arXiv subjects

U. Burgaz

Publications and source records attributed to U. Burgaz.

At least 19 recordsLinked to original sources

arXiv2026

SN 2024afyu interpreted as a Pair Instability Supernova

Pair-instability supernovae (PISNe) are the predicted explosions of very massive stars triggered by electron-positron pair production. Numerous transients have been proposed as PISN candidates, yet none has provided unambiguous confirmation of this explosion mechanism. The predicted strengths of nebular emission lines offer a powerful means of testing the PISN scenario. We investigate the nature of SN 2024afyu, a nearby (z = 0.0085), long-lived (trise = 85 +- 11.7 days) SN with peculiar spectral evolution, with the aim of identifying its powering mechanism. We analyse multi-band photometry and optical and near-infrared spectroscopy from shortly after explosion to the nebular phase (around 500 days past peak). Besides early appearance of [Ca II] features, we identify a number of sulfur and silicon emission lines, for which we estimate electron temperatures and elemental masses. SN 2024afyu has an inferred 56Ni mass of around 0.4 to 1.0 solar masses and an inferred sulfur mass of the order of 3 solar masses, substantially larger than expected for conventional core-collapse explosions. SN 2024afyu is photometrically similar (although fainter, M_Peak(r) = -18.9 +- 0.04 mag) but spectroscopically distinct to other proposed PISNe. Yet, existing PISN models broadly reproduce several key characteristics, including the overall spectral appearance and broad photometric evolution. SN 2024afyu is a strong PISN candidate, since alternative scenarios would struggle to explain the combination of broad light curve, large intermediate-mass-element abundance, and general spectroscopic evolution. The discrepancies between the observations and currently available theoretical models highlight the need for new PISN calculations spanning a wider range of progenitor masses, metallicities, mixing prescriptions, and circumstellar environments.

astro-ph.SR↗

arXiv2026

Supernovae Ia ejecta velocities and host galaxy environments: the role of survey-selection effects

The origin of the near-maximum-light Si II $\lambda$6355 velocity diversity among supernovae Ia (SNe Ia) remains uncertain. Previous studies have suggested that high-velocity (HV) and normal-velocity (NV) SNe Ia occupy systematically different host galaxy environments, implying differences in their progenitor populations. We re-examine this hypothesis using a sample of 354 nearby ($z\leq0.04$) spectroscopically normal SNe Ia, comprising 239 NV and 115 HV events, identified in targeted and untargeted surveys. For each SN, we determine the host galaxy morphology, galactocentric distance, and physical size, and obtain homogeneous stellar mass estimates while assessing the influence of survey-selection effects. The Si II velocity distribution is well described by two Gaussian components, confirming the NV and HV populations. We find no statistically significant differences between the galactocentric distance, host galaxy size, or stellar mass distributions of the two velocity subgroups. The environmental trends reported in previous studies are recovered only with marginal statistical significance in the targeted subsample, whereas they disappear almost entirely in the untargeted subsample, consistent with survey-selection effects playing a major role in the previously inferred environmental differences between the NV and HV events. Our results weaken the interpretation that the observed Si II velocity diversity is primarily driven by systematic differences in the global host properties examined here. Instead, they favour a scenario in which intrinsic explosion asymmetries and viewing-angle effects account for a substantial fraction of the observed velocity diversity, without excluding a possible contribution from local environmental conditions or progenitor properties.

astro-ph.GA↗

arXiv2026

Eruptive mass loss less than a year before the explosion of superluminous supernovae. II. A systematic search for pre-explosion eruptions with VLT/X-shooter

We present X-shooter observations of a sample of 21 hydrogen-poor superluminous supernovae (SLSNe-I), spanning a redshift range of z=0.13-0.95, aimed at searching for shells of circumstellar material (CSM). Specifically, we focus on identifying broad Mg II absorption features that are blueshifted by several thousand kilometers per second and have previously been interpreted as arising from resonance line scattering of the SLSN continuum by rapidly expanding CSM ejected shortly before explosion. Utilizing high-quality spectra, we model the region around 2800A to characterize the Mg II line profiles, enabling us to either confirm their presence or place constraints on undetected CSM shells. We identify five objects in our sample that show broad Mg II absorption features consistent with the presence of CSM. While SN2018ibb, SN2020xga and SN2022xgc have been previously reported, we identify previously undiscovered CSM shells in DES15S2nr and DES16C3ggu. These shells were likely expelled approximately two and three months, respectively, before the explosion of their associated SNe, timescales consistent with late-stage mass-loss episodes. We do not find any correlations between the shell properties and the SN properties, except for a marginal correlation between the light curve decline time scale and the shell velocities. We further demonstrate that CSM configurations similar to the majority of the detected shells would have been observable in spectra with signal-to-noise >8 per resolution element, and that the lines from a shell are in general detectable except in the cases where the shell is either very geometrically and/or optically thin. Therefore, we conclude that the detection of CSM shells is not a selection effect, but may instead point to the existence of a subclass of SLSNe-I undergoing late-stage shell ejections shortly before explosion.

astro-ph.HE↗

arXiv2026

Photometric classification of supernovae detected by the Zwicky Transient Facility using noise augmentation

Modern time-domain surveys, such as the Zwicky Transient Facility (ZTF), detect far more extragalactic transients than can be spectroscopically classified. Photometric classification offers a scalable alternative, enabling the identification of larger, fainter, and higher-redshift supernova samples suitable for applications such as Type Ia supernova (SN Ia) cosmology. We present a feature-based photometric classifier for SNe detected by ZTF, with the primary goal of constructing a photometric SN Ia sample for cosmological analyses. Our approach utilises the autoencoder architecture of ParSNIP (Boone 2021) to capture the intrinsic diversity of SN light curves. We trained the model on a spectroscopically classified ZTF SN sample, incorporating a realistic noise augmentation procedure that simulates the flux uncertainties of fainter sources. Light curve features were used to train a gradient-boosted decision tree classifier, implemented in both binary (SN Ia vs. non-Ia) and multi-class configurations. We validated our classifier on independent, fainter ZTF data with and without noise augmentation. To evaluate real-time performance, we also applied our classifier to live ZTF alerts and conducted a spectroscopic classification survey within the ePESSTO+ collaboration. We found that noise augmentation significantly improves classification performance, particularly for fainter sources. Our binary classifier achieves an SN Ia recall of (98.1 $\pm$ 0.4)%, averaged across five train-test splits. SN Ia recall exceeds 98% for events with a peak apparent magnitude up to 20 and more than 10 detections, and remains above 96% up to magnitude 20.5. Overall, 95% of sources were correctly classified in both binary and multi-class modes. Our classifier performs efficiently on real ZTF data and enables construction of a large photometric SN Ia sample for cosmology.

astro-ph.IM↗

arXiv2026

Study of the anisotropy of cosmic expansion on ZTF type Iasupernovae simulations

The cosmological principle assumes the isotropy of the Universe at large scales. It is a foundational assumption in the $Λ$CDM model, which is the current standard model of cosmology. Recent tensions give legitimacy to investigating the possibility of anisotropies in the Universe. The large sky coverage achieved by the Zwicky Transient Facility survey (ZTF) allows us to test the veracity of the cosmological principle using observations of Type Ia supernovae (SNe Ia). In this article, we develop a methodology to measure potential anisotropies in the Hubble constant $H_0$. We test our method on realistic simulations of the second data release (DR2) of ZTF SNe Ia in which we introduce a dipole. We develop an unbiased method both to introduce a dipole in the simulations and to recover it. We test a potential $H_0$ dependency of our method while varying the dipole amplitude. We analyse the impact of introducing large-scale structures in the simulations and the efficiency of using a volume-limited sample, which is an unbiased subsample of the ZTF SNe Ia sample. Finally, we build an error model applied to the recovered dipole amplitude ($ΔH_0$) and its direction ($α_0$, $δ_0$). Our analysis allows us to recover a dipole with an error on the amplitude of $0.33\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$, and uncertainties of $3.4^\circ$ and $6.1^\circ$ on the right ascension and declination, respectively, for an initial dipole amplitude of $ΔH_0 = 3\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$. The resulting dipole is independent of the chosen $H_0$ value and sky coverage. This paper paves the way for a future precise ZTF dipole investigation.

astro-ph.CO↗

arXiv2025

Constraining Lens Masses in Moderately to Highly Magnified Microlensing Events from Gaia

Microlensing events provide a unique way to detect and measure the masses of isolated, non-luminous objects, particularly dark stellar remnants. Under certain conditions, it is possible to measure the mass of these objects using photometry alone, specifically when a microlensing light curve displays a finite source (FS) effect. This effect generally occurs in highly magnified light curves, i.e. when the source and the lens are very well aligned. In this study, we analyse Gaia Alerts and Gaia Data Release 3 datasets, identifying four moderate-to-high-magnification microlensing events without a discernible FS effect. The absence of this effect suggests a large Einstein radius, implying substantial lens masses. In each event, we constrained the FS effect, and therefore established lower limits for the angular Einstein radius and lens mass. Additionally, we used the DarkLensCode software to obtain the mass, distance, and brightness distribution for the lens based on the Galactic model. Our analysis established lower mass limits of $\sim 0.7$ $M_{\odot}$ for one lens and $\sim 0.3-0.5$ $M_{\odot}$ for two others. A DarkLensCode analysis supports these findings, estimating lens masses in the range of $\sim 0.42-1.70$ $M_{\odot}$ and dark lens probabilities exceeding 80%. These results strongly indicate that the lenses are stellar remnants, such as white dwarfs or neutron stars. While further investigations are required to confirm the nature of these lenses, we demonstrate a straightforward yet effective approach to identifying stellar remnant candidates.

astro-ph.GA↗

arXiv2025

ZTF-SEDm Type Ia supernova sample for Twins Embedding spectrophotometric standardisation

This paper has two aims: the first one is to build a large homogeneous spectrophotometric Type Ia supernova (SN Ia) sample, using 3069 spectra from the second Zwicky Transient Facility data release (ZTF DR2). Using this sample we reproduce, as the second objective of the paper, the Twins Embedding (TE) spectrophotometric standardisation method, which led to an exceptionally low value of 0.073 mag for the intrinsic scatter. We improve the flux-calibration accuracy of the SEDm SN Ia spectral sample using the ZTF photometric data, which are calibrated at the percent level. We then apply the three steps of the TE parameterisation to a subset of 783 ZTF SN spectra near maximum light, and analyse the resulting standardisation methods. The precision of the phase correction model, which is the first step of the TE, is estimated at 0.01 mag in g band, using ZTF data. Despite the challenge posed by the ZTF spectrum extraction pipeline, we apply a first standardisation in color based on the second step of the TE, the Read Between The Lines (RBTL). When considering the scatter due to the redshift error and the flux calibration error, we estimate a 0.129 mag Hubble residual scatter for this ZTF sample as an upper limit. As expected from the low spectral quality, the final TE standardisation based on three non-linear parameters did not improve the overall dispersion. We release 1897 flux calibrated spectra of 1607 SNe Ia with an estimated photometric accuracy of 0.07 mag. We further demonstrate the ability to apply a spectrophotometric standardisation with limited quality spectra. The RBTL standardisation is more efficient than that of SALT with one less parameter, and the resulting host steps are consistent with zero, making it less prone to astrophysical bias. For future spectroscopic surveys, a better spectral quality would enable the full TE standardisation to be computed. (Abridged)

astro-ph.CO↗

arXiv2025

ZTF SNe Ia DR2: Towards cosmology-grade ZTF supernova light curves using scene modeling photometry

The Zwicky Transient Facility (ZTF) is conducting a wide-field survey of the northern sky in three optical bands and the collaboration cosmology working group has released 3628 spectroscopically confirmed Type Ia supernovae (SNe Ia) discovered during its first 2.5 years of operation. This "ZTF SN Ia DR2" sample is the largest SN Ia dataset to date. Fully exploiting this dataset to improve understanding of the properties of dark energy requires a photometric accuracy of O(0.1%). This can be achieved using Scene Modeling Photometry (SMP), which is optimal to extract a transient signal (SN) from a complex background (its host), while ensuring a common flux estimator with nearby stars used as calibration reference. In this paper, we present the status of the SMP development and use it to assess the precision and accuracy of the ZTF SN Ia DR2 force photometry light curves. We reach a repeatability of the star observations better than 1%. However, we have identified a new sensor effect, dubbed "pocket-effect", which distorts the Point Spread Function (PSF) in a flux-dependent manner leading to non-linearities in the photometry of a few percent. Correcting for this effect requires time- and sensor-dependent corrections to be applied at the pixel level, which is currently under development. This effects affects all light curve releases to date -- both from forced photometry and scene modelling preventing ZTF SN Ia DR2 to be used for accurate cosmological inference. Comparing the SMP and forced photometry measurements, we find that stretch and color estimated from both processings are consistent, aside from a 10 mmag shift in color. This assess the robustness of results presented as part of the the ZTF SN Ia DR2 release. The absolute calibration however shifts by 90 mmag. A reprocessing of the full ZTF SN Ia DR2 dataset using the SMP method is currently in progress.

astro-ph.CO↗

arXiv2025

ZTF SN Ia DR2 follow-up: Exploring the origin of the Type Ia supernova host galaxy step through Si II velocities

The relationship between Type Ia supernovae (SNe Ia) and their host galaxy stellar masses is well documented. In particular, Hubble residuals display a luminosity shift based on host mass, known as the mass step, which is often used as an extra correction in the standardisation of SN Ia luminosities. Here we investigate Hubble residuals and the mass step in the context of Si II $λ6355$ velocities, using 277 near-peak SNe Ia from ZTF DR2. We divide the sample into high-velocity (HV) and normal-velocity (NV) SNe Ia, separated at 12,000 km/s, resulting in 70 HV and 207 NV objects. We then examine links between Si II $λ$6355 velocities, light-curve stretch $x_{1}$, colour $c$, and host properties to explore potential environmental and/or progenitor-related effects. Although we only find a marginal difference between the Hubble residuals of HV and NV SNe Ia, the NV mass step is $0.149 \pm 0.024$ mag ($6.3σ$), while HV SNe Ia show $0.046 \pm 0.041$ mag ($1.1σ$), consistent with zero. The NV-HV mass-step difference is $\sim 2.2σ$. The clearest subtype difference is seen in central regions ($d_{DLR} < 1$), where NV SNe Ia show a strong mass step but HV SNe Ia none, yielding a $3.1-3.6σ$ difference. A host-colour step appears for both: NV $0.142 \pm 0.024$ mag ($5.9σ$) and HV $0.158 \pm 0.042$ mag ($3.8σ$). Overall, NV and HV colour steps are consistent. HV SNe Ia show modest ($\sim 2.5$-$3σ$) steps in outer regions ($d_{DLR} > 1$), while NV SNe show stronger environmental trends. Thus, NV SNe Ia appear more environmentally sensitive, especially in central, likely metal-rich and older regions, while HV SNe Ia show weaker, subset-dependent trends, and applying a universal mass-step correction could introduce biases. Refined classifications or environment-dependent factors may improve future cosmological analyses beyond standard $x_{1}$ and $c$ cuts.

astro-ph.GA↗

arXiv2025

AT2021uey: A planetary microlensing event outside the Galactic bulge

We report the analysis of a planetary microlensing event AT2021uey. The event was observed outside the Galactic bulge and was alerted by both space- (Gaia) and ground-based (ZTF and ASAS-SN) surveys. From the observed data, we find that the lens system is located at a distance of 1 kpc and comprises an M-dwarf host star of about half a solar mass, orbited by a Jupiter-like planet beyond the snowline. The source star could be a metal-poor giant located in the halo according to the spectral analyses and modelling. Hence, AT2021uey is a unique example of the binary-lens event outside the bulge that is offered by a disc-halo lens-source combination.

astro-ph.EP↗

arXiv2025

ZTF SN Ia DR2: Environmental dependencies of stretch and luminosity of a volume limited sample of 1,000 Type Ia Supernovae

To get distances, Type Ia Supernovae magnitudes are corrected for their correlation with lightcurve width and colour. Here we investigate how this standardisation is affected by the SN environment, with the aim to reduce scatter and improve standardisation. We first study 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 look at the standardisation parameter $α$, which accounts for the correlation between residuals and stretch, along with its environment dependence and linearity. We finally compute magnitude offsets between SNe in different astrophysical environments after colour and stretch standardisation, aka steps. This analysis is made possible due to the unprecedented statistics of the ZTF SN Ia DR2 volume-limited sample. The stretch distribution exhibits a bimodal behaviour, as previously found in literature. However, we find the distribution means to decrease with host stellar mass at a 9.2$σ$ significance. We demonstrate, at the 13.4$σ$ level, that the stretch-magnitude relation is non-linear, challenging the usual linear stretch-residuals relation. Fitting for a broken-$α$ model, we indeed find two different slopes between stretch regimes ($x_1<-0.48\pm0.08$): $α_{low}=0.27\pm0.01$ and $α_{high}=0.08\pm0.01$, a $Δ_α=-0.19\pm0.01$ difference. As the relative proportion of SNe Ia in the high-/low-stretch modes evolves with redshift and environment, this implies that a linear $α$ also evolves with 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 $\geq 5σ$ level, increased to $\sim0.17\pm0.01$ mag when accounting for the stretch-non linearity.

astro-ph.CO↗

arXiv2025

ZTF SN Ia DR2: Improved SN Ia colors through expanded dimensionality with SALT3+

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.

astro-ph.CO↗

arXiv2025

ZTF SN Ia DR2: High-velocity components in the Si II $λ$6355

The ZTF SN Ia Data Release 2 provides a perfect opportunity to perform a thorough search for, and subsequent analysis of, high-velocity components in the Si II $λ$6355 feature in the pre-peak regime. The source of such features remains unclear, with potential origins in circumstellar material or density/abundance enhancements intrinsic to the SN ejecta. Therefore, they may provide clues to the elusive progenitor and explosion scenarios of SNe Ia. We employ a MCMC fitting method followed by BIC testing to classify single and double Si II $λ$6355 components in the DR2. The detection efficiency of our classification method is investigated through the fitting of simulated features, allowing us to place cuts upon 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 that we investigate, we identify 85 spectra exhibiting Si II $λ$6355 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 are seen to fade earlier leaving only the double components with smaller separations as we move towards maximum light. Our findings suggest that around three quarters of SN Ia spectra before -11 d show high-velocity components in the Si II $λ$6355 with this dropping to around one third in the six days before maximum light. We observe no difference between the populations of SNe Ia that do and do not form Si II $λ$6355 HVFs in terms of 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.

astro-ph.HE↗

arXiv2025

ZTF SN Ia DR2: Colour standardisation of Type Ia Supernovae and its dependence on environment

As type Ia supernova (SN Ia) cosmology transitions from a statistics-dominated to a systematics-dominated era, it is crucial to understand the remaining unexplained uncertainties that affect their luminosity, such as those stemming from astrophysical biases. SNe Ia are standardisable candles whose absolute magnitude reaches a scatter of 0.15 mag when empirical correlations with their light-curve stretch and colour and with their environment are accounted for. We investigate the dependence of the standardisation process of SNe Ia on the astrophysical environment, focusing on colour standardisation. We used the volume-limited ZTF SN Ia DR2 sample, which offers unprecedented statistics for the low-redshift ($z < 0.06$) range. We first studied the colour distribution, to then select a dustless subsample of objects. We then examined the colour-residual relation and its associated parameter $β$. Finally, we investigated the colour dependence of the environment-dependent magnitude offsets (steps) to separate their intrinsic and extrinsic components. Our sample of nearly 1,000 SNe probes the red tail of the colour distribution up to $c = 0.8$. The dustless sample exhibits a significantly shorter red tail ($4.3σ$) than the whole sample, but the distributions around $c\sim0$ are similar for both samples. This suggests that the reddening above $c\geq0.2$ is dominated by interstellar dust absorption of the host. The colour-residual relation is linear with SN colour. We found indications of a potential evolution of $β$ with the stellar host mass, with $β\sim3.6$ for low-mass galaxies, compared to $β=3.05\pm0.06$ for the full sample. Finally, in contrast to recent claims from the literature, we found no evolution of steps as a function of SN colour. This suggests that dust may not be the dominating mechanism for the dependence on the environment of the magnitude of SNe Ia.

astro-ph.CO↗

arXiv2025

ZTF SN Ia DR2: The diversity and relative rates of the thermonuclear SN population

The Zwicky Transient Facility SN Ia Data Release 2 (ZTF SN Ia DR2) contains more than 3,000 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.

astro-ph.HE↗

arXiv2024

ZTF SN Ia DR2: Properties of the low-mass host galaxies of Type Ia supernovae in a volume-limited sample

In this study, we explore the characteristics of `low-mass' ($\log(M_{\star}/M_{\odot}) \leq 8$) and `intermediate-mass' ($8 \lt \log(M_{\star}/M_{\odot}) \leq 10$) host galaxies of Type Ia supernovae (SNe Ia) from the second data release (DR2) of the Zwicky Transient Facility survey and investigate their correlations with different sub-types of SNe Ia. We use the photospheric velocities measured from the Si II $λ$6355 feature, SALT2 light-curve stretch ($x_1$) and host-galaxy properties of SNe Ia to re-investigate the existing relationship between host galaxy mass and Si II $λ$6355 velocities. We also investigate sub-type preferences for host populations and show that while the more energetic and brighter 91T-like SNe Ia tends to populate the younger host populations, 91bg-like SNe Ia populate in the older populations. Our findings suggest High Velocity SNe Ia (HV SNe Ia) not only comes from the older populations but they also come from young populations as well. Therefore, while our findings can partially provide support for HV SNe Ia relating to single degenerate progenitor models, they indicate that HV SNe Ia other than being a different population, might be a continued distribution with different explosion mechanisms. We lastly investigate the specific rate of SNe Ia in the volume-limited SN Ia sample of DR2 and compare with other surveys.

astro-ph.HE↗

arXiv2024

ZTF SN Ia DR2: Study of Type Ia Supernova lightcurve fits

Type Ia supernova (SN Ia) cosmology relies on the estimation of lightcurve parameters to derive precision distances that leads to the estimation of cosmological parameters. The empirical SALT2 lightcurve modeling that relies on only two parameters, a stretch x1, and a color c, has been used by the community for almost two decades. In this paper we study the ability of the SALT2 model to fit the nearly 3000 cosmology-grade SN Ia lightcurves from the second release of the Zwicky Transient Facility (ZTF) cosmology science working group. While the ZTF data was not used to train SALT2, the algorithm is modeling the ZTF SN Ia optical lightcurves remarkably well, except for lightcurve points prior to -10 d from maximum, where the training critically lacks statistics. We find that the lightcurve fitting is robust against the considered choice of phase-range, but we show the [-10; +40] d range to be optimal in terms of statistics and accuracy. We do not detect any significant features in the lightcurve fit residuals that could be connected to the host environment. Potential systematic population differences related to the SN Ia host properties might thus not be accountable for by the addition of extra lightcurve parameters. However, a small but significant inconsistency between residuals of blue- and red-SN Ia strongly suggests the existence of a phase-dependent color term, with potential implications for the use of SNe Ia in precision cosmology. We thus encourage modellers to explore this avenue and we emphasize the importance that SN Ia cosmology must include a SALT2 retraining to accurately model the lightcurves and avoid biasing the derivation of cosmological parameters.

astro-ph.CO↗

arXiv2024

ZTF SN Ia DR2: An environmental study of Type Ia supernovae using host galaxy image decomposition

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 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 disk/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. We find that although this image decomposition technique leads to a significant bias towards elliptical galaxies in our final sample of galaxies, the overall results are robust. By successfully modelling 728 host galaxies, we find that the photometric properties of SNe Ia found in disks 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$σ$) and disk (5.1$σ$) subpopulations of SNe Ia. Lower stretch SNe Ia are found in redder environments, which we identify as an age/metallicity effect. Within the subpopulation of SNe Ia found in disk containing galaxies, we find a significant linear trend (6.1$σ$) between light-curve stretch and model-derived local $r$-band surface brightness, which we link to the age/metallicity gradients found in disk galaxies. SN Ia colour shows little correlation with host environment as seen in the literature. We identify a possible dust effect in our model-derived surface brightness (3.3$σ$), for SNe Ia in disk galaxies.

astro-ph.GA↗