JWST Detects a Dusty AGB-like Source Before the Type Ia-CSM Supernova 2026sqf

Abstract: Supernova (SN) 2026sqf recently appeared in the nearby face-on spiral galaxy NGC 3310 and has shown signs of strong interaction with a circumstellar medium (CSM). Such intense interaction, rare among nearby SNe, offers a valuable opportunity to reveal details on the origin and nature of its progenitor system. We analyzed the early-phase spectra and light curves (LCs) of SN 2026sqf. The general shapes of the observed spectra, the strengths of the emission lines, and the LC evolution all suggest that SN 2026sqf belongs to the rare SN Ia-CSM subclass. If confirmed, this would be the closest known member of this class, at D~19 Mpc. We also report the first candidate progenitor system for a thermonuclear supernova identified in JWST pre-explosion imaging, and the first evidence for a (probable) carbon-rich AGB donor to the exploding white dwarf (WD). Our results are consistent with the expectation that SNe Ia-CSM emerge from a system consisting of a WD and an AGB star undergoing a common-envelope phase. The CSM mass and dust content are consistent with expectations for an AGB environment, but the narrow-line width exceeds superwind expansion velocities, favoring an episodic ejection. Binary interaction shortly before the explosion is a natural explanation, though the channel and timescale remain uncertain. Late-time follow-up, especially with JWST, will test the identification and the conclusions of our early-phase analysis.

• The paper identifies a red, dusty pre-explosion source with an infrared luminosity of approximately 17,800 solar luminosities, consistent with an extreme AGB star and a possible white dwarf companion.
• Spectral features, Balmer-line ratios, and light-curve evolution favor a Type Ia-CSM classification, while estimated mass-loss rates of 0.01–0.04 solar masses per year suggest episodic binary-driven ejection rather than a steady AGB wind.
• The proposed progenitor system could represent the first directly identified Ia-CSM progenitor and the first thermonuclear-supernova progenitor detected in JWST pre-explosion imaging, although positional, modeling, and classification uncertainties remain.

Context and motivation

SN 2026sqf was discovered on 2026 July 8 in the nearby face-on spiral galaxy NGC 3310 ($D = 19.0 \pm 1.5$ Mpc), making it the fourth supernova recorded in this host. Early classification spectra showed narrow hydrogen and helium emission suggestive of flash-ionized circumstellar material (CSM), initially resembling a Type II event, but the subsequent evolution diverged from normal core-collapse behavior. This paper presents early-time optical photometry and spectroscopy, together with an analysis of pre-explosion JWST and HST imaging of the explosion site, and argues that SN 2026sqf is a member of the rare Type Ia-CSM subclass — and, if confirmed, the closest known one by a wide margin. The prior closest event of this class, SN 2012ca, lies at roughly 80 Mpc, and most known members are beyond 100 Mpc.

The broader motivation is the unresolved question of Type Ia progenitor channels. Direct pre-explosion imaging is the most decisive test of the donor's nature, but for thermonuclear supernovae it has so far yielded only the special He-rich donor case of the Type Iax SN 2012Z. No progenitor identification has previously been achieved for any SN Ia-CSM.

Classification as a Type Ia-CSM event

Four NOT/ALFOSC spectra obtained between +5 and +20 days after discovery were compared against SNID-SAGE and REDBACK template libraries. The two latest spectra match the Ia-CSM templates SNe 2005gj, 2016iks, and LSQ15adm, and increasingly display overluminous SN Ia characteristics — Fe III, Si II, Ca II, and weak S II — alongside strong hydrogen emission. Two quantitative diagnostics distinguish SNe Ia-CSM from SNe IIn: weaker H$\beta$ and a larger H$\alpha$/H$\beta$ ratio. For SN 2026sqf the measured values are EW(H$\beta$) $\sim 5{-}7$ Å and $F_{\mathrm{H}\alpha}/F_{\mathrm{H}\beta} \sim 5$, both closer to the Ia-CSM averages of Sharma et al. than to those of SNe IIn. The early $g$- and $r$-band light curves also track known ZTF Ia-CSM events closely. Taken together — spectral shape, Balmer line diagnostics, and light-curve evolution — the evidence favors the Ia-CSM classification, though the paper notes it is not yet formally confirmed.

The pre-explosion point source

Archival JWST/NIRCam imaging from March 2026 (only about four months before explosion) and MIRI imaging from 2024, plus multiple HST epochs, cover the SN site. After astrometric alignment (uncertainties of $\lesssim 0.25''$), the authors identify a red point source coincident with the SN position in NIRCam, MIRI, and — marginally, at $\beta$0 — in HST F814W images. Photometry was performed independently with space_phot and DOLPHOT, agreeing to within $\beta$1 mag except for MIRI F770W; averaged fluxes were adopted.

Integrating the dereddened SED yields $\beta$2 and $\beta$3. This places the source well above RGB luminosities ($\beta$4) and in the regime of luminous AGB stars, while falling below typical RSG progenitors of Type II SNe ($\beta$5). The SED resembles those of the brightest known carbon-rich AGB stars (e.g., AFGL 3068, MSX LMC 220), though it is more luminous in the mid-IR. Three-component modeling (extinguished stellar blackbody plus warm and cool dust shells, using carbonaceous and silicate optical constants with escape-probability corrections) gives stellar radii of $\beta$6 and $\beta$7 K near the Hayashi limit. Luminosity, radius, and temperature jointly converge on an (extreme) AGB classification, and pure carbon dust provides a reasonable fit — which would make this the first evidence for a probable carbon-rich AGB donor to an exploding white dwarf. Silicate dust, however, cannot be excluded.

Implications for the progenitor channel

If the identification holds, SN 2026sqf is the first candidate progenitor system for a thermonuclear supernova detected in JWST pre-explosion imaging and the first direct progenitor identification for the Ia-CSM class. The source properties are consistent with a white dwarf plus AGB companion undergoing common-envelope evolution (CEE) at the time of explosion.

From the two-component H$\beta$8 profile, the intermediate component ($\beta$9 erg s$\alpha$0, FWHM $\alpha$1 km s$\alpha$2) implies a mass-loss rate of $\alpha$3, consistent with other SNe Ia-CSM. Preliminary light-curve modeling suggests a total CSM mass of $\alpha$4, implying a deposition timescale of only a few decades. Such rates far exceed standard AGB superwind values ($\alpha$5), and the narrow-line width ($\alpha$6 km s$\alpha$7) exceeds superwind expansion velocities — both favoring episodic ejection, for which binary interaction and RLOF or merger with the AGB core are natural explanations. The paper is explicit that the specific channel and timescale remain uncertain.

Limitations and open questions

Several caveats bear directly on the central claims. The astrometric error circle ($\alpha$8) admits the possibility that an unrelated star within it is the true counterpart, although the identified source is the most promising candidate on photometric and physical grounds. The SED modeling assumes a single, time-independent model for data taken years apart — an assumption that may fail for an evolving system in its final phases — and relies on simplified blackbody-plus-spherical-dust-shell prescriptions; a mixed dust composition remains possible. The mass-loss estimate assumes a spherical, $\alpha$9 CSM with constant $\beta$0 and a fixed H$\beta$1 efficiency factor, all of which the authors acknowledge as oversimplified; as prior work emphasizes, such conversions depend sensitively on CSM structure and microphysical parameters. The distance itself carries $\beta$2 Mpc uncertainty from inconsistent literature values. Finally, the Ia-CSM classification, while strongly supported, is not yet confirmed. Late-time JWST mid-IR photometry and spectroscopy — tracking dust temperature and mass as the shock sweeps up the shell — together with optical, radio, and X-ray follow-up, are identified as the decisive tests of both the progenitor identification and the spherical, constant-wind assumption.

Conclusion

SN 2026sqf combines three firsts: the nearest known candidate SN Ia-CSM at $\beta$3 Mpc, the first candidate progenitor identification for this subclass, and the first thermonuclear-supernova progenitor detected in JWST pre-explosion imaging. The pre-explosion source is consistent with a luminous, dusty, likely carbon-rich AGB star, supporting a white dwarf plus AGB common-envelope progenitor channel, while the inferred high mass-loss rate and narrow line widths point to episodic, binary-driven mass ejection. The identification and the CSM interpretation remain provisional pending late-time observations, for which this event offers an unusually favorable target.