Fixing the U-band photometry of Type Ia supernovae

TL;DR

This paper improves U-band photometry of Type Ia supernovae using spectroscopic corrections, reducing discrepancies and enabling better use of ultraviolet data for cosmological distance measurements.

Contribution

It introduces spectroscopically-derived corrections for U-band photometry, reconciling light curves from different telescopes and addressing the U-band anomaly in supernova cosmology.

Findings

Reduced the 0.4 mag spread in U-band light curves

Identified a 0.12 mag U-band anomaly affecting distance moduli

Enabled more accurate utilization of U-band data at redshift 0.3-0.8

Abstract

We present previously unpublished photometry of supernovae 2003gs and 2003hv. Using spectroscopically-derived corrections to the U-band photometry, we reconcile U-band light curves made from imagery with the Cerro Tololo 0.9-m, 1.3-m and Las Campanas 1-m telescopes. Previously, such light curves showed a 0.4 mag spread at one month after maximum light. This gives us hope that a set of corrected ultraviolet light curves of nearby objects can contribute to the full utilization of rest frame U-band data of supernovae at redshift ~0.3 to 0.8. As pointed out recently by Kessler et al. in the context of the Sloan Digital Sky Survey supernova search, if we take the published U-band photometry of nearby Type Ia supernovae at face value, there is a 0.12 mag U-band anomaly in the distance moduli of higher redshift objects. This anomaly led the Sloan survey to eliminate from their analyses all photometry obtained in the rest frame U-band. The Supernova Legacy Survey eliminated observer frame U-band photometry, which is to say nearby objects observed in the U-band, but they used photometry of high redshift objects no matter in which band the photons were emitted.