Two c's in a pod: cosmology-independent measurement of the Type Ia supernova colour–luminosity relation with a sibling pair

Creators: Biswas, Rahul; Goobar, Ariel; Dhawan, Suhail; Schulze, Steve; Johansson, Joel; Bellm, Eric C.; Dekany, Richard; Drake, Andrew J.; Duev, Dmitry A.; Fremling, Christoffer; Graham, Matthew; Kim, Young-Lo; Kool, Erik C.; Kulkarni, Shrinivas R.; Mahabal, Ashish A.; Perley, Daniel; Rigault, Mickael; Rusholme, Ben; Sollerman, Jesper; Shupe, David L.; Smith, Matthew; Walters, Richard S.

Abstract

Using Zwicky Transient Facility (ZTF) observations, we identify a pair of 'sibling' Type Ia supernovae (SNe Ia), i.e. hosted by the same galaxy at z = 0.0541. They exploded within 200 d from each other at a separation of 0.6arcsec corresponding to a projected distance of only 0.6 kpc. Performing SALT2 light-curve fits to the gri ZTF photometry, we show that for these equally distant 'standardizable candles', there is a difference of 2 mag in their rest-frame B-band peaks, and the fainter supernova (SN) has a significantly red SALT2 colour c = 0.57 ± 0.04, while the stretch values x₁ of the two SNe are similar, suggesting that the fainter SN is attenuated by dust in the interstellar medium of the host galaxy. We use these measurements to infer the SALT2 colour standardization parameter, β = 3.5 ± 0.3, independent of the underlying cosmology and Malmquist bias. Assuming the colour excess is entirely due to dust, the result differs by 2σ from the average Milky Way total-to-selective extinction ratio, but is in good agreement with the colour–brightness corrections empirically derived from the most recent SN Ia Hubble–Lemaitre diagram fits. Thus we suggest that SN 'siblings', which will increasingly be discovered in the coming years, can be used to probe the validity of the colour and light-curve shape corrections using in SN Ia cosmology while avoiding important systematic effects in their inference from global multiparameter fits to inhomogeneous data sets, and also help constrain the role of interstellar dust in SN Ia cosmology.

Additional Information

© 2021 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model). Accepted 2021 October 2. Received 2021 October 1; in original form 2021 June 29. Published: 14 October 2021. RB was supported by the research project grant 'Understanding the Dynamic Universe' funded by the Knut and Alice Wallenberg Foundation under Dnr KAW 2018.0067. AG acknowledges support from the Swedish Research Council under Dnr VR 2016-03274 and 2020-03444. SD acknowledges support from the Isaac Newton Trust and the Kavli Foundation through Newton–Kavli fellowship. MR, MS, and Y-LK have received funding from the European Research Council (ERC) under the European Union's Horizon 2020 Framework Programme (grant agreement no. 759194 – USNAC). ECK acknowledges support from the G.R.E.A.T research environment funded by Vetenskapsrådet, the Swedish Research Council, under project number 2016-06012, and support from The Wenner-Gren Foundation. This study is based on observations obtained with the Samuel Oschin Telescope 48- and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility (ZTF) project. ZTF is supported by the National Science Foundation under Grant No. AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, the University of Washington, Deutsches Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, and Lawrence Berkeley National Laboratories. Operations are conducted by COO, IPAC, and UW. The ZTF forced-photometry service was funded under the Heising-Simons Foundation grant. This work was supported by the GROWTH project funded by the National Science Foundation under Grant No. 1545949. This study is also based on observations made with the Nordic Optical Telescope, owned in collaboration by the University of Turku and Aarhus University, and operated jointly by Aarhus University, the University of Turku, and the University of Oslo, representing Denmark, Finland, and Norway, the University of Iceland and Stockholm University at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias. The software used for the analysis and is described below. Software: NUMPY (van der Walt, Colbert & Varoquaux 2011), ASTROPY (Astropy Collaboration 2013, 2018), SNCOSMO (Barbary et al. 2016), PHOTUTILS (Bradley et al. 2019), ZTFQUERY (Rigault 2018), SWARP (Bertin 2010), HOTPANTS (Becker 2015), ZUDS, FRINGEZ (Medford et al. 2021), LAMBDAR (Wright et al. 2016), PROSPECTOR (Leja et al. 2017), EMCEE (Foreman-Mackey et al. 2013), PYGTC (Bocquet & Carter 2016), MATPLOTLIB (Hunter 2007), and IPAC forced-photometry service. Data Availability: The ZTF images used for forced photometry are available at the Infrared Science Archive (IRSA) https://irsa.ipac.caltech.edu/Missions/ztf.html and through the ZTF data portal https://www.ztf.caltech.edu/page/dr5#12. The alerts used for initial processing are made available by ZTF in the form of an alert archive https://ztf.uw.edu/, as well as through public brokers that access that ZTF alerts in near real time. The results of running the forced-photometry pipelines that form the main data used in the analysis of this paper are provided in the form of tables in Appendix C.

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