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.

Strongly lensed supernovae (SNe) are a rare class of transient that can offer tight cosmological constraints that are complementary to methods from other astronomical events. We present a follow-up study of one recently discovered strongly lensed SN, the quadruply imaged type Ia SN 2022qmx (aka "SN Zwicky"), at z = 0.3544. We measure updated, template-subtracted photometry for SN Zwicky and derive improved time delays and magnifications. This is possible because SNe are transient, fading away after reaching their peak brightness. Specifically, we measure point-spread-function photometry for all four images of SN Zwicky in three Hubble Space Telescope WFC3/UVIS passbands (F475W, F625W, and F814W) and one WFC3/IR passband (F160W), with template images taken ∼11 months after the epoch in which the SN images appear. We find consistency to within 2σ between lens-model-predicted time delays (≲1 day) and measured time delays with HST colors (≲2 days), including the uncertainty from chromatic microlensing that may arise from stars in the lensing galaxy. The standardizable nature of SNe Ia allows us to estimate absolute magnifications for the four images, with images A and C being elevated in magnification compared to lens model predictions by about 6σ and 3σ, respectively, confirming previous work. We show that millilensing or differential dust extinction is unable to explain these discrepancies, and we find evidence for the existence of microlensing in images A, C, and potentially D that may contribute to the anomalous magnification.

The first James Webb Space Telescope (JWST) Near InfraRed Camera imaging in the field of the galaxy cluster PLCK G165.7+67.0 (z = 0.35) uncovered a Type Ia supernova (SN Ia) at z = 1.78, called “SN H0pe.” Three different images of this one SN were detected as a result of strong gravitational lensing, each one traversing a different path in spacetime, thereby inducing a relative delay in the arrival of each image. Follow-up JWST observations of all three SN images enabled photometric and rare spectroscopic measurements of the two relative time delays. Following strict blinding protocols which oversaw a live unblinding and regulated postunblinding changes, these two measured time delays were compared to the predictions of seven independently constructed cluster lens models to measure a value for the Hubble constant, H0 = 71.8 + 9.2 − 8.1 km s−1 Mpc−1. The range of admissible H0 values predicted across the lens models limits further precision, reflecting the well-known degeneracies between lens model constraints and time delays. It has long been theorized that a way forward is to leverage a standard candle, but this has not been realized until now. For the first time, the lens models are evaluated by their agreement with the SN absolute magnifications, breaking degeneracies and producing our best estimate, H0 = 75.7 − 5.5 + 8.1 km s−1 Mpc−1. This is the first precise measurement of H0 from a multiply imaged SN Ia and only the second from any multiply imaged SN.

For approximately a decade, we have finally entered the era of discoveries of multiply imaged gravitationally lensed supernovae. To date, all cluster-lensed supernovae, very distant, faint and spatially resolved, have been found from space. In contrast, those deflected by individual galaxies have been very compact and bright enough to be identified with wide-field ground-based surveys through the magnification of the ‘standard candles’ method, i.e. without the need to spatially resolve the individual images. We review the challenges in identifying these extremely rare events, as well as the unique opportunities they offer for two major applications: time-delay cosmography and the study of the properties of deflecting bodies acting as lenses.

This article is part of the Theo Murphy meeting issue ‘Multi-messenger gravitational lensing (Part 1)’.

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 (16.8ÏÂ) and disc (5.1ÏÂ) 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 (6.1ÏÂ) 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 (3.3ÏÂ) for SNe Ia in disc galaxies.

Context. Type Ia supernova (SN Ia) cosmology studies will soon be dominated by systematic, uncertainties, rather than statistical ones. Thus, it is crucial to understand the unknown phenomena potentially affecting their luminosity that may remain, such as astrophysical biases. For their accurate application in such studies, SN Ia magnitudes need to be standardised; namely, they must be corrected for their correlation with the light-curve width and colour.

Aims. Here, we investigate how the standardisation procedure used to reduce the scatter of SN Ia luminosities is affected by their environment. Our aim is to reduce scatter and improve the standardisation process.

Methods. We first studied the SN Ia stretch distribution, as well as its dependence on environment, as characterised by local and global (g − z) colour and stellar mass. We then looked at the standardisation parameter, α, which accounts for the correlation between residuals and stretch, along with its environment dependency and linearity. Finally, we computed the magnitude offsets between SNe in different astrophysical environments after the colour and stretch standardisations (i.e. steps). This analysis has been made possible thanks to the unprecedented statistics of the volume-limited Zwicky Transient Facility (ZTF) SN Ia DR2 sample.

Results. The stretch distribution exhibits a bimodal behaviour, as previously found in the literature. However, we find the distribution to be dependent on environment. Specifically, the mean stretch modes decrease with host stellar mass, at a 9.2σ significance. We demonstrate, at the 13.4σ level, that the stretch-magnitude relation is non-linear, challenging the usual linear stretch-residuals relation currently used in cosmological analyses. In fitting for a broken-α model, we did indeed find two different slopes between stretch regimes (x₁ ≶  with  = −0.48 ± 0.08): α_low = 0.271 ± 0.011 and α_high = 0.083 ± 0.009, comprising a difference of Δα = −0.188 ± 0.014. As the relative proportion of SNe Ia in the high-stretch and low-stretch modes evolves with redshift and environment, this implies that a single-fitted α also evolves with the redshift and environment. Concerning the environmental magnitude offset γ, we find it to be greater than 0.12 mag, regardless of the considered environmental tracer used (local or global colour and stellar mass), all measured at the ≥5σ level. When accounting for the non-linearity of the stretch, these steps increase to ∼0.17 mag, measured with a precision of 0.01 mag. Such strong results highlight the importance of using a large volume-limited dataset to probe the underlying SN Ia-host correlations.

Context. Type Ia supernova (SNIa) are excellent probes of local distance and the growing sample sizes of SNIa have driven an increased propensity to study the associated systematic uncertainties and improve standardisation methods in preparation for the next generation of cosmological surveys into the dark energy equation of state, w. Aims. We aim to probe the potential change in the SNIa standardisation parameter, c, with redshift and the host-galaxy of the supernova. Improving the standardisation of SNIa brightness measurements will require the relationship between the host and the SNIa to be accounted for. In addition, potential shifts in the SNIa standardisation parameters with redshift will cause biases in the recovered cosmology. Methods. In this work, we assembled a volume-limited sample of 3000 likely SNIa across a redshift range from z=0.015 to z=0.36. This sample was fitted with changing mass and redshift bins to determine the relationship between the intrinsic properties of SNe Ia and their redshift and host galaxy parameters. We then investigated the colour-luminosity parameter, β, as a subsequent test of the SNIa standardisation process. Results. We find that the changing colour distribution of SNe Ia with redshift is driven by dust at a confidence of > 4ÏÂ. Additionally, we show a strong correlation between the host galaxy mass and the colour-luminosity coefficient β (> 4ÏÂ), even when accounting for the quantity of dust in a host galaxy. Conclusions. These results indicate that the observed colour distribution of SNe Ia does change with redshift. However, we note that this is an observational effect, rather than an intrinsic change. Future cosmological measurements with SNe Ia must take into account these changing dust distributions to reduce the number of potential sources of systematic uncertainty.

The Zwicky Transient Facility SN Ia Data Release 2 provides a perfect opportunity to perform a thorough search for and subsequent analysis of SiIIλ6355 high-velocity features (HVFs) in the pre-peak regime. The source of such features remains unclear, but potential origins include circumstellar material, as well as enhancements to the abundances or densities intrinsic to the supernova (SN) ejecta. Therefore, they may provide clues to the elusive progenitor and explosion scenarios of Type Ia SNe (SNe Ia). We employed a Markov chain Monte Carlo fitting method followed by Bayesian information criterion testing to classify single and double SiIIλ6355 components in the DR2. The detection efficiency of our classification method was investigated through the fitting of simulated features, which allowed us to place cuts on the spectral quality required for reliable classification. These simulations were also used to perform an analysis of the recovered parameter uncertainties and potential biases in the measurements. Within the 329 spectra sample we investigated, we identified 85 spectra exhibiting SiIIλ6355 HVFs. We find that HVFs decrease in strength with phase relative to their photospheric counterparts; however, this decrease can occur at different phases for different objects. HVFs with larger velocity separations from the photosphere were observed to fade earlier, leaving only the double components with smaller separations as we moved towards maximum light. Our findings suggest that around three quarters of SN Ia spectra before - 11 d show high-velocity components in the SiIIλ6355, with this dropping to around one third in the six days before maximum light. We observed no difference between the populations of SNe Ia that do and do not form SiIIλ6355 HVFs in terms of the SALT2 light curve parameter x1, peak magnitude, decline rate, host mass, or host colour, supporting the idea that these features are ubiquitous across the SN Ia population.

Context. 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. Aims. 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. Methods. We constructed a new SALT model, that 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. Results. 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. Conclusions. 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.

Type Ia supernovae (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 e_ect is. We used realistic simulations of SNe Ia observed by the Zwicky Transient Facility (ZTF) to investigate the effect of different methods of taking PVs into account. We studied 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 the sample variance into account. Considering the results we have obtained using simulations, we determine the PV systematic e_ects in the context of the ZTF SN Ia DR2 sample. We determine the PV impact on the intercept of the Hubble diagram, aB, which is directly linked to the measurement of H0. We show that not taking into account PVs and their correlations results in a shift in the H0 value of about 1.0 km s-1 Mpc-1 and a slight underestimation of the H0 error bar.