SALT3: An Improved Type Ia Supernova Model for Measuring Cosmic Distances

Abstract

A spectral-energy distribution (SED) model for Type Ia supernovae (SNe Ia) is a critical tool for measuring precise and accurate distances across a large redshift range and constraining cosmological parameters. We present an improved model framework, SALT3, which has several advantages over current models including the leading SALT2 model (SALT2.4). While SALT3 has a similar philosophy, it differs from SALT2 by having improved estimation of uncertainties, better separation of color and light-curve stretch, and a publicly available training code. We present the application of our training method on a cross-calibrated compilation of 1083 SNe with 1207 spectra. Our compilation is 2.5× larger than the SALT2 training sample and has greatly reduced calibration uncertainties. The resulting trained SALT3.K21 model has an extended wavelength range 2000-11000 Å (1800 Å redder) and reduced uncertainties compared to SALT2, enabling accurate use of low-z I and iz photometric bands. Including these previously discarded bands, SALT3.K21 reduces the Hubble scatter of the low-z Foundation and CfA3 samples by 15% and 10%, respectively. To check for potential systematic uncertainties we compare distances of low (0.01 < z < 0.2) and high (0.4 < z < 0.6) redshift SNe in the training compilation, finding an insignificant 2 ± 14 mmag shift between SALT2.4 and SALT3.K21. While the SALT3.K21 model was trained on optical data, our method can be used to build a model for rest-frame NIR samples from the Roman Space Telescope. Our open-source training code, public training data, model, and documentation are available at https://saltshaker.readthedocs.io/en/latest/, and the model is integrated into the sncosmo and SNANA software packages.

The Pantheon+ Analysis: Forward-Modeling the Dust and Intrinsic Colour Distributions of Type Ia Supernovae, and Quantifying their Impact on Cosmological Inferences

“…Dust2Dust will be a crucial tool in future cosmological analyses. Recalibration and the retraining of light-curve fitting tools, such as SALT2 (Taylor et al 2021) and SALT3 (Kenworthy et al 2021), can significantly affect scatter model parameters. This shift in calibration and light-curve fitting will necessitate inferring new scatter model parameters for use in generating bias correcting simulations.…”

Cosmological Results from the RAISIN Survey: Using Type Ia Supernovae in the Near Infrared as a Novel Path to Measure the Dark Energy Equation of State

“…In the last decade, only the SALT2 (Guy et al 2007(Guy et al , 2010 and SiFTO (Conley et al 2008) light curve fitters have been used for measurements of w, with SALT2 being by far the most common model. SALT2, however, extends only to a central filter wavelength of approximately 7000 Å, with the recentlydeveloped SALT3 model (Kenworthy et al 2021) now extending this wavelength to 8700 Å to include the restframe z band.…”

Spectroscopic Studies of Type Ia Supernovae Using LSTM Neural Networks

“…To showcase the fidelity improvement of our method, we compared the spectral templates constructed by LSTM with those generated by two empirical models of SNe Ia, i.e., the template of Hsiao et al (2007) and the SALT3 model (Kenworthy et al 2021).…”

“…Beyond giving an average spectral evolution of SNe Ia, SALT2 also models the variations from supernova "stretch" and includes a modulation term encoding the time-invariant color component. SALT3 (Kenworthy et al 2021) is an improved version of SALT2 using the same framework but has better uncertainty estimation and better Rest-Frame Wavelength ( Å)…”

“…disentanglement of color and supernova "stretch". The SALT3 baseline model used in our work is the one pre-sented in Kenworthy et al (2021), which was trained on a dataset of 1207 spectra from 1083 SNe. Throughout the paper, we used the Python implementation of Hsiao template and SALT3 model in sncosmo library (Barbary et al 2016) for the baseline comparisons.…”