We present space-based follow-up observations of the superluminous Type I supernova (SLSN-I) SN 2025wny at redshift z_SN=2.0151, gravitationally lensed by two galaxies at redshifts z_G1=0.3755 and z_G2=0.3766 into five resolved images. SN 2025wny is the first strongly lensed SLSN discovered and the first galaxy-scale lensed supernova for which both photometric and spectroscopic time-delay measurements are feasible. As such, it opens a new observational window for precision cosmology and the study of stellar explosions near the epoch of peak cosmic star formation. Our follow-up observations comprise two epochs of Hubble Space Telescope (HST) imaging, together with near-infrared imaging and spectroscopy obtained with the James Webb Space Telescope (JWST). From these data, we measure precise astrometry and multi-band photometry for the five resolved supernova images, the host galaxy, and the two deflecting galaxies. HST provides accurate relative image positions and rest-frame ultraviolet photometry, while JWST delivers complementary near-infrared imaging and spectroscopy probing the rest-frame optical at high signal-to-noise ratio. Together they yield a detailed characterization of both the lensing configuration and the supernova spectral energy distribution over a broad wavelength range. The data presented here provide the observational foundation for the accompanying analyses of the supernova properties, lens modeling, and time-delay cosmography, including the astrometric, photometric, and spectroscopic information required to measure H_0.

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 θ_ E,1≃ 1.6" and θ_ E,2≃ 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 μ_ tot∼ 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 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 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.

Understanding the impact of the astrophysical environment on Type Ia supernova (SN Ia) properties is crucial to minimize systematic uncertainties in cosmological analyses based on this probe. We investigate the dependence of the SN Ia SALT2.4 light-curve stretch on the distance from their nearest galaxy cluster to study a potential effect of the intracluster medium (ICM) environment on SN Ia intrinsic properties. We use the largest SN Ia sample to date and cross-match it with existing X-ray, Sunyaev-Zel'dovich, and optical cluster catalogs in order to study the dependence between stretch and distance to the nearest detected cluster from each SN Ia. We model the underlying stretch distribution with a Gaussian mixture with relative amplitudes that depend on redshift and cluster-centric distance. We find a significant improvement of the fit quality of the stretch distribution if we include the distance-dependant term in the model with a variation of the Akaike information criterion $\rm{\Delta AIC} = -10.2$. Because of the known correlation between galaxy age and distance from cluster center, this supports previous evidence that the age of the stellar population is the underlying driver of the bimodial shape of the SN Ia stretch distribution. We further compute the evolution of the fraction of quenched galaxies as a function of distance with respect to cluster center from our best-fit model of the SNe Ia stretch distribution and compare it to previous results obtained from $H\alpha$ line measurements, optical broadband photometry, and simulations. We find our estimate to be compatible with these results. The results of this work indicate that SNe Ia searches at high redshift targeted towards clusters to maximize detection probability should be considered with caution as the stretch distribution of the detected sample would be strongly biased towards the old sub-population of SNe Ia.

More than 3000 spectroscopically confirmed Type Ia supernovae (SNe Ia) are presented in the second data release (DR2) of the Zwicky Transient Facility survey. In this paper we detail the spectral properties of 482 SNe Ia near maximum light, up to a redshift limit of z <= 0.06. We measured the velocities and pseudo-equivalent widths (pEW) of key spectral features (Si II lambda 5972 and Si II lambda 6355) and investigated the relation between the properties of the spectral features and the photometric properties from the SALT2 light-curve parameters as a function of spectroscopic sub-class. We discuss the non-negligible impact of host galaxy contamination on SN Ia spectral classifications, and we investigate the accuracy of spectral template matching of the DR2 sample. We define a new subclass of underluminous SNe Ia (04gs-like) that lie spectroscopically between normal SNe Ia and transitional 86G-like SNe Ia (stronger Si II lambda 5972 than normal SNe Ia, but significantly weaker Ti II features than 86G-like SNe). We model these 04gs-like SN Ia spectra using the radiative-transfer spectral synthesis code TARDIS and show that cooler temperatures alone are unable to explain their spectra; some changes in elemental abundances are also required. However, the broad continuity in spectral properties seen from bright (91T-like) to faint normal SN Ia, including the transitional and 91bg-like SNe Ia, suggests that variations within a single explosion model may be able to explain their behaviour.

Context. Gravitationally lensed type Ia supernovae (glSNe Ia) are unique astronomical tools that can be used to study cosmological parameters, distributions of dark matter, the astrophysics of the supernovae, and the intervening lensing galaxies themselves. A small number of highly magnified glSNe Ia have been discovered by ground-based telescopes such as the Zwicky Transient Facility (ZTF), but simulations predict that a fainter, undetected population may also exist. Aims. We present a systematic search for glSNe Ia in the ZTF archive of alerts distributed from June 1 2019 to September 1 2022. Methods. Using the AMPEL platform, we developed a pipeline that distinguishes candidate glSNe Ia from other variable sources. Initial cuts were applied to the ZTF alert photometry (with constraints on the peak absolute magnitude and the distance to a catalogue-matched galaxy, as examples) before forced photometry was obtained for the remaining candidates. Additional cuts were applied to refine the candidates based on their light curve colours, lens galaxy colours, and the resulting parameters from fits to the SALT2 SN Ia template. The candidates were also cross-matched with the DESI spectroscopic catalogue. Results. Seven transients were identified that passed all the cuts and had an associated galaxy DESI redshift, which we present as glSN Ia candidates. Although superluminous supernovae (SLSNe) cannot be fully rejected as contaminants, two events, ZTF19abpjicm and ZTF22aahmovu, are significantly different from typical SLSNe and their light curves can be modelled as two-image glSN Ia systems. From this two-image modelling, we estimate time delays of 22 +/- 3 and 34 +/- 1 days for the two events, respectively, which suggests that we have uncovered a population of glSNe Ia with longer time delays. Conclusions. The pipeline is efficient and sensitive enough to parse full alert streams. It is currently being applied to the live ZTF alert stream to identify and follow-up future candidates while active. This pipeline could be the foundation for glSNe Ia searches in future surveys, such as the Rubin Observatory Legacy Survey of Space and Time.

The second data release of Type Ia supernovae (SNe Ia) observed by the Zwicky Transient Facility has provided a homogeneous sample of 3628 SNe Ia with photometric and spectral information. This unprecedented sample size enables us to better explore our currently tentative understanding of the dependence of the host environment on SN Ia properties. In this paper, we make use of two-dimensional image decomposition to model the host galaxies of SNe Ia. We model elliptical galaxies as well as disc and spiral galaxies with or without central bulges and bars. This allows for the categorisation of SN Ia based on their morphological host environment, as well as the extraction of intrinsic galaxy properties corrected for both cosmological and atmospheric effects, through point-spread-function (PSF) convolution. We find that although this image decomposition technique leads to a significant bias towards elliptical galaxies in our final sample of processed galaxies, the overall results are still robust. By successfully modelling 728 host galaxies, we find that the photometric properties of SNe Ia found in discs and in elliptical galaxies correlate fundamentally differently with their host environment. We identified strong linear relations between light-curve stretch and our model-derived galaxy colour for both the elliptical (16.8 sigma) and disc (5.1 sigma) subpopulations of SNe Ia. Lower-stretch SNe Ia are found in redder environments, which we identify as an age and/or metallicity effect. Within the subpopulation of SNe Ia found in disc-containing galaxies, we find a significant linear trend (6.1 sigma) between light-curve stretch and model-derived local r-band surface brightness, which we link to the age and metallicity gradients found in disc galaxies. SN Ia colour shows little correlation with the host environment, as is seen in the literature. We do identify a possible dust effect in our model-derived surface brightness (3.3 sigma) for SNe Ia in disc galaxies.

Type Ia supernovae (SNe Ia) constitute a historical probe for deriving cosmological parameters through the fit of the Hubble-Lema & icirc;tre diagram, that is, the SN Ia distance modulus versus their redshift. In the era of precision cosmology, realistic simulation of SNe Ia for any survey entering an Hubble-Lema & icirc;tre diagram is a key tool for addressing observational systematics, such as the Malmquist bias. As the distance modulus of SNe Ia is derived from the fit of their light curves, a robust simulation framework is required. In this paper, we present the performances of the simulation framework skysurvey with the aim to reproduce the Zwicky Transient Facility (ZTF) SN Ia DR2, which covers the first phase of the ZTF and ran from March 2018 to December 2020. The ZTF SN Ia DR2 sample corresponds to almost 3000 classified SNe Ia of cosmological quality. We simulated individual light curves of the ZTF SN Ia DR2 sample to confirm the validity of the framework while taking the observing conditions and instrument performances into account. After the ZTF SN Ia DR2 selection criteria were applied, we found that the simulated fluxes and associated uncertainties agre well with the measured uncertainties when the sky-noise deduced from the observed science magnitude limits is corrected for by a factor 1.23 for the g band, 1.17 for the r band, and 1.20 for the i band. In addition, we accounted for an error floor of 2.5%, 3.5%, and 6% of the flux level in the g, r, and i bands, respectively. Furthermore a redshift dependence of the SALT2 light-curve parameters (stretch and colour) was conducted to deduce the redshift limit that defines a volume-limited sample, that is, an unbiased SNe Ia sample. We found that the ZTF SN Ia DR2 volume-limited sample is characterized by z <= 0.06. This volume-limited sample of about 1000 SNe Ia is unique, and an astrophysical analysis can be carried out based on it, or the standardisation procedure can be tested with unprecedented precision (these analyses are presented in companion papers).

The Zwicky Transient Facility SN Ia Data Release 2 (ZTF SN Ia DR2) contains more than 3000 Type Ia supernovae (SNe Ia), providing the largest homogeneous low-redshift sample of SNe Ia. Having at least one spectrum per event, this data collection is ideal for large-scale statistical studies of the photometric, spectroscopic and host-galaxy properties of SNe Ia, particularly of the rarer ‘peculiar’ sub-classes. In this paper we first present the method we developed to spectroscopically classify the SNe in the sample, and the techniques we used to model their multi-band light curves and explore their photometric properties. We then show a method to distinguish between the peculiar sub-types and the normal SNe Ia. We also explore the properties of their host galaxies and estimate their relative rates, focusing on the peculiar sub-types and their connection to the cosmologically useful SNe Ia. Finally, we discuss the implications of our study with respect to the progenitor systems of the peculiar SN Ia events.

This competition in high-energy physics (HEP) and machine learning was the first to strongly emphasise uncertainties in (H → τ^+ τ^-) cross-section measurement. Participants were tasked with developing advanced analysis techniques capable of dealing with uncertainties in the input training data and providing credible confidence intervals. The accuracy of these intervals was evaluated using pseudo-experiments to assess correct coverage. The dataset is now published in Zenodo, and the winning submissions are fully documented.

Astrophysical observations of the cosmos allow us to probe extreme physics and answer foundational questions on our universe. Modern astronomy is increasingly operating under a holistic approach, probing the same question with multiple diagnostics including how sources vary over time, how they appear across the electromagnetic spectrum, and through their other signatures, including gravitational waves, neutrinos, cosmic rays, and dust on Earth. Astrophysical observations are now reaching the point where approximate physics models are insufficient. Key sources of interest are explosive transients, whose understanding requires multidisciplinary studies at the intersection of astrophysics, gravity, nuclear science, plasma physics, fluid dynamics and turbulence, computation, particle physics, atomic, molecular, and optical science, condensed matter and materials science, radiation transport, and high energy density physics. This white paper provides an overview of the major scientific advances that lay at the intersection of physics and astronomy and are best probed through time-domain and multimessenger astrophysics, an exploration of how multidisciplinary science can be fostered, and introductory descriptions of the relevant scientific disciplines and key astrophysical sources of interest.

The HiggsML Uncertainty Challenge is a machine learning competition aimed at improving uncertainty-aware AI techniques in high-energy physics. Part of the FAIR Universe initiative, focuses on estimating the Higgs boson signal strength while accounting for systematic uncertainties affecting collider experiments. Unlike traditional classification tasks, participants must construct confidence intervals that properly cover systematic distortions. The HiggsML Uncertainty Challenge establishes a benchmark for uncertainty-aware AI, with applications in high-energy physics and beyond. The competition is hosted on Codabench, an open AI benchmarking platform, and uses highperformance computing resources at NERSC Perlmutter for scalable and reproducible model evaluation. The dataset and evaluation framework will remain publicly available for continued research.

Understanding Type Ia supernovae (SNe Ia) and the empirical standardisation relations that make them excellent distance indicators is vital to improving cosmological constraints. SN Ia 'siblings, i.e. two or more SNe Ia in the same host or parent galaxy, offer a unique way to infer the standardisation relations and their scatter across the population. We analysed a sample of 25 SN Ia pairs observed homogeneously by the Zwicky Transient Facility (ZTF) to infer the SNe Ia light curve width-luminosity and colour-luminosity parameters, alpha and beta. Using the pairwise constraints from siblings, which allow for a scatter in the standardisation relations, we found alpha = 0.218 +/- 0.055 and beta = 3.084 +/- 0.312, respectively, with a dispersion in alpha and beta of <= 0.195 and <= 0.923, respectively, at a 95% confidence level. While the median dispersion is large, the values within similar to 1 sigma are consistent with no dispersion. Hence, fitting for a single global standardisation relation, we found alpha = 0.228 +/- 0.029 and beta = 3.160 +/- 0.191. We also found a very small intrinsic scatter of the siblings sample sigma(int) <= 0.10 mag at a 95% confidence level compared to sigma(int) = 0.22 +/- 0.04 mag when computing the scatter using the Hubble residuals without comparing them as siblings. When comparing to large samples used in cosmological measurements, we found an alpha that is similar to 2-3 sigma higher, while the beta values are consistent. The high alpha is driven by low x(1) pairs, potentially suggesting that the slow and fast declining SN Ia have different slopes for the width-luminosity relation. We found no difference in alpha and beta when dividing the sample by host galaxy mass. The finding of a higher alpha with increased statistics can be confirmed or refuted through upcoming time-domain surveys. If confirmed, this finding can improve the cosmological inference from SNe Ia and be used to infer properties of the progenitors for subpopulations of SNe Ia.

A star that was stripped of its outer layers shortly before it exploded as a supernova offers astronomers an unprecedented glimpse of a stellar interior. A star that was stripped of its outer layers shortly before it exploded as a supernova offers astronomers an unprecedented glimpse of a stellar interior.

Type Ia supernova (SN Ia) light curves have a secondary maximum that exists in the r, i, and near-infrared filters. The secondary maximum is relatively weak in the r band, but holds the advantage that it is accessible, even at high redshift. We used Gaussian process fitting to parameterise the light curves of 893 SNe Ia from the Zwicky Transient Facility's (ZTF) second data release (DR2), and we were able to extract information about the timing and strength of the secondary maximum. We found > 5 sigma correlations between the light curve dec rate (Delta m(15)(g)) and the timing and strength of the secondary maximum in the r band. Whilst the timing of the secondary maximum in the i band is also correlated with Delta m(15)(g), the strength of the secondary maximum in the i band shows significant scatter as a function of Delta m(15)(g). We found that the transparency timescales of 97 per cent of our sample are consistent with double detonation models and that SNe Ia with small transparency timescales (< 32 d) reside predominantly in locally red environments. We measured the total ejected mass for the normal SNe Ia in our sample using two methods and both were consistent with medians of 1.3 +/- 0.3 and 1.2 +/- 0.2 M-circle dot. We find that the strength of the secondary maximum is a better standardisation parameter than the SALT light curve stretch (x(1)). Finally, we identified a spectral feature in the r band as Fe II, which strengthens during the onset of the secondary maximum. The same feature begins to strengthen at < 3 d post maximum light in 91bg-like SNe. Finally, the correlation between x1 and the strength of the secondary maximum was best fit with a broken, with a split at x(1)(0) = - 0.5 +/- 0.2, suggestive of the existence of two populations of SNe Ia.

The current standard model of cosmology successfully describes a variety of measurements, but the nature of its main ingredients, dark matter and dark energy, remains unknown. Euclid is a medium-class mission in the Cosmic Vision 2015-2025 programme of the European Space Agency (ESA) that will provide high-resolution optical imaging, as well as near-infrared imaging and spectroscopy, over about 14,000 deg^2 of extragalactic sky. In addition to accurate weak lensing and clustering measurements that probe structure formation over half of the age of the Universe, its primary probes for cosmology, these exquisite data will enable a wide range of science. This paper provides a high-level overview of the mission, summarising the survey characteristics, the various data-processing steps, and data products. We also highlight the main science objectives and expected performance.

SNe Ia are used to determine the distance-redshift relation and build the Hubble diagram. Neglecting their host-galaxy peculiar velocities (PVs) may bias the measurement of cosmological parameters. The smaller the redshift, the larger the effect is. We use realistic simulations of SNe Ia observed by the Zwicky Transient Facility (ZTF) to investigate the effect of different methods to take into account PVs. We study the impact of neglecting galaxy PVs and their correlations in an analysis of the SNe Ia Hubble diagram. We find that it is necessary to use the PV full covariance matrix computed from the velocity power spectrum to take into account the sample variance. Considering the results we have obtained using simulations, we determine the PV systematic effects in the context of the ZTF DR2 SNe Ia sample. We determine the PV impact on the intercept of the Hubble diagram, a_B, which is directly linked to the measurement of H_0. We show that not taking into account PVs and their correlations results in a shift of the H_0 value of about 1.0km.s^-1.Mpc^-1 and a slight underestimation of the H_0 error bar.

As Type Ia supernova cosmology transitions from a statistics dominated to a systematics dominated era, it is crucial to understand leftover unexplained uncertainties affecting their luminosity, such as the ones stemming from astrophysical biases. Indeed, SNe Ia are standardisable candles, whose absolute magnitude reach a 0.15~mag scatter once empirical correlations with their lightcurve stretch and colour and with their environment are accounted for. In this paper, we investigate how the standardisation process of SNe Ia depends on environment, to ultimately reduce their scatter in magnitude, focusing on colour standardisation. We use the volume-limited ZTF SN Ia DR2 sample, which offers unprecedented statistics for the low redshift ($z<0.06$) range. We first study the colour distribution, focusing on the effects of dust, to then select a dustless subsample of objects from low stellar mass environments and from the outskirts of their host galaxies. We then look at the colour-residuals relation and its associated parameter $\beta$. Finally, we investigate the colour dependency of the environment-dependent magnitude offsets (steps), to try to disentangle intrinsic and extrinsic colour origin. Our sample probes well the red tail of the colour distribution, up to $c=0.8$. The dustless sample exhibits a significantly lower red tail ($4.6\sigma$) in comparison to the whole sample. This suggests that reddening above $c\geq0.2$ is dominated by host interstellar dust absorption. Looking at the colour-residuals relation, we find it to be linear with lightcurve colour. We show hints of a potential evolution of $\beta$ with host stellar mass at a $2.5\sigma$ level. Finally, unlike recent claims from the literature, we see no evolution of steps as a function of lightcurve colour, suggesting that dust may not be the dominating mechanism responsible for the environmental dependency of SNe Ia magnitude.

Type Ia supernovae (SNe Ia) are a key probe in modern cosmology, as they can be used to measure luminosity distances at gigaparsec scales. Models of their light-curves are used to project heterogeneous observed data onto a common basis for analysis. The SALT model currently used for SN Ia cosmology describes SNe as having two sources of variability, accounted for by a color parameter c, and a "stretch parameter" x1. We extend the model to include an additional parameter we label x2, to investigate the cosmological impact of currently unaddressed light-curve variability. We construct a new SALT model, which we dub "SALT3+". This model was trained by an improved version of the SALTshaker code, using training data combining a selection of the second data release of cosmological SNe Ia from the Zwicky Transient Facility and the existing SALT3 training compilation. We find additional, coherent variability in supernova light-curves beyond SALT3. Most of this variation can be described as phase-dependent variation in g-r and r-i color curves, correlated with a boost in the height of the secondary maximum in i-band. These behaviors correlate with spectral differences, particularly in line velocity. We find that fits with the existing SALT3 model tend to address this excess variation with the color parameter, leading to less informative measurements of supernova color. We find that neglecting the new parameter in light-curve fits leads to a trend in Hubble residuals with x2 of 0.039 +/- 0.005 mag, representing a potential systematic uncertainty. However, we find no evidence of a bias in current cosmological measurements. We conclude that extended SN Ia light-curve models promise mild improvement in the accuracy of color measurements, and corresponding cosmological precision. However, models with more parameters are unlikely to substantially affect current cosmological results.

In this study, we explore the characteristics of 'low-mass' (log(M star/M circle dot) <= 8) and 'intermediate-mass' (8 < log(M star/M circle dot) <= 10) host galaxies of Type Ia supernovae (SNe Ia) from the second data release (DR2) of the Zwicky Transient Facility survey. We investigated their correlations with different sub-types of SNe Ia. We used the photospheric velocities measured from the Si II lambda 6355 feature, SALT2 light-curve stretch (x1), and host-galaxy properties of SNe Ia to re-investigate the existing relationship between host galaxy mass and Si II lambda 6355 velocities. We also investigated sub-type preferences for host populations. We show that the more energetic and brighter 91T-like SNe Ia tend to reside among the younger host populations, while 91bg-like SNe Ia are found among the older populations. Our findings suggest that high-velocity SNe Ia (HV SNe Ia) do indeed come from older populations, but they can also come from young populations as well. Therefore, while our findings can partly provide support for HV SNe Ia in the context of single degenerate progenitor models, they indicate that HV SNe Ia (rather than comprising a different population) might be a continued distribution with different explosion mechanisms. Lastly, we investigate the specific rate of SNe Ia in the volume-limited SN Ia sample of DR2 and compare our results with other surveys.