The broad-lined type Ic supernova 2020lao experienced an energetic explosion with no central-engine signatures

¹ Department of Physics and Astronomy, Chalmers University of Technology, SE-412 96 Göteborg, Sweden
² Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C, Denmark
³ National Astronomical Observatory of Japan, National Institutes of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁴ Graduate Institute for Advanced Studies, SOKENDAI, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁵ School of Physics and Astronomy, Monash University, Clayton, VIC 3800, Australia
⁶ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁷ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching bei München, Germany
⁸ The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova 10691, Stockholm, Sweden
⁹ Institute for Astronomy, University of Hawai’i, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
¹⁰ Institut d’Estudis Espacials de Catalunya (IEEC), E-08034 Barcelona, Spain
¹¹ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, E-08193 Barcelona, Spain
¹² Planetary Science Institute, 1700 East Fort Lowell Road, Suite 106 Tucson, AZ 85719-2395, USA
¹³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218-2410, USA
¹⁴ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy
¹⁵ Department of Physics, Florida State University, 77 Chieftain Way, Tallahassee, FL 32306, USA
¹⁶ INAF – Astrophysics and Space Science Observatory, Via P. Gobetti 101, 40129 Bologna, Italy
¹⁷ School of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK

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Received: 3 December 2025
Accepted: 7 March 2026

Abstract

We present infant-phase observations of the broad-line Type Ic supernova (SN Ic-BL) 2020lao, including optical spectroscopy beginning within about 48 hours of the inferred explosion epoch and extending to nearly 100 days. The explosion time was constrained by power-law fits to the rising TESS and ZTF light curves, with the first ZTF detection occurring only ∼27 hours after explosion. The optical light curves show a rapid rise that lasted for ≈8.8 days and a peak luminosity typical of SNe Ic-BL (i.e., M_r ≃ −18.5 mag). Unlike some engine-driven SN Ic-BL events, the early light curve of SN 2020lao shows no evidence of an optical afterglow or excess emission, and the absence of any detectable shock–cooling component in the TESS and ZTF data constrains the progenitor to a compact Wolf-Rayet-like star whose R_★ is less than or equal to a few times the R_⊙, ruling out any extended envelope. The spectra resemble those of the X-ray-flash-associated SN 2006aj but with systematically higher expansion velocities. From Arnett-type fits to the bolometric light curve and measured Fe IIλ5169 line velocities, we infer a ⁵⁶Ni mass of 0.23 ± 0.03 M_⊙, an ejecta mass (M_ej) of 3.2 ± 0.8 M_⊙, and a kinetic energy (E_K) of ∼(23.1 ± 12.4)×10⁵¹ erg, corresponding to a specific kinetic energy (E_K/M_ej) of ≈(7.2 ± 3.5)×10⁵¹ erg M_⊙⁻¹. Spectral synthesis modeling broadly reproduces the photospheric-phase spectra of SN 2020lao and suggests E_K/M_ej ≈ 4.9 × 10⁵¹ erg M_⊙⁻¹. SN 2020lao and SN 2006aj synthesized comparable amounts of ⁵⁶Ni, yet SN 2020lao exhibits E_K/M_ej values on the order of 5–10 times larger. Published VLA and Swift/XRT non-detections reveal no afterglow emission, allowing us to place stringent limits on relativistic ejecta and dense circumstellar material. Given that SN 2020lao reaches a specific kinetic energy typical of engine-driven SNe Ic-BL, the lack of an early optical excess together with the non-detections in the radio and X-ray bands suggests that if a relativistic jet was launched, the explosion must have been viewed far off axis or the jet was choked before breakout. If there was no relativistic jet, SN 2020lao would therefore be an extreme nonrelativistic SN Ic-BL. This underscores the importance of continued infant-phase, multiwavelength monitoring of these explosions.