AstroNote 2026-65

AT2026fgk was initially detected by the Gravitational-wave Optical Transient Observer (GOTO) on MJD 61109.21 (2026-03-10 05:14:31 UT) at L = 18.84 +/- 0.06 mag (ID 294132). The transient rapidly brightened in Asteroid Terrestrial-impact Last Alert System (ATLAS), at atlaso =  16.71 +/- 0.02 mag on MJD 61109.51, indicating a rise of >6 mag/d. Subsequent Zwicky Transient Facility (ZTF) observations reveal rapid fading, with ztfg = 18.09 +/- 0.05 on MJD 61110.41 and ztfr = 17.64 +/- 0.05 on MJD 61110.49. The latest ZTF epoch yields a red color g-r ~ 0.4 (color is corrected for Galactic extinction). 

This event was identified as a candidate of interest because of its fast initial rise in ZTF (ztfg > 1 mag/d). The nearest catalogued host is identified in the Legacy Survey (LS) with a photometric redshift of z = 0.11 +/- 0.02, at an angular offset of ~0.8 arcseconds from the transient position. At this redshift, the implied peak absolute magnitude would be M_o ~ -22 mag.

At the present time, the association of the transient with the galaxy is plausible but not yet conclusive. Given its rapid nature and red colour, we encourage further community follow-up to study this rapidly-fading red transient.

This AstroNote is 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.