SNEMO: Improved Empirical Models for Type Ia Supernovae

Abstract

arXiv:1810.09476v1 [astro-ph.CO] 22 Oct 2018 2 Saunders et al.Type Ia supernova cosmology depends on the ability to fit and standardize observations of supernova magnitudes with an empirical model. We present here a series of new models of Type Ia Supernova spectral time series that capture a greater amount of supernova diversity than possible with the models that are currently customary. These are entitled SuperNova Empirical MOdels (SNEMO a) ).The models are constructed using spectrophotometric time series from 172 individual supernovae from the Nearby Supernova Factory, comprising more than 2000 spectra. Using the available observations, Gaussian Processes are used to predict a full spectral time series for each supernova. A matrix is constructed from the spectral time series of all the supernovae, and Expectation Maximization Factor Analysis is used to calculate the principal components of the data. K-fold cross-validation then determines the selection of model parameters and accounts for color variation in the data. Based on this process, the final models are trained on supernovae that have been dereddened using the Fitzpatrick and Massa extinction relation. Three final models are presented here: SNEMO2, a twocomponent model for comparison with current Type Ia models; SNEMO7, a seven component model chosen for standardizing supernova magnitudes which results in a total dispersion of 0.100 mag for a validation set of supernovae, of which 0.087 mag is unexplained (a total dispersion of 0.113 mag with unexplained dispersion of 0.097 mag is found for the total set of training and validation supernovae); and SNEMO15, a comprehensive 15 component model that maximizes the amount of spectral time series behavior captured.

Initial Evaluation of SNEMO2 and SNEMO7 Standardization Derived from Current Light Curves of Type Ia Supernovae

“…The resulting Hubble-Lemaître diagram confirms the result from Saunders et al (2018) that SNEMO7 reduces the scatter around the assumed cosmology from an Figure 6. The difference in distance between SALT2 and SNEMO7 vs redshift.…”

“…The Hubble-Lemaître diagram using SALT2 (blue Xs) and SNEMO7 (red points). As seen inSaunders et al (2018), the scatter for the same SNe Ia is smaller when using SNEMO7. This figure assumes a fiducial cosmology and does not correct for any Malmquist biases.…”

“…For a further understanding of the similarities and difference between SALT2 and SNEMO, we plot the correlations between the model parameters in Figure 1. Figure 13 of Saunders et al (2018) shows this same plot but for SNEMO2 parameters measured from spectrophotometric data. This work focuses on SNEMO7 parameters derived from photometric data, so Figure 1 uses our nominal SNEMO7 data set.…”

“…Table 3 shows the estimated standardization coefficients after applying various data quality cuts. Taking the data with no uncertainty model added, we determine that they differ at ∼ 2σ with the original estimates produced when using the SNfactory data set (Saunders et al 2018, and forthcoming erratum). When evaluating seven parameters, it is expected to see some variability.…”

“…The blue lines for σ unexplained and γ are from the SALT2 fits of the same data set. Whereas, the other lines, αi and β, are the values presented in Saunders et al (2018). Note-The SN Ia used in the SALT2 analysis are the ones that passed the σ i ≤ 2 for SNEMO2 with no error model and were successfully fit with SALT2.…”