A Natural $\gtrsim 100\times$ Telescope: Discovery of the Strongly Lensed Type II SN 2025mkn at $z=1.37$
astro-ph.CO
/ Authors
Cameron Lemon, Ariel Goobar, Joel Johansson, Edvard Mörtsell, Steve Schulze, Igor Andreoni, Aleksandra Bochenek, Seán J. Brennan, Malte Busmann, Michael Coughlin
and 83 more authors
Kaustav K. Das, Suhail Dhawan, Christoffer Fremling, Anjasha Gangopadhyay, Daniel Gruen, Xander J. Hall, Anna Y. Q. Ho, Mansi M. Kasliwal, Daniel A. Perley, Mickael Rigault, Genevieve Schroeder, Mathew Smith, Jesper Sollerman, Jean J. Somalwar, Robert Stein, Stephen Thorp, Alice Townsend, Jacob L. Wise, Lin Yan, Nikki Arendse, Eric C. Bellm, Tracy X. Chen, Andrew Drake
/ Abstract
We present the discovery of SN 2025mkn, a gravitationally lensed Type II supernova. First detected as a blue transient in ZTF, 0.83$^{\prime\prime}$ from a $z=0.42$ elliptical galaxy, follow-up SNIFS/UH2.2m 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 $\sim 0.07^{\prime\prime}$, and a 30 times fainter counterimage opposite the lens, for which NIRSpec reveals strong H$α$ emission also at $z=1.371$. The light curves and spectra are consistent with the Type II supernova source being magnified $\gtrsim 100$ times, with $\sim 250$ 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. Light-curve-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.