Follow-up of SN 2025wny V: Lens Modelling and Cosmography of a Strongly Lensed Superluminous Supernova at $z = 2.015$ using Space Data

Alice Townsend; Ariel Goobar; Cameron Lemon; Christoffer Fremling; Edvard M\"ortsell; Graham P. Smith; Hannah C.Turner; Jacob Osman Hjortlund; Jakob Nordin; Joel Johansson

REVIEW

Follow-up of SN 2025wny V: Lens Modelling and Cosmography of a Strongly Lensed Superluminous Supernova at $z = 2.015$ using Space Data

Edvard Mörtsell, Joel Johansson, Ariel Goobar, Alice Townsend, Hannah C.Turner, Suhail Dhawan, Cameron Lemon, Peter Nugent

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Thomas E. Collett, Stephen Thorp, Jacob Osman Hjortlund, Jakob Nordin, Lin Yan, Graham P. Smith, Christoffer Fremling

Cosmology and Nongalactic Astrophysics · Submitted 28 Aug 2026

Pith's verdict: SN 2025wny is a genuinely new lensed supernova at z=2 that yields H₀ from time delays, but the abstract’s silence on κ_ext makes the central numbers less robust than they appear.

T0 · AI review

How this review was made: An AI referee (deepseek/deepseek-v4-flash) read the paper on September 9, 2026 and answered a fixed list of questions about it. T0 is the starting level every paper gets. The levels describe how deeply Pith has checked the paper, never the authors.

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T3 Formal statement The claims written out in a formal math language a computer can read.
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arXiv ID2608.28430 (version 1)

Pith IDOP6T2ND3, Pith's permanent record of this paper and its review

CitationAlice Townsend, Ariel Goobar, Cameron Lemon, Christoffer Fremling, Edvard M\"ortsell, Graham P. Smith, Hannah C.Turner, Jacob Osman Hjortlund, Jakob Nordin, Joel Johansson, Lin Yan, Peter Nugent, Stephen Thorp, Suhail Dhawan, Thomas E. Collett. "Follow-up of SN 2025wny V: Lens Modelling and Cosmography of a Strongly Lensed Superluminous Supernova at $z = 2.015$ using Space Data." arXiv:2608.28430 (2026). https://pith.science/paper/OP6T2ND3

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Claim paper

Why Pith says this

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Editor's note

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Thanks for sending this over. Here’s my take based on the abstract alone—no full text to check, so take this with that grain of salt.

The new thing is real: a lensed Type I SLSN at z=2.015, imaged by two foreground galaxies, with HST and JWST data, time delays, and a concrete H₀ estimate. That’s a genuinely new object, and the abstract’s clarity about conditionality—mass-profile parameterization, slope priors, environment—is a good sign. They’re not overselling. The reported magnifications and Einstein radii are internally consistent across filters, and they explicitly fold in microlensing corrections. That’s careful.

The soft spot is the one the stress-test flagged: κ_ext. The abstract says the result is conditional on “possible additional lensing contributions from the surrounding large-scale environment,” but gives no bound, estimate, or even a placeholder for κ_ext. In time-delay lensing that’s not a minor detail—κ_ext directly rescales H₀ by 1/(1–κ_ext), and typical values of 0.05–0.15 shift H₀ by 5–15 km/s/Mpc, comparable to the quoted statistical errors. Without any treatment, the central H₀ values are less informative than they appear. This may be addressed in the full paper, but the abstract should at least flag that the reported error bars do not include this term.

The other limitation is the reliance on two power-law profiles plus shear. That’s standard, but the abstract itself notes that moving from fixed isothermal to free slopes shifts H₀ by ~4 km/s/Mpc. That’s a reasonable sensitivity exercise, but it doesn’t cover the full systematic space (e.g., substructure, more complex mass distributions). Again, the full paper might do this.

Bottom line: this is a legitimate, potentially important new measurement from a well-motivated target. The authors are upfront about many assumptions. But the missing κ_ext treatment is a real weakness that prevents the quoted H₀ from being taken at face value. If I were editing, I’d send it to a referee who can push on that specific point. For my own work, I wouldn’t cite it until I see how κ_ext is handled.

Recommendation: engage with it, but ask the referee to press on the environment term. The paper is worth reviewing.

Referee report

Significance

If the reported H0 constraint survives a full systematic-error treatment, it would provide an independent, high-redshift (z = 2.015) H0 measurement that does not use the local distance ladder, in a redshift regime where only a few time-delay lenses exist. The abstract shows several positive features: filter-consistent lens models, explicit inclusion of microlensing corrections when comparing flux ratios, a stated conditional interpretation of the results, and an acknowledgment that current kinematic data have only a modest effect. These choices indicate care in presenting the analysis. The principal limitations are the lack of a quantitative external-convergence (kappa_ext) systematic budget and the absence of a full text to verify the lens-modeling details, time-delay provenance, and data-reduction choices.

Major objections (3)

[Abstract, H0 results] The central H0 values (66.7^{+7.6}_{-6.3} and 70.8^{+8.2}_{-6.1} km/s/Mpc) are quoted as filter-marginalized constraints but include no estimate or bound on the external convergence kappa_ext for the two lens lines of sight. In time-delay lensing H0 scales as 1/(1 - kappa_ext), and typical line-of-sight contributions from galaxy counts are of order 0.05-0.15, which would shift H0 by approximately 5-15 km/s/Mpc, comparable to or exceeding the quoted statistical uncertainties. The abstract's statement that the results are conditional on 'possible additional lensing contributions from the surrounding large-scale environment' is an important caveat but does not replace a quantitative systematic-error budget. Without such a budget, the claim of an independent H0 measurement is not yet fully supported.
[Abstract, time-delay inputs] The abstract states that the lens models are combined with spectroscopically and photometrically measured time delays, but it provides no details on where these time delays were measured, their uncertainties, or how those uncertainties propagate into the H0 constraint. Since any systematic error in the time delays enters directly into H0, the full manuscript must present the time-delay measurements and a clear treatment of their error budget. This comment is a request for completeness rather than an assertion of a flaw.
[Abstract, magnification and flux-ratio consistency] The abstract reports that the posterior spans total magnifications of approximately mu_tot ~ 5-50 and that flux ratios are consistent with observations after microlensing corrections. The wide range of total magnification raises the question of how informative the flux-ratio data are for the mass model; the full text should show the relevant likelihoods and explain which images are used, as this bears on the robustness of the derived H0. This point is secondary to the kappa_ext concern but directly affects how strongly the quoted result constrains the mass model.

How Pith checks papers → · Full audit of this paper →

The paper

The paper claims

SN 2025wny provides a new independent measurement of the Hubble constant at high redshift using strong gravitational lensing.

The reading

This paper models the strongly lensed superluminous supernova SN 2025wny using HST and JWST data to constrain the Hubble constant H0. By combining lens models with measured time delays, they obtain H0 = 66.$7^{{+7.6}}$_{-6.3} km/s/Mpc for isothermal mass profiles, or 70.$8^{{+8.2}}$_{-6.1} km/s/Mpc when density slopes are allowed to vary. This is one of the first H0 measurements from a superluminous supernova at redshift 2, independent of the local distance ladder. The result demonstrates the potential of such supernovae as cosmological probes at high redshift.

What carries the argument

The key machinery is lens modelling using two elliptical power-law mass distributions for the foreground galaxies plus an external shear term, combined with spectroscopically and photometrically measured time delays between the multiple images. The surface-brightness distribution of the lensed host galaxy also constrains the mass model.

Extended reading notes

Core claim

The central discovery is that SN 2025wny, multiply imaged by two foreground galaxies at z=0.376, yields Hubble constant constraints consistent with both Planck CMB and local distance ladder measurements, but with larger uncertainties. The inferred Einstein radii are approximately 1.6 and 0.7-0.8 arcseconds, and the total magnification after microlensing correction ranges from about 5 to 50. The filter-marginalized H0 values are 66.$7^{{+7.6}}$_{-6.3} km/s/Mpc for a fiducial isothermal model and 70.$8^{{+8.2}}$_{-6.1} km/s/Mpc when the density slopes of the lens galaxies are allowed to vary over a broad range.

Load-bearing premise

The load-bearing premise is that the lens mass distribution is correctly described by two elliptical power-law profiles plus an external shear, with no significant additional line-of-sight mass contributions.

Editorial extensions

If this is right

SN 2025wny provides an independent high-redshift H0 measurement that does not rely on the local distance ladder, offering a new anchor for cosmography.
Superluminous supernovae are shown to be viable sources for time-delay cosmography at z~2, opening a new population of standardizable sources.
Future kinematic measurements of the lens galaxies and characterization of the lens environment will tighten the H0 constraints.
The consistency with both Planck and local measurements suggests no strong tension in H0 at high redshift from this single object.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

If refined with better kinematics and environmental data, this method could help resolve the Hubble tension by providing a high-redshift anchor independent of the CMB and local distance ladder.
The large magnification (up to 50×) implies that similar superluminous supernovae could be detected at even higher redshifts with JWST, extending cosmography beyond z=2.
The 4 km/s/Mpc shift in H0 when freeing the density slope highlights how strongly the result depends on the adopted mass-model parameterization, motivating direct kinematic constraints.

Original abstract

We present a lensing and cosmographic analysis of the strongly lensed Type I superluminous supernova SN 2025wny at redshift $z=2.015$, multiply imaged by two foreground galaxies at $z=0.376$. Using imaging obtained with the Hubble Space Telescope and the James Webb Space Telescope, we model the lens system with two elliptical power-law mass distributions and an external shear component. In addition to the supernova image positions, the modelling incorporates the surface-brightness distribution of the lensed host galaxy. The inferred Einstein radii are $\theta_{\rm E,1}\simeq 1.6$" and $\theta_{\rm E,2}\simeq 0.7$ - $0.8$", with broadly consistent results across all filters. After accounting for microlensing by stars in the lens galaxies, the posterior distribution spans total magnifications of approximately $\mu_{\rm tot}\sim 5$--$50$, with flux ratios of the multiple images consistent with observations. Combining the lens models with spectroscopically and photomerically measured time delays yields a filter-marginalized constraint of \[ H_0 = 66.7^{+7.6}_{-6.3}\; {\rm km\,s^{-1}\,Mpc^{-1}}, \] for a fiducial model with isothermal mass profiles. Allowing the density slopes of the lens galaxies to vary over a broad range results in \[ H_0 = 70.8^{+8.2}_{-6.1}\; {\rm km\,s^{-1}\,Mpc^{-1}}. \] These values are conditional on the adopted parameterization of the lens mass distribution, the assumed priors on the density slopes, and possible additional lensing contributions from the surrounding large-scale environment. We find that incorporating the currently available stellar kinematic measurements has only a modest effect on the inferred value of $H_0$. Future measurements of the lens-galaxy kinematics and a detailed characterization of the lens environment will further strengthen the utility of SN 2025wny as a cosmological probe.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-09-23: 1 request, 8,125 input tokens.

What the abstract itself states, as probabilities: claims first 0.03 · states limitation 0.97 · human participants 0.01 · controlled comparison 0.04 · randomized assignment 0.01 · states quantitative result 0.99 · compares with prior results 0.24 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; show every role.

We present a lensing and cosmographic analysis of the strongly lensed Type I superluminous supernova SN 2025wny at redshift $z=2.015$, multiply imaged by two foreground galaxies at $z=0.376$. Using imaging obtained with the Hubble Space Telescope and the James Webb Space Telescope, we model the lens system with two elliptical power-law mass distributions and an external shear component. In addition to the supernova image positions, the modelling incorporates the surface-brightness distribution of the lensed host galaxy. The inferred Einstein radii are $\theta_{\rm E,1}\simeq 1.6$" and $\theta_{\rm E,2}\simeq 0.7$ - $0.8$", with broadly consistent results across all filters. After accounting for microlensing by stars in the lens galaxies, the posterior distribution spans total magnifications of approximately $\mu_{\rm tot}\sim 5$--$50$, with flux ratios of the multiple images consistent with observations. Combining the lens models with spectroscopically and photomerically measured time delays yields a filter-marginalized constraint of \[ H_0 = 66.7^{+7.6}_{-6.3}\; {\rm km\,s^{-1}\,Mpc^{-1}}, \] for a fiducial model with isothermal mass profiles. Allowing the density slopes of the lens galaxies to vary over a broad range results in \[ H_0 = 70.8^{+8.2}_{-6.1}\; {\rm km\,s^{-1}\,Mpc^{-1}}. \] These values are conditional on the adopted parameterization of the lens mass distribution, the assumed priors on the density slopes, and possible additional lensing contributions from the surrounding large-scale environment. We find that incorporating the currently available stellar kinematic measurements has only a modest effect on the inferred value of $H_0$. Future measurements of the lens-galaxy kinematics and a detailed characterization of the lens environment will further strengthen the utility of SN 2025wny as a cosmological probe.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#	sentence	role	the authors' own work	states the main conclusion	strength
s0	We present a lensing and cosmographic analysis of the strongly lensed Type I superluminous supernova SN 2025wny at redshift $z=2.015$, multiply imaged by two foreground galaxies at $z=0.376$.	contribution 1.00	0.86	0.08	asserted 1.00
s1	Using imaging obtained with the Hubble Space Telescope and the James Webb Space Telescope, we model the lens system with two elliptical power-law mass distributions and an external shear component.	method 1.00	0.96	0.02	asserted 1.00
s2	In addition to the supernova image positions, the modelling incorporates the surface-brightness distribution of the lensed host galaxy.	method 0.98	0.83	0.02	asserted 1.00
s3	The inferred Einstein radii are $\theta_{\rm E,1}\simeq 1.6$" and $\theta_{\rm E,2}\simeq 0.7$ - $0.8$", with broadly consistent results across all filters.	result 1.00	0.94	0.04	asserted 0.97
s4	After accounting for microlensing by stars in the lens galaxies, the posterior distribution spans total magnifications of approximately $\mu_{\rm tot}\sim 5$--$50$, with flux ratios of the multiple images consistent with observations.	result 0.99	0.94	0.05	asserted 0.83
s5	Combining the lens models with spectroscopically and photomerically measured time delays yields a filter-marginalized constraint of \[ H_0 = 66.7^{+7.6}_{-6.3}\; {\rm km\,s^{-1}\,Mpc^{-1}}, \] for a fiducial model with isothermal mass profiles.	result 1.00	0.95	0.50	asserted 1.00
s6	Allowing the density slopes of the lens galaxies to vary over a broad range results in \[ H_0 = 70.8^{+8.2}_{-6.1}\; {\rm km\,s^{-1}\,Mpc^{-1}}.	result 1.00	0.95	0.21	asserted 0.99
s7	\] These values are conditional on the adopted parameterization of the lens mass distribution, the assumed priors on the density slopes, and possible additional lensing contributions from the surrounding large-scale environment.	limitation 1.00	0.37	0.06	asserted 0.74
s8	We find that incorporating the currently available stellar kinematic measurements has only a modest effect on the inferred value of $H_0$.	result 0.93	0.95	0.07	asserted 1.00
s9	Future measurements of the lens-galaxy kinematics and a detailed characterization of the lens environment will further strengthen the utility of SN 2025wny as a cosmological probe.	future work 1.00	0.15	0.05	hedged possibility 0.72

What kind of paper the classifier reports this is: empirical study 1.00 · evidence from analysis of existing data 0.85.

What was examined, and what was not

sources read

arXiv listing abstract v1

not examined

full text
figures and tables
references and citing sentences
relations to other papers

levels run

P abstract

not run

S sections
T body sentences
C citations
R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

References

How the literature uses it

What this paper does with the arXiv papers it cites

Each citing sentence is taken from the citing paper's LaTeX source and typed by Pith's classifier. A label appears at probability 0.8 or more (0.95 for support), a contest at 0.7 or more; the quoted words are the citing authors'. How each label was checked · JSON

What it cites

Uses data or values from 1 paper

The transient is located in close projection to a luminous galaxy with an archival spectrum from the Dark Energy Spectroscopic Instrument (DESI), which places the galaxy at a redshift of $z=0.3754 ±0.0001$ [ref] .

Data Release 1 of the Dark Energy Spectroscopic Instrument · Discovery and spectroscopy · 0.98

Compares with 1 paper

These values are in excellent agreement with the earlier image-position-based modelling of [ref] and [ref] .

2602.16620 · New HST/JWST Lens Model · 1.00

Cites as background 1 paper

The first lens models of SN 2025wny were presented by [ref] , based on adaptive-optics imaging obtained with the Large Binocular Telescope.

2602.16620 · Previous Lens Modelling Results · 0.95

Discussion

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