Understanding type Ia supernovae through their U-band spectra^★

¹ Institut fur Physik, Humboldt-Universitat zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
e-mail: jakob.nordin@physik.hu-berlin.de
² Physics Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA
³ Laboratoire de Physique Nucléaire et des Hautes Énergies, Université Pierre et Marie Curie Paris 6, Université Paris Diderot Paris 7, CNRS-IN2P3, 4 place Jussieu, 75252 Paris Cedex 05, France
⁴ Department of Physics, Yale University, New Haven, CT 06250-8121, USA
⁵ Berkeley Center for Cosmological Physics, University of California Berkeley, Berkeley, CA 94720, USA
⁶ Department of Physics, University of California Berkeley, 366 LeConte Hall MC 7300, Berkeley, CA 94720-7300, USA
⁷ Université de Lyon, 69622, Lyon, France; Université de Lyon 1, Villeurbanne; CNRS/IN2P3, Institut de Physique Nucléaire de Lyon, France
⁸ Department of Physics and Astronomy, University of Southampton, Southampton, Hampshire SO17 1BJ, UK
⁹ The Oskar Klein Centre, Department of Physics, AlbaNova, Stockholm University, SE-106 91 Stockholm, Sweden
¹⁰ Aix Marseille Université, CNRS/IN2P3, CPPM UMR 7346, 13288 Marseille, France
¹¹ Clermont Université, Université Blaise Pascal, CNRS/IN2P3, Laboratoire de Physique Corpusculaire, BP 10448, 63000 Clermont-Ferrand, France
¹² Max-Planck Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
¹³ Deutsches Elektronen-Synchrotron, 15735 Zeuthen, Germany
¹⁴ Tsinghua Center for Astrophysics, Tsinghua University, Beijing 100084, PR China
¹⁵ Centre de Recherche Astronomique de Lyon, Université Lyon 1, 9 Avenue Charles André, 69561 Saint Genis Laval Cedex, France
¹⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁷ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
¹⁸ Computational Cosmology Center, Computational Research Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road MS 50B-4206, Berkeley, CA 94720, USA
¹⁹ Kavli Institute for the Physics and Mathematics of the Universe, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583, Japan

Received: 19 October 2017
Accepted: 3 January 2018

Abstract

Context. Observations of type Ia supernovae (SNe Ia) can be used to derive accurate cosmological distances through empirical standardization techniques. Despite this success neither the progenitors of SNe Ia nor the explosion process are fully understood. The U-band region has been less well observed for nearby SNe, due to technical challenges, but is the most readily accessible band for high-redshift SNe.

Aims. Using spectrophotometry from the Nearby Supernova Factory, we study the origin and extent of U-band spectroscopic variations in SNe Ia and explore consequences for their standardization and the potential for providing new insights into the explosion process.

Methods. We divide the U-band spectrum into four wavelength regions λ(uNi), λ(uTi), λ(uSi) and λ(uCa). Two of these span the Ca H&K λλ 3934, 3969 complex. We employ spectral synthesis using SYNAPPS to associate the two bluer regions with Ni/Co and Ti.

Results. The flux of the uTi feature is an extremely sensitive temperature/luminosity indicator, standardizing the SN peak luminosity to 0.116 ± 0.011 mag root mean square (RMS). A traditional SALT2.4 fit on the same sample yields a 0.135 mag RMS. Standardization using uTi also reduces the difference in corrected magnitude between SNe originating from different host galaxy environments. Early U-band spectra can be used to probe the Ni+Co distribution in the ejecta, thus offering a rare window into the source of light curve power. The uCa flux further improves standardization, yielding a 0.086 ± 0.010 mag RMS without the need to include an additional intrinsic dispersion to reach χ²∕dof ~ 1. This reduction in RMS is partially driven by an improved standardization of Shallow Silicon and 91T-like SNe.