AstroNote 2026-36

AT2026dbl was first detected by the Zwicky Transient Facility on MJD 61085.35 at a magnitude of g = 18.68 ± 0.10, and reported to TNS at MJD 61085.48 under the identifier ZTF26aaglqak (TNS Report No. 291251). The transient is located off-nucleus from a faint coincident host candidate with an SDSS photometric redshift of z = 0.518 ± 0.121 and a Legacy Survey photometric redshift of z = 0.411 ± 0.164. As the last photometric upper limit for this event was r > 20.26 on MJD 61080.32, we could not place a robust constraint on its rise time. However, its extremely blue colour of g - r ~ -0.5 mag on MJD 61085.35 motivated us to obtain photometric observations with the IO:O imager on the Liverpool Telescope (LT).

We observed the transient with LT IO:O on MJD 61087.00 using SDSS ugriz filters. Image-subtraction was performed using Pan-STARRS1 reference imaging for griz filters, and using SDSS reference imaging for u-band, followed by PSF photometry. This showed that the transient had faded slightly since discovery, with a g-band magnitude of g = 18.84 ± 0.04. Its colour was exceptionally blue, with g - r ~ -0.4 mag and u - g ~ -0.6 mag. In a subsequent epoch on MJD 61087.89, we found that it had faded to g = 19.09 ± 0.06 whilst maintaining blue colours of g - r ~ -0.2 mag and u - g ~ -0.7 mag.

Following the confirmation of its blue colour and fading lightcurve, a spectrum was obtained on MJD 61088.25 using the Low Resolution Imaging Spectrometer on the Keck I telescope (PI: Jacobson-Galan) and reduced using LPipe (Perley et al. 2019, PASP,  131, 4503). This spectrum showed a featureless blue continuum with absorption features matching the Mg II doublet at 2796Å and 2803Å at a redshift of z = 0.190 ± 0.001. We also detect emission features from the host galaxy from [OII], [OIII], and H-alpha at a consistent redshift at a separate extraction at the host location. This implies a MW extinction corrected (E(B-V) = 0.03) and K-corrected absolute magnitude of M_u = -21.62 ± 0.10 at MJD 61087.00.

AT2026dbl's luminosity, blue colours, featureless spectrum and promptly fading lightcurve are consistent with behaviour seen in luminous fast blue optical transients (LFBOTs) such as AT2024wpp and AT2018cow at a similar phase, although we cannot currently rule out other scenarios. We are currently pursuing additional follow-up observations to constrain the nature of the transient.

Based on observations obtained with the Samuel Oschin Telescope 48-inch 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.