A Natural ≳100× Telescope : Discovery of the Strongly Lensed Type II SN 2025mkn at z = 1.37

Lemon, Cameron and Goobar, Ariel and Johansson, Joel and Mörtsell, Edvard and Schulze, Steve and Andreoni, Igor and Bochenek, Aleksandra and Brennan, Seán J. and Busmann, Malte and Coughlin, Michael and Das, Kaustav K. and Dhawan, Suhail and Fremling, Christoffer and Gangopadhyay, Anjasha and Gruen, Daniel and Hall, Xander J. and Ho, Anna Y. Q. and Kasliwal, Mansi M. and Perley, Daniel A. and Rigault, Mickael and Schroeder, Genevieve and Smith, Mathew and Sollerman, Jesper and Somalwar, Jean J. and Stein, Robert and Thorp, Stephen and Townsend, Alice and Wise, Jacob L. and Yan, Lin and Arendse, Nikki and Bellm, Eric C. and Chen, Tracy X. and Drake, Andrew and Masci, Frank J. and Purdum, Josiah and Smith, Roger and Hinkle, Jason T. and Rivera-Thorsen, T. Emil and Shappee, Benjamin J. and Tucker, Michael A. and Aguilar, Jessica and Ahlen, Steven and Aldering, Greg and BenZvi, Segev and Bianchi, Davide and Brooks, David and Claybaugh, Todd and de la Macorra, Axel and Della Costa, John and Dey, Arjun and Doel, Peter and Flaugher, Brenna and Font-Ribera, Andreu and Forero-Romero, Jaime E. and Gaztañaga, Enrique and Gontcho A. Gontcho, Satya and Gutierrez, Gaston and Huterer, Dragan and Ishak, Mustapha and Jimenez, Jorge and Joyce, Dick and Juneau, Stephanie and Kehoe, Robert and Kim, Alex G. and Kirkby, David and Kisner, Theodore and Kremin, Anthony and Lahav, Ofer and Landriau, Martin and Le Guillou, Laurent and Levi, Michael E. and Manera, Marc and Meisner, Aaron and Miquel, Ramon and Moustakas, John and Nadathur, Seshadri and O’Connor, Brendan and Palanque-Delabrouille, Nathalie and Palmese, Antonella and Percival, Will J. and Pérez-Ràfols, Ignasi and Poppett, Claire and Prada, Francisco and Rossi, Graziano and Sanchez, Eusebio and Schlegel, David and Schubnell, Michael and Shafieloo, Arman and Silber, Joseph and Sprayberry, David and Tarlé, Gregory and Weaver, Benjamin A. and Zou, Hu (2026) A Natural ≳100× Telescope : Discovery of the Strongly Lensed Type II SN 2025mkn at z = 1.37. Astrophysical Journal Letters, 1003 (2): L47. ISSN 2041-8205

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Abstract

We present the discovery of SN 2025mkn, a gravitationally lensed Type II supernova. First detected as a blue transient in Zwicky Transient Facility (ZTF), 0 .″ 83 from a z = 0.42 elliptical galaxy, the follow-up SNIFS/UH2.2 m and LRIS/Keck spectra revealed absorption lines at z = 1.371. Later JWST NIRCam imaging shows that the bright transient is a close pair of point sources separated by ∼0.″07 , and a 30 times fainter counterimage opposite the lens, for which NIRSpec reveals strong Hα emission also at z = 1.371. The lightcurves and spectra are consistent with the Type II supernova source being magnified ≳100 times, with ∼250 required to reconcile its luminosity with that of nearby events such as SN 2023ixf. Lens models are consistent with such high magnifications, and always show that the faint image arrived first (undetected in earlier ZTF imaging), consistent with the later spectral phase of this fainter image. A fourth image is also predicted and possibly detected in the NIRSpec data. Lightcurve-based time-delay measurements are not possible due to the first image being the faintest; however, the resolved NIRSpec spectra offer a future opportunity for time-delay cosmography through supernova phase measurements.