve using the analytical model from ref. ²⁶ and use the resulting mass-transfer rates to fit the radio detections.

Extended Data Fig. 6 shows the bolometric light curve of SN 2020eyj, with the initial peak described by the SN Ia template fit (solid line), and for the tail phase the luminosities inferred from the Keck spectra at 131 and 251 days and the ALFOSC epoch at 383 days (see ‘Light-curve fits’ section). From the light curve described by the SN Ia component alone (solid and dotted lines), it is also clear that the late-time light curve of SN 2020eyj cannot be powered by ⁵⁶Ni decay, as the flux integrated across the Keck spectrum at 131 days is already at least ten times larger than what the radioactive decay delivers. Also plotted are the CSM-interaction model fits to the light-curve tail, for both E(B − V) = 0 and 0.5 mag, as discussed in the ‘Light-curve fits’ section. Assuming a pre-SN wind velocity of 1,000 km s⁻¹, the CSM-powered tail of SN 2020eyj is consistent with mass-transfer rates between 10⁻³ M_⊙ yr⁻¹ (E(B − V) = 0 mag) and 3 × 10⁻² M_⊙ yr⁻¹ (E(B − V) = 0.5 mag), which is 1–2 orders of magnitude larger than that considered in the original study²⁶. At these very high mass-transfer rates, the critical mass accretion rate by the WD is exceeded and the excess is ejected as an optically thick wind, resulting in an extended He envelope²⁶. In the model, the forward shock reaches approximately 10¹⁷ cm in 800 days. If we assume a wind velocity of 1,000 km s⁻¹, the CSM mass within 10¹⁷ cm in the models range from 0.3 M_⊙ to 1 M_⊙, for E(B − V) = 0 and 0.5 mag, respectively.

Extended Data Fig. 6 shows the wind model radio light curves fitted to the radio detections at 5.1 GHz, adopting ϵ_e = 0.1 and mass-transfer rates of 10⁻³ M_⊙ yr⁻¹ and 3 × 10⁻² M_⊙ yr⁻¹, for E(B − V) = 0 and 0.5 mag, respectively. We consider both synchrotron emission with synchrotron self-absorption and free–free absorption but note that, at the late phase of the radio detection, free–free absorption has only a minor impact. The radio light curve with an adopted mass-transfer rate of 10⁻³ M_⊙ yr⁻¹ is consistent with the radio detections of SN 2020eyj at 5.1 GHz, with microphysics parameter ϵ_B = 1.7 × 10⁻³. For the high-extinction scenario, with a mass-transfer rate of 3 × 10⁻² M_⊙ yr⁻¹, the model fits when ϵ_B = 1.5 × 10⁻⁵. In either case, the late time evolution follows the observed power-law decline rate of the observed radio luminosity of β = −1.6, which is comparable with that for hydrogen-free SNe Ibc (ref. ¹³⁵).

It is worth noting that the bolometric light curve only extends to 400 days, whereas the first detection of SN 2020eyj at 5 GHz took place at 605 days. Furthermore, it has been argued that the mass-transfer rates associated with the optically thick wind phase (>10⁻⁷ M_⊙ yr⁻¹) do not lead to SNe Ia but rather to accretion-induced collapse of the WD (refs. ^136,137), although alternative wind models have been suggested to overcome this problem¹³⁸.

CSM shells

The CSM surrounding the H-rich analogue of SN 2020eyj, PTF11kx, was argued to be concentrated in shells¹⁵. Other SNe Ia have shown evidence for CSM concentrated in thin shells, albeit at distances (about 10¹⁶ cm) that no interaction with the ejecta is expected^{139,140,141,142}. Shells have also been invoked for the configuration of the CSM in core-collapse H-rich type IIn SNe and typically attributed to ejection events by their massive progenitors. One noteworthy example is the well-studied SN 2014C, which transitioned from a stripped-envelope SN to a type IIn SN owing to interaction with a distant shell, and was detected in the radio^143,144. Models for the radio emission of SNe Ia colliding with a constant-density shell of CSM have been previously presented in the literature, along with approximate functional forms to describe the evolution of the optically thick synchrotron light curve¹⁴⁵. Because those models assume hydrogen-rich material, for our calculations, we modify n_e = ρ/m_p to n_e = ρ/(2m_p); otherwise, we use the default parameters, notably ϵ_B = 0.1. We explore shell models with a range of CSM masses M_csm = (0.01–1) M_⊙ and interaction end times from t_end = 328 days (the spectrum that does not show prominent He i lines) to t_end = 763 days (the second radio detection)—in this model, interaction must have ended before the second radio detection for the radio emission to have declined between the two observations. We assume a shell inner radius of R_in = (30,000 km s⁻¹)(50 days) = 1.3 × 10¹⁶ cm to close the system of equations in the model; then, the ranges of M_csm and t_end correspond to a range of shell widths ΔR/R_in = 3.4–7.5. For each model, we calculate the representative model error as ${\sigma }_{{\rm{mod}}}=\max \left(\left|{L}_{\nu ,{\rm{obs}}}({t}_{i})-{L}_{\nu ,{\rm{mod}}}({t}_{i})\right|/\Delta {L}_{\nu ,{\rm{obs}}}({t}_{i})\right)$, in which subscripts ‘obs’ and ‘mod’ refer to observed and modelled values, respectively, L_ν is spectral luminosity and ΔL_ν is the error on the luminosity (flux error only; error in distance is not included). The best-fit model by this metric has M_csm = 0.36 M_⊙ and t_end = 665 days, which is a very similar mass to what is found for PTF11kx based on analysis of its optical spectra¹⁴⁶. We find that models with σ_mod ≤ 3 have t_end ≈ (500–763) days and M_csm ≈ (0.2–0.5) M_⊙, whereas those with σ_mod ≤ 1 (that is, a better fit) have t_end ≳ 580 days and M_csm ≈ (0.3–0.4) M_⊙. The best-fit shell model is shown in Fig. 4.

V445 Puppis

The nova outburst of V445 Pup in the year 2000 lacked any Balmer emission in the spectra of its ejecta, but instead was characterized by He and carbon emission lines^34,147, making it the first and so far only known He nova system. On the basis of light-curve modelling, a mass (≥1.35 M_⊙) close to the Chandrasekhar limit was inferred for the WD in V445 Pup (ref. ³⁶), consistent with the observed high ejecta velocities up to 8,450 km s⁻¹ (ref. ³⁵). Combined with a high mass-transfer rate >10⁻⁷ M_⊙ yr⁻¹, in which half of the accreted matter remains on the WD (ref. ³⁶), V445 Pup is considered to be a prime candidate progenitor for the SD He + WD SN Ia progenitor channel.

On the basis of infrared spectra showing prominent carbon lines^34,148 and a rapid decline in the light curve of V445 Pup, it was shown that a carbon-rich thick dust shell must have formed in the nova ejecta^34,147. High-resolution NIR images resolved the nova event into an expanding narrow bipolar shell with bulk velocities of about 6,700 km s⁻¹ and a perpendicular central dust disk that strongly attenuates the optical He i emission lines arising from the receding shell³⁵. Seven years after the outburst, the bipolar shell of V445 Pup, as imaged in the NIR, extended to around 10¹⁷ cm and the central dust torus had an outer radius (perpendicular to the lobes) of ≳10¹⁶ cm (ref. ³⁵). An outer dust shell in a V445 Pup-like system could survive dust sublimation from a SN Ia explosion, depending on peak luminosity and grain composition¹²⁷. A recent study of the long-lived radio evolution of V445 Pup showed that the system was continuously synchrotron luminous for years after the outburst³⁹. The synchrotron emission originated from the inner edge of the equatorial disk and was interpreted as interaction between a wind coming off the WD from nuclear burning and the surviving disk. The persistence of the disk through the nova outburst suggests that the disk is at least comparable in mass with the mass of the nova ejecta, which was estimated to be about 10⁻⁴ M_⊙ (ref. ³⁶). In turn, the mass of the WD in V445 Pup, close to the Chandrasekhar limit, limits the ejecta mass in the system to not more than about 10⁻³ M_⊙ (Fig. 7 in ref. ³⁶).

ISM

Radio emission can potentially arise from a SN Ia in the DD scenario as a result of interaction with the ISM. We have modelled the radio light curve from such a merger scenario in the same way as in refs. ^31,134, that is, we assume that the supernova is the result of two merging WDs with masses 0.9 and 1.1 M_⊙, as described in ref. ¹⁴⁹. The outermost ejecta has a density slope ∝ ρ⁻ⁿ with n = 13 (see ref. ³¹ for a discussion on n). The microphysics parameters are the standard values ϵ_e = 0.1 and ϵ_B = 0.01. The modelled radio emission increases with time (Fig. 4) and, to agree with the observed 5.1-GHz fluxes at 605 and 741 days, the ISM electron density has to be 660 cm⁻³ and 450 cm⁻³, respectively, assuming fully ionized hydrogen and helium with He/H = 0.1. For n = 13 and fixed ϵ_e, the electron density scales roughly as ${{\epsilon }}_{{\rm{B}}}^{-0.74}$, so other ISM densities are possible accordingly. For a probable upper limit on ϵ_B of 0.1, the ISM density would be n_e = 85 cm⁻³ to fit the flux at the second epoch, and for ϵ_B of 0.001, n_e = 2,570 cm⁻³. The increase in radio flux with time is opposite to what is observed and is a property for all our ISM models with n > 7.1. Lower n values are not expected³¹ and the densities required in our ISM models are much higher than normal ISM densities. Moreover, for the n = 13, ϵ_B = 0.01 model, in which n_e = 450 cm⁻³, the modelled flux for the first epoch undershoots by 2σ (Fig. 4). In summary, our radio observations and their modelling argue strongly against an ISM scenario, which arises from a DD progenitor system. Furthermore, the observed strong helium lines are also at odds with an ISM scenario¹⁵⁰. We therefore conclude that SN 2020eyj did not result from the thermonuclear runaway of a WD in a DD progenitor system, leaving the SD scenario as the only viable alternative.

Precursor search

The CSM surrounding SN 2020eyj could have originated from one or more novae, such as observed in V445 Pup. We investigate if a similar outburst at the location of SN 2020eyj can be found in ZTF data going back more than 2 years. The position of SN 2020eyj was observed 772 times (after quality cuts) in the g, r and i bands across 202 different nights in the final 2.29 years before the SN explosion. There are no notable pre-explosion detections in unbinned or binned light curves (1-day-long to 90-day-long bins) following the search method described in ref. ¹⁵¹. When combining observations in week-long bins, we reach a median limiting absolute magnitude of −14.28 in the r band (−14.26 in the g band). We can, hence, rule out precursors that are brighter than −14 magnitude 21% of the time in the r band (16% of the time in the g band). Precursors brighter than magnitude −15 can be ruled out 49% of the time in the r band (39% for the g band) in the final 2.29 years before the SN. The nova outburst of V445 Pup peaked at m_V = 8.6 (ref. ¹⁵²), which—at a distance of 8.2 kpc (ref. ³⁵)—equates to an absolute magnitude of M_V = −1, far below the detection threshold of the ZTF.