ZTF SN Ia DR2: Secondary maximum in type Ia supernovae

¹ School of Physics, Trinity College Dublin, College Green, Dublin 2, Ireland
² GSI Helmholtzzentrum für Schwerionenforschung, Planckstraße 1, 64291 Darmstadt, Germany
³ Univ Lyon, Univ Claude Bernard Lyon 1, CNRS, IP2I Lyon/IN2P3, UMR 5822, F-69622 Villeurbanne, France
⁴ Department of Physics, Lancaster University, Lancaster LA1 4YB, UK
⁵ Oskar Klein Centre, Department of Physics, Stockholm University, SE-10691 Stockholm, Sweden
⁶ Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
⁷ National Research Council of Canada, Herzberg Astronomy & Astrophysics Research Centre, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
⁸ Institute of Astronomy and Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁹ IPAC, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
¹⁰ 247-17 Caltech, Pasadena, CA 91125, USA
¹¹ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, E-08193 Barcelona, Spain
¹² Institut d’Estudis Espacials de Catalunya (IEEC), E-08034 Barcelona, Spain
¹³ Lawrence Berkeley National Laboratory, 1 Cyclotron Road MS 50B-4206, Berkeley, CA 94720, USA
¹⁴ Department of Astronomy, University of California, Berkeley, 501 Campbell Hall, Berkeley CA 94720, USA
¹⁵ Department of Physics, Drexel University, Disque Hall, Office No. 808 32 S. 32nd St., Philadelphia, PA 19104, USA
¹⁶ Caltech Optical Observatories, California Institute of Technology, Pasadena, CA 91125, USA
¹⁷ Aix Marseille Université, CNRS/IN2P3, CPPM, Marseille, France
¹⁸ Oskar Klein Centre, Department of Astronomy, Stockholm University, SE-10691 Stockholm, Sweden

^⋆ Corresponding author; deckersm@tcd.ie

Received: 15 April 2024
Accepted: 26 June 2024

Abstract

Type Ia supernova (SN Ia) light curves have a secondary maximum that exists in the r, i, and near-infrared filters. The secondary maximum is relatively weak in the r band, but holds the advantage that it is accessible, even at high redshift. We used Gaussian process fitting to parameterise the light curves of 893 SNe Ia from the Zwicky Transient Facility’s (ZTF) second data release (DR2), and we were able to extract information about the timing and strength of the secondary maximum. We found > 5σ correlations between the light curve dec rate (Δm₁₅(g)) and the timing and strength of the secondary maximum in the r band. Whilst the timing of the secondary maximum in the i band is also correlated with Δm₁₅(g), the strength of the secondary maximum in the i band shows significant scatter as a function of Δm₁₅(g). We found that the transparency timescales of 97 per cent of our sample are consistent with double detonation models and that SNe Ia with small transparency timescales (< 32 d) reside predominantly in locally red environments. We measured the total ejected mass for the normal SNe Ia in our sample using two methods and both were consistent with medians of 1.3  ±  0.3 and 1.2  ±  0.2 M_⊙. We find that the strength of the secondary maximum is a better standardisation parameter than the SALT light curve stretch (x₁). Finally, we identified a spectral feature in the r band as Fe II, which strengthens during the onset of the secondary maximum. The same feature begins to strengthen at < 3 d post maximum light in 91bg-like SNe. Finally, the correlation between x₁ and the strength of the secondary maximum was best fit with a broken, with a split at x₁⁰  =   − 0.5  ±  0.2, suggestive of the existence of two populations of SNe Ia.