AstroNote 2025-196

Here we present a classification of AT2025kkb/ZTF25aaofttb as a superluminous supernova (SLSN), with an apparent peak magnitude of m_g = 19.4, a K-corrected peak absolute magnitude of M_g = -22.8, and a redshift of z = 0.58. ZTF25aaofttb was first detected by ZTF on 2025-04-24 at an apparent magnitude of m_g = 19.99 and reported to TNS by the ZTF team on 2025-05-14. ZTF25aaofttb was identified and reported as a hostless candidate by ELEPHANT (Pessi+2024) on 2025-05-27 (AstroNote 2025-159) and a SLSN candidate by ZTF (Astronote 2025-179) on 2025-06-11.

According to ZTF photometry, the SLSN peaked on 2025-05-11 and has since exhibited a slow decline to an apparent magnitude m_g = 20.3 with a K-corrected absolute magnitude of M_g = -21.9 over at least ~50 days.

Spectra of ZTF25aaofttb were recorded by the Dark Energy Spectroscopic Instrument’s (DESI) spare fiber target of opportunity program, “DESI Transients Survey” (Astronote 2025-84), on 2025-05-30 and the Gemini Multi-Object Spectrograph (GMOS) at the Gemini South Telescope on 2025-06-25 (PI: Hall). The spectra reveal a blue continuum with broad Mg II and comb-like OII lines and no H. The presence of broad Mg II and OII steps was matched through Next Generation Superfit (NGSF; Astronote 2022-191) with the best template match being SLSN-Ic PTF-09cnd. A redshift of z = 0.58 was determined due to narrow Mg II absorption lines from the IGM/ISM of the host galaxy present in both the Gemini and DESI spectrum.

Swift observations were triggered, and the first observations took place on 2025-06-08. The source was not detected with XRT, yielding an upper limit of 1.7 * 10^-2 cts/s at 0.3 - 10 keV. Given the redshift of z = 0.58, this gives a luminosity upper limit of approximately 7.5 * 10⁴⁴ erg/s. We performed forced photometry on the Swift/UVOT images yielding weak detections with uvw1 = 22.1±0.5 mag on 2025-06-08 and uvm2 = 21.7±0.4 mag on 2025-06-15.