AstroNote 2025-306

The transient SN2025wny (ZTF25abnjznp), initially discovered by the Zwicky Transient Facility (ZTF) and the Gravitational-Wave Optical Transient Observer (GOTO), was recently observed to show multiple optical images in follow-up imaging with the Liverpool Telescope (Wise et al. 2025, AstroNote 2025-296).  The source was initially targeted due to its location in a candidate lensing system with a lens redshift of z=0.3754 (from DESI observations).

We have obtained multiple spectroscopic observations of the system dating back to early September, most of which have been largely featureless except for unclear broad features in the blue region.  On the night of 2025-10-21 (UT), we obtained a deep observation using the Low Resolution Imaging Spectrometer on the Keck I telescope (PI: Qin), covering images “A” and “B” (using the labels from the finder image of Wise et al.), as well as additional spectra at other orientations covering other candidate images.

The spectrum of the brightest image (“A”) was reduced using LPipe (Perley et al. 2019, PASP,  131, 4503).  The resulting spectrum shows a strong blue continuum with several broad features, as well as a series of narrow absorption features matching standard ISM/IGM lines at a common redshift of z=2.01, including HI 1217, Si II 1260, CII 1335, Si IV 1394,1403, Si II 1527, C IV 1550, Al II 1670, and possibly Mg II 2800.  The continuum is well matched by reference HST UV spectra of Type I superluminous supernovae (SN2017egm, SN2016eay) at the same redshift, although the continuum of SN 2025wny is bluer than that seen in either reference events.

We therefore classify the event as a strongly lensed Type I superluminous supernova.  At a redshift of z=2.01, the peak absolute magnitude as inferred from our LT i-band light curve (including a basic 1+z k-correction, but without lensing or extinction corrections) is approximately -25.3 at a rest-frame wavelength of 2560 Angstroms.   This would suggest a magnification factor of about 10 for a superluminous supernova at the high end of the luminosity distribution (see e.g. Angus et al. 2019, MNRAS, 487, 2215), or possibly more for a lower-luminosity SLSN or a SLSN in the presence of some host extinction.

Further observations are planned.