The Pantheon+ Analysis: SuperCal-fragilistic Cross Calibration, Retrained SALT2 Light-curve Model, and Calibration Systematic Uncertainty

Abstract

We present a recalibration of the photometric systems in the Pantheon+ sample of Type Ia supernovae (SNe Ia) including those in the SH0ES distance-ladder measurement of H 0. We utilize the large and uniform sky coverage of the public Pan-STARRS stellar photometry catalog to cross calibrate against tertiary standards released by individual SN Ia surveys. The most significant updates over the “SuperCal” cross calibration used for the previous Pantheon and SH0ES analyses are: (1) expansion of the number of photometric systems (now 25) and filters (now 105), (2) solving for all filter offsets in all systems simultaneously to produce a calibration uncertainty covariance matrix for cosmological-model constraints, and (3) accounting for the change in the fundamental flux calibration of the Hubble Space Telescope CALSPEC standards from previous versions on the order of 1.5% over a Δλ of 4000 Å. We retrain the SALT2 model and find that our new model coupled with the new calibration of the light curves themselves causes a net distance modulus change (d μ/dz) of 0.04 mag over the redshift range 0 < z < 1. We introduce a new formalism to determine the systematic impact on cosmological inference by propagating the covariance in the fitted calibration offsets through retraining simultaneously with light-curve fitting and find a total calibration uncertainty impact of σ w = 0.013; roughly half the size of the sample statistical uncertainty. Similarly, we find the systematic SN calibration contribution to the SH0ES H 0 uncertainty is less than 0.2 km s−1 Mpc−1, suggesting that SN Ia calibration cannot resolve the current level of the “Hubble Tension.”

The Pantheon+ Analysis: Cosmological Constraints

“…Third is the change resulting from improved modeling of the SN Ia intrinsic scatter; while in this work we adopt the BS21 model, we include the models developed for JLA (G10 and C11) as systematics. Each of these changes has been motivated externally by previous works (e.g., Schlafly & Finkbeiner 2011;Brout et al 2022); however, they nonetheless cause shifts in dμ/dz of ∼0.05, or ∼0.04 in w. Finally, because all of three of these changes have the same sign of dμ/dz slope, rather than canceling each other, when combined in this work, they result in a ∼0.1 difference in the constraint on w relative to JLA (after combining with CMB).…”

“…To standardize the SN Ia brightnesses, we fit light curves using SNANA with the SALT2 model as originally developed by Guy et al (2010) and updated in Brout et al (2022), hereafter SALT2-B22. For each SN, the SALT2 light-curve fit returns four parameters: the light-curve amplitude x 0 where m x 2.5 log ;…”

“…We therefore include a systematic uncertainty where the mass step occurs at 10 10.2 M e . Baseline: The calibration of all 25 photometric systems used in this work is discussed in Brout et al (2022). The outputs of Fragilistic are a best-fit calibration solution for each of the 105 passbands and a joint 105 × 105 covariance matrix that describes the covariance between the zero-point calibrations of all passbands that arise from using a single common stellar catalog to tie all surveys together (PS1).…”

“…Top: updating the extinction curve used in the lightcurve fitting from CCM to F99. Middle: updating the SALT2 model, as discussed inBrout et al (2022). Bottom: changing the baseline assumption for the intrinsic scatter to the P21, G10, and C11 models.…”

Uniform Recalibration of Common Spectrophotometry Standard Stars onto the CALSPEC System Using the SuperNova Integral Field Spectrograph

“…Thus, the calibration must be transferred to fainter and redder stars. The CALSPEC network (Bohlin 2007;Bohlin et al 2014Bohlin et al , 2020 has met this need, providing a practical intermediary between the fundamental WDs and the stars used to flux calibrate essentially all astronomical surveys (Bohlin et al 2011;Betoule et al 2013;Rubin et al 2015;Scolnic et al 2015;Bohlin 2016;Currie et al 2020;Brout et al 2022). Despite its success, CALSPEC is observationally expensive to expand, with the highest-quality optical observations coming only from the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS).…”