AstroNote 2025-333

2025afcr was first detected by the Zwicky Transient Facility (ZTF; Bellm et al. 2019, Graham et al. 2019) on UT 2025-11-24 at a magnitude of g = 18.816 ± 0.108, and reported to TNS on UT 2025-11-28 under the identifier ZTF25acemaph (TNS Astronomical Transient Report No. 278583). The transient is offset by ~0.4" from a compact blue source which is classified as a star by the Sloan Digital Sky Survey (SDSS). However, this object was categorised in the Legacy Survey DR10 as a potentially distant object with a photometric redshift of z = 0.441 ± 0.132. Two days after first detection by ZTF, the transient had also brightened by ~0.3 mag in g, and had a colour of g - r ~ -0.45. This motivated us to follow-up this transient as a candidate superluminous event.

An initial spectrum was obtained using the SED Machine (SEDM; Blagorodnova et al. 2018, Rigault et al. 2019) ~4 days post-first detection on MJD 61007.50, showing a blue continuum. This motivated us to obtain a further follow-up spectrum using the Next Generation Palomar Spectrograph (NGPS; AstroNote 2024-340) on the Palomar 5.1m Hale (P200) telescope, which was obtained on MJD 61009.53 with a spectral coverage between ~5620-10240Å. This spectrum displayed a mostly featureless, blue continuum with superimposed narrow H-alpha and [OIII] emission features from star formation in the host galaxy at a redshift of z = 0.1335. At approximately the same time of this observation the transient had risen to a magnitude of g = 18.016 ± 0.073 in ZTF photometry, corresponding to an absolute magnitude of M_g ~ -21, confirming its superluminous nature.

On MJD 61011.25, we obtained photometry and spectroscopy using the IO:O camera (Steele et al. 2004) and the SPectrograph for the Rapid Acquisition of Transients (SPRAT; Piascik et al. 2014) on the Liverpool Telescope (LT), utilising a blue-optimised setup for SPRAT with a spectral coverage between ~4000-8100Å. Our imaging showed that the transient had continued to rise and continued to show blue colours, with host-subtracted photometric measurements of u = 17.634 ± 0.033, g = 17.839 ± 0.026, r = 18.220 ± 0.032, i = 18.425 ± 0.037 and z = 18.708 ± 0.065. Our spectrum showed a blue continuum with OII absorption troughs in the blue characteristic of Type-I superluminous supernova (SLSN-I) events. Fits of this spectrum to templates from previously classified events also yielded good matches to pre-maximum spectra of the SLSNe-I 2017egm, 2010gx and Gaia16apd. This motivated us to classify 2025afcr as a SLSN-I (TNS Classification Report No. 22496).

On MJD 61011.34, we also obtained an observation using the Ultraviolet-Optical Telescope (UVOT; Roming et al. 2005) on the Neil Gehrels Swift Observatory (Gehrels et al. 2004). This observation yielded AB magnitudes of u = 17.45 ± 0.09, uvw1 = 17.50 ± 0.06, uvm2 = 17.71 ± 0.05 and uvw2 = 17.93 ± 0.05. Combined with our LT photometry, this UV brightness suggests a very high photospheric temperature for 2025afcr.

As this event is still rising to peak, we encourage follow-up observations.

Classification performed using Next Generation Superfit (NGSF; AstroNote 2022-191, Howell et al. 2005).

Based on observations made with the Liverpool Telescope operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias with financial support from the UK Science and Technology Facilities Council.

Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Award #2407588 and a partnership including Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, USA; University of Wisconsin at Milwaukee, USA; Cornell University, USA; Drexel University, USA; University of North Carolina at Chapel Hill, USA; Institute of Science and Technology, Austria; National Central University, Taiwan, and OKC, University of Stockholm, Sweden. Operations are conducted by Caltech's Optical Observatory (COO), Caltech/IPAC, and the University of Washington at Seattle, USA.