AstroNote 2025-302

AT2025aarn was initially detected by the Zwicky Transient Facility (ZTF) on MJD 60964.478113 (2025-10-16 11:28:29) at g = 20.96 ± 0.30 mag (internal name ZTF25abxjgzg), although it was not flagged as an alert. One night later, it had brightened by 3.7 magnitudes and exhibited blue colors (g-r ~ -0.3), which triggered the release of a ZTF alert. It was flagged as a transient of interest due to its rapid rise and proximity (5” offset) to a diffuse galaxy in DESI DR1, which exhibits several emission lines (Hα, Hβ, [OIII], [OII], [NII], [SII]) at a common redshift of 0.0157. Photometric observations by the SED Machine (g = 16.69 ± 0.02, r = 17.01 ± 0.04, i = 17.17 ± 0.01) on the P60 telescope at Palomar obtained around the time of the first ZTF alert further confirmed the transient’s blue color.

On MJD 60966.380, we observed the transient using the Kast double spectrograph on the 3.05-meter C. Donald Shane telescope at the Lick Observatory. We exposed for a total of 1570s, and covered a wavelength range of 3510Å to 10850Å. We used the 300/7500 grating on the red side, which has a dispersion of 2.55 Å/pixel, and the 600/4310 grism on the blue side, which has a dispersion of 1.02 Å/pixel. We reduced the spectrum with the UCSC spectral pipeline (Siebert et al., 2020) using the standard star BD+28 4211, taken on the previous night, for flux calibration. This spectrum shows a blue, featureless continuum with faint, narrow H-alpha emission consistent with the redshift of the coincident host candidate.

We also observed the transient at MJD 60967.046 with the SPectrograph for the Rapid Acquisition of Transients (SPRAT) on the 2.0-meter Liverpool Telescope (LT) with a blue-optimised setup. We exposed on the target for a total exposure time of 1200s, with a wavelength range of 4000Å to 8000Å. Observations were reduced using a custom Python reduction pipeline, using the spectroscopic standard star Hiltner 102 for flux calibration. This spectrum also revealed a blue, featureless continuum.

On MJD 60967.040, we obtained photometry using the IO:O camera on the LT. We exposed for 40s in SDSS g-band, and 50s in SDSS r-band and i-band. We reduced the observations using a custom Python reduction pipeline, using Pan-STARRS1 (PS1) foreground reference stars to calculate zeropoints. We find magnitudes of g = 16.66 ± 0.02, r = 16.86 ± 0.04 and i = 17.12 ± 0.03. This is broadly consistent with the Palomar P60 photometry obtained the previous night, with a slight possible rise in r-band from the first P60 detection. 

We also obtained a Swift observation totaling 4ks at 2025 Oct 18, 22:10:12 UTC. The source was marginally detected (3-sigma) at 1.3 * 10^-3 cts/s at 0.3 - 10 keV. Given the DESI host galaxy’s redshift of z = 0.0157, this gives a rough luminosity of 3.9 * 10⁴⁰ erg/s. 

The luminosity, rise time, and featureless spectrum are characteristic of a young Type II supernova, but the X-ray detection is unexpected. The initial blue color, fast rise, and X-ray detection could also be explained as an LFBOT, but the luminosity is much lower than that of AT2018cow and the color is reddening.  A foreground CV would normally show a faster rise to peak, but a CV has not strictly been ruled out. Given the rapid rise and possible extragalactic nature of the transient, particularly the tentative X-ray detection, we encourage additional follow-up observations to determine its nature.

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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.

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.

We would like to thank the staff at Lick Observatory for enabling these observations.