ZTF SN Ia DR2: Cosmology-independent constraints on Type Ia supernova standardisation from supernova siblings

¹ Institute of Astronomy and Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
² The Oskar Klein Centre for Cosmoparticle Physics, Department of Physics, Stockholm University, SE-10691 Stockholm, Sweden
³ Université de Lyon, Université Claude Bernard Lyon 1, CNRS/IN2P3, IP2I Lyon, F-69622 Villeurbanne, France
⁴ Department of Physics, Lancaster University, Lancs LA1 4YB, UK
⁵ School of Physics, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland
⁶ Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
⁷ Lawrence Berkeley National Laboratory, 1 Cyclotron Road MS 50B-4206, Berkeley CA 94720, USA
⁸ Department of Astronomy, University of California, Berkeley, 501 Campbell Hall, Berkeley CA 94720, USA
⁹ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, E-08193 Barcelona, Spain
¹⁰ Institut d’Estudis Espacials de Catalunya (IEEC), E-08034 Barcelona, Spain
¹¹ The Oskar Klein Centre, Department of Astronomy, Stockholm University, Albanova University Center, Stockholm SE-106 91, Sweden
¹² LPNHE, (CNRS/IN2P3, Sorbonne Université, Université Paris Cité), Laboratoire de Physique Nucléaire et de Hautes Énergies, 75005 Paris, France
¹³ Nordic Optical Telescope, Rambla José Ana Fernández Pérez 7 ES-38711 Breña Baja, Spain
¹⁴ Université Clermont Auvergne, CNRS/IN2P3, LPCA, F-63000 Clermont-Ferrand, France
¹⁵ IPAC, California Institute of Technology, Pasadena CA 91125, USA
¹⁶ Caltech Optical Observatories, California Institute of Technology, Pasadena CA 91125, USA
¹⁷ School of Physics and Astronomy, University of Minnesota Minneapolis MN 55455, USA

^⋆ Corresponding author: suhail.dhawan@ast.cam.ac.uk

Received: 15 April 2024
Accepted: 26 March 2025

Abstract

Understanding Type Ia supernovae (SNe Ia) and the empirical standardisation relations that make them excellent distance indicators is vital to improving cosmological constraints. SN Ia ‘siblings, i.e. two or more SNe Ia in the same host or parent galaxy, offer a unique way to infer the standardisation relations and their scatter across the population. We analysed a sample of 25 SN Ia pairs observed homogeneously by the Zwicky Transient Facility (ZTF) to infer the SNe Ia light curve width-luminosity and colour-luminosity parameters, α and β. Using the pairwise constraints from siblings, which allow for a scatter in the standardisation relations, we found α = 0.218 ± 0.055 and β = 3.084 ± 0.312, respectively, with a dispersion in α and β of ≤0.195 and ≤0.923, respectively, at a 95% confidence level. While the median dispersion is large, the values within ∼1σ are consistent with no dispersion. Hence, fitting for a single global standardisation relation, we found α = 0.228 ± 0.029 and β = 3.160 ± 0.191. We also found a very small intrinsic scatter of the siblings sample σ_int ≤ 0.10 mag at a 95% confidence level compared to σ_int = 0.22 ± 0.04 mag when computing the scatter using the Hubble residuals without comparing them as siblings. When comparing to large samples used in cosmological measurements, we found an α that is ∼2-3 σ higher, while the β values are consistent. The high α is driven by low x₁ pairs, potentially suggesting that the slow and fast declining SN Ia have different slopes for the width-luminosity relation. We found no difference in α and β when dividing the sample by host galaxy mass. The finding of a higher α with increased statistics can be confirmed or refuted through upcoming time-domain surveys. If confirmed, this finding can improve the cosmological inference from SNe Ia and be used to infer properties of the progenitors for subpopulations of SNe Ia.