SN Encore: Lensed Type Ia Supernova

• SN Encore is a strongly lensed Type Ia supernova at z=1.95, exhibiting multiple images that make it unique for cluster-scale time-delay cosmography.
• Its spectroscopic confirmation and light-curve analysis, using JWST data, affirm its standardizability and reliability for precise Hubble constant measurements.
• Robust strong-lensing reconstructions incorporating cluster dynamics and microlensing systematics position SN Encore as a premier probe for cosmological studies.

SN Encore is a strongly lensed Type Ia supernova at $z = 1.95$ in the multiply imaged galaxy MRG-M0138 behind the galaxy cluster MACS J0138.0$-$2155, and is the second multiply imaged supernova discovered in the same host after SN Requiem. The system is the first known lensing configuration to produce more than one multiply imaged supernova in a single host galaxy, and it has become a central case for cluster-scale time-delay cosmography because the combination of precise supernova light-curve timing, a richly constrained cluster mass model, and multiple background sources permits a direct inference of the Hubble constant $H_0$ (Pierel et al., 2024, Bazzanini et al., 23 Jun 2026).

1. Discovery, host galaxy, and system uniqueness

SN Encore was discovered on 2023 November 17 in JWST/NIRCam imaging and was absent in archival HST/WFC3-IR F160W data from 2016. The discovery paper identified it as a bright, strongly lensed supernova candidate with $m_{\rm F150W,AB}\approx 24$ mag, detected in three locations after host-galaxy subtraction, in a host that is quintuply imaged by the cluster potential (Pierel et al., 2024). Subsequent work established that both Encore and the earlier SN Requiem occurred in the same host galaxy, MRG-M0138, at source redshift $z_s = 1.949$, behind the cluster MACS J0138.0$-$2155 at lens redshift $z_d = 0.336$ (Bazzanini et al., 23 Jun 2026).

The host galaxy was already known as a massive, quiescent system. Its stellar mass was reported as $\log_{10}(M_\ast/M_\odot)=11.7\pm0.2$, with low current star formation, and parametric host fitting gave a probability of detecting two SNe Ia in this host over a $\sim10$ year window of $\approx 3\%$ (Pierel et al., 2024). This configuration is therefore unusual but not excluded by the host’s mass and delay-time-distribution-based SN Ia rate estimates.

A recurrent point of clarification is that the host is quintuply imaged, whereas the supernova event is discussed operationally in terms of the images that are detected or likely detectable. Early analyses described three visible SN images and two additional predicted images, while later strong-lensing reconstructions labeled the robustly relevant sequence as 1a, 1b, 1c, 1d, and a model-dependent central image 1e (Pierel et al., 2024, Bazzanini et al., 23 Jun 2026). This suggests that the geometric multiplicity of the host and the observational practicality of SN-image detection are related but not identical.

2. Transient classification and spectroscopic properties

JWST/NIRSpec IFU spectroscopy established SN Encore as a spectroscopically confirmed Type Ia supernova. The discovery analysis found that both SNID and NGSF classify Encore as a SN Ia with $-$0 confidence, with nine of the top ten matches being normal SNe Ia (Pierel et al., 2024). A dedicated spectroscopic study then analyzed two JWST/NIRSpec IFU spectra of image A, obtained 39 days apart in the observer frame, and inferred rest-frame ages of $-$1 and $-$2 days post maximum using SNID template matching (Dhawan et al., 2024).

The same spectroscopic analysis compared the spectra to composite low-$-$3 SN Ia spectra and found strong evidence for similarity between SN Encore and the local sample. The line velocities of Si II $-$4 and the Ca II near-infrared triplet were consistent with the low-$-$5 distribution and with other lensed SNe Ia such as iPTF16geu and SN H0pe (Dhawan et al., 2024). No significant deviations or evolutionary trends were detected in the late-phase rest-frame optical region covered by the observations. In this sense, SN Encore is not only a cosmographic source but also a high-redshift test of SN Ia spectral stability.

The light-curve characterization used SALT-family modeling and later BayesSN-based inference. The discovery paper reported a SALT3 stretch of $-$6, placing Encore in the fast-declining but cosmology-usable SN Ia regime (Pierel et al., 2024). Later time-delay work used BayesSN spectral energy distribution modeling, with the rest-frame near-infrared secondary maximum playing an important role in breaking shape–magnification degeneracies when redder JWST bands were included (Pierel et al., 15 Sep 2025). A plausible implication is that SN Encore’s combination of Type Ia standardizability, high signal-to-noise JWST coverage, and multiple imaging makes it unusually informative relative to previously known cluster-lensed transients.

3. Lensing configuration and image phenomenology

In the multiple-image catalog used for lens modeling, SN Encore is System 1. The reference Gravity.jl model identifies at least four physically relevant images: 1a and 1b are the observed JWST/HST images from late 2023 and early 2024, 1c is a fainter earlier image that was not reliably photometered in the current data, and 1d is a highly delayed future counter-image. A demagnified central image 1e is also possible but is too faint to be of practical use in the Gravity.jl analysis (Bazzanini et al., 23 Jun 2026).

The short-delay pair 1a and 1b is the most important observed configuration. The measured time delay is

$-$7

where the negative sign means that 1b arrives earlier than 1a (Bazzanini et al., 23 Jun 2026). Under the Gravity.jl reference model with fixed $-$8, the predicted value is $-$9 days, showing consistency between lens modeling and direct light-curve inference (Bazzanini et al., 23 Jun 2026).

The same model predicts $H_0$0 days and $H_0$1 days, placing image 1d in approximately May–September 2032 for $H_0$2 (Bazzanini et al., 23 Jun 2026). Representative magnifications are $H_0$3, $H_0$4, $H_0$5, and $H_0$6; the negative sign indicates odd parity for 1a and 1d (Bazzanini et al., 23 Jun 2026). Independent model comparisons gave similar long-delay predictions, typically $H_0$7–$H_0$8 days for the best-fitting models (Suyu et al., 15 Sep 2025).

A common misconception is that the current cosmographic leverage comes mainly from the long future reappearance. In fact, the presently measured short delay between 1b and 1a already supplies most of the statistical weight in current $H_0$9 inferences, while the long-delay image 1d is primarily a future opportunity for a much sharper measurement (Bazzanini et al., 23 Jun 2026). The physical reason the 1d delay is so long is the deep, extended potential of the cluster lens: cluster-scale caustic topology produces large Fermat-potential differences and therefore year-to-decade delays (Bazzanini et al., 23 Jun 2026).

4. Strong-lensing reconstruction and control of systematics

The MACS J0138.0$m_{\rm F150W,AB}\approx 24$02155 mass distribution has been modeled with several independent pipelines. A blind comparison assembled seven independent mass models using six software packages—glafic, GLEE, Lenstool I, Lenstool II, Zitrin-analytic, 0.8 (MrMARTIAN hybrid), and WSLAP+—all based on a common high-quality dataset (Suyu et al., 15 Sep 2025). Restricting to the “gold” sample of 23 spectroscopically confirmed multiple images from 8 sources spanning $m_{\rm F150W,AB}\approx 24$1, the well-fitting models gave mutually consistent predictions for the positions, magnifications, and time delays of Encore and Requiem, especially for models with $m_{\rm F150W,AB}\approx 24$2 (Suyu et al., 15 Sep 2025).

The observational basis for these reconstructions was substantially sharpened by VLT/MUSE spectroscopy. A spectroscopic catalog of 107 reliable redshifts included 50 spectroscopically confirmed cluster members within $m_{\rm F150W,AB}\approx 24$3 and 13 spectroscopically confirmed multiple images from four background systems, while stellar kinematics for 14 bright cluster members were used to calibrate the Faber–Jackson relation with slope $m_{\rm F150W,AB}\approx 24$4 (Granata et al., 2024). Later lens-model papers extended the member catalog to 84 galaxies and incorporated two line-of-sight perturbers, a foreground galaxy at $m_{\rm F150W,AB}\approx 24$5 and a background galaxy at $m_{\rm F150W,AB}\approx 24$6 (Ertl et al., 12 Mar 2025, Acebron et al., 12 Mar 2025).

The 2026 Gravity.jl analysis implemented a Bayesian parametric reconstruction of the total projected mass using one cluster-scale dark-matter halo modeled as a Non-singular Isothermal Ellipsoid, galaxy-scale subhalos for 81 cluster members as spherical dPIE profiles scaled by luminosity via a Faber–Jackson-like relation, three jellyfish galaxies modeled individually, two spectroscopic line-of-sight galaxies in a multi-plane framework, and an external shear term

$m_{\rm F150W,AB}\approx 24$7

Its fixed-cosmology reference mass model reproduced the image positions with an image-plane rms residual of $m_{\rm F150W,AB}\approx 24$8 and a reduced chi-square of about $m_{\rm F150W,AB}\approx 24$9 (Bazzanini et al., 23 Jun 2026).

Across model families, several systematic themes recur. Cored-isothermal cluster profiles fit the data well, whereas a model with a Navarro–Frenk–White primary cluster halo had an image-position $z_s = 1.949$0 value four times higher (Ertl et al., 12 Mar 2025). Inclusion of the foreground perturber FG is crucial: excluding it biases magnifications and worsens fit quality (Acebron et al., 12 Mar 2025). The remaining key systematics are the mass-sheet degeneracy, radial-profile freedom, substructure, and external convergence from line-of-sight structure; future mitigation strategies identified in the literature include more spectroscopy, explicit line-of-sight modeling, incorporation of extended surface brightness directly in the likelihood, and stellar kinematics of MRG-M0138 (Granata et al., 2024, Bazzanini et al., 23 Jun 2026).

5. Time-delay cosmography and the measurement of $z_s = 1.949$1

The cosmographic formalism is the standard time-delay relation for strong lensing: $z_s = 1.949$2 with

$z_s = 1.949$3

For a fixed lens model in flat $z_s = 1.949$4CDM with fixed $z_s = 1.949$5 and $z_s = 1.949$6, the time-delay distance satisfies $z_s = 1.949$7, so measured delays directly constrain $z_s = 1.949$8 once the Fermat-potential differences are determined (Bazzanini et al., 23 Jun 2026).

The direct time-delay measurement for Encore was obtained from Type Ia light-curve modeling using BayesSN, with two independent fitting frameworks and explicit inclusion of microlensing systematics. The final value,

$z_s = 1.949$9

was then combined with the ensemble of lens models to produce a blind cosmographic inference (Pierel et al., 15 Sep 2025). Using seven independent mass models weighted by the likelihood of the observed image positions, the 2025 blind comparison reported

$-$0

with the uncertainty dominated by the time-delay measurement rather than model spread (Suyu et al., 15 Sep 2025).

The later Gravity.jl analysis sampled $-$1 jointly with the lens parameters and also incorporated the measured Requiem delays. It found $-$2 using Encore only, and

$-$3

when Encore and Requiem were used jointly (Bazzanini et al., 23 Jun 2026). This is fully consistent with the blind-model combination result and places SN Encore among the small set of lensed supernovae that already yield an explicit $-$4 measurement.

The dominant limitation at present is not the cluster model itself but the delay precision. Current errors are driven by the $-$5 relative uncertainty in the measured SN time delays, especially Encore’s $-$6, which carries most of the constraining power (Bazzanini et al., 23 Jun 2026). This is an important correction to the common assumption that cluster-lens cosmography is already systematics-limited in this system; the present regime is still largely statistical, although model systematics must be controlled to exploit future long-baseline delays.

6. Future reappearances and scientific role

The most immediate future event in the system is not an Encore image but the delayed image 2d of SN Requiem, predicted at roughly February–July 2027 with $-$7 days and $-$8 in the Gravity.jl model (Bazzanini et al., 23 Jun 2026). Even so, SN Encore remains central, because its short observed delay already anchors the time-delay distance, and its long-delay image 1d will add an independent long-baseline measurement several years later.

For Encore itself, the delayed image 1d is predicted to appear in roughly May–September 2032 for $-$9, with modest magnification $z_d = 0.336$0 (Bazzanini et al., 23 Jun 2026). Earlier multi-model work similarly placed the next Encore appearance at $z_d = 0.336$1 days, with the reappearance date shifting in the expected way under $z_d = 0.336$2 scaling (Suyu et al., 15 Sep 2025). The future central image 1e remains model-dependent and observationally difficult: it is predicted in about half of the posterior samples in the Gravity.jl analysis, in 35% of chain samples in one multi-plane model combination, and in only $z_d = 0.336$3 of MCMC realizations in another parametric study (Bazzanini et al., 23 Jun 2026, Ertl et al., 12 Mar 2025, Acebron et al., 12 Mar 2025).

The broader significance of SN Encore lies in the conjunction of several features rarely available in a single system: a spectroscopically confirmed Type Ia supernova at very high redshift, multiple observed images, an unusually rich set of spectroscopic strong-lensing constraints, and a sibling multiply imaged SN in the same host galaxy. The current analyses therefore describe MACS J0138.0$z_d = 0.336$42155 as a premier anchor for cluster-scale time-delay cosmography and present a near-term path to $z_d = 0.336$5–$z_d = 0.336$6 statistical precision on $z_d = 0.336$7 provided that extended surface-brightness modeling, additional spectroscopy, stellar kinematics, and line-of-sight systematics are brought under comparable control (Bazzanini et al., 23 Jun 2026). In that sense, SN Encore functions both as a measured cosmographic source and as the short-delay anchor of a longer program whose decisive leverage will come from the forthcoming decade-scale reappearances.