Early light curve excess in Type IIb supernovae observed with ATLAS

Qualitative constraints on progenitor systems

¹ Instituto de Astrofísica, Departamento de Física, Facultad de Ciencias Exactas, Universidad Andrés Bello, Fernández Concha 700, Las Condes, Santiago RM, Chile
² Millennium Institute of Astrophysics, Nuncio Monseñor Sotero Sanz 100, Of. 104, Providencia, Santiago, Chile
³ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
⁴ Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica, Chile
⁵ Data and Artificial Intelligence Initiative (IDIA), Faculty of Physical and Mathematical Sciences, Universidad de Chile, Santiago, Chile
⁶ Center for Mathematical Modeling, Universidad de Chile, Beauchef 851, Santiago 8370456, Chile
⁷ Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
⁸ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, N. Ireland BT7 1NN, United Kingdom
⁹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore MD 21218, USA
¹⁰ Physics and Astronomy Department, Johns Hopkins University Baltimore MD 21218, USA
¹¹ Astronomical Observatory Institute, Faculty of Physics and Astronomy, Adam Mickiewicz University, ul. Słoneczna 36, 60-286 Poznań, Poland
¹² South African Astronomical Observatory, Cape Town 7925, South Africa
¹³ Department of Physics, Stellenbosch University, Stellenbosch 7602, South Africa

^⋆ Corresponding author: b.ayalainostroza@gmail.com

Received: 4 March 2025
Accepted: 11 June 2025

Abstract

Context. Type IIb supernovae (SNe IIb) often exhibit an early light curve excess (EE) preceding the main peak powered by ⁵⁶Ni decay. The physical origin of this early emission remains an open question. Among the proposed scenarios, shock cooling (SC) emission, resulting from the interaction of the shock wave with extended envelopes, is considered the most plausible mechanism. However, the occurrence rate of such events has yet to be reliably constrained.

Aims. This study aims to quantify the frequency of EE in SNe IIb and investigate its physical origin by analysing optical light curves from the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey, ultimately providing qualitative constraints on their progenitor systems.

Methods. We identified 74 potential SNe IIb from 153 spectroscopically classified events reported in the Transient Name Server (TNS), observed by ATLAS with peak fluxes exceeding 150 μJy (18.46 mag) and explosion epoch uncertainties below six days. Using a spectral reclassification method, we selected a sample of 66 SNe IIb and a cleaned sample of 59 SNe IIb for analysis. We then applied light curve model fitting and outlier analysis to identify objects exhibiting EE emission and studied their photometric properties.

Results. We identify 20 SNe IIb with EE, corresponding to a frequency of approximately 30.5% to 50%, the higher value being obtained under the most stringent observational data-quality cuts. The duration and colour evolution of the early excess support its interpretation as shock cooling in extended envelopes. We also find that EE SNe IIb exhibit faster post-peak declines than non-EE events, while both groups show similar peak absolute magnitudes and rise-time distributions.

Conclusions. Our findings suggest that EE and non-EE SNe IIb likely share similar initial progenitor masses but differ in their ejecta properties, potentially due to varying degrees of binary interaction. This study constrains EE SNe frequency and photometric properties, paving the way for future theoretical work, such as hydrodynamical modelling of EE SNe light curves, which could corroborate these results and contribute to constraining the evolutionary pathways of SNe IIb progenitor systems.