AstroNote 2023-345

AT2023zzk (ZTF23abtycgb/) was discovered by Zwicky Transient Facility (ZTF) on MJD == 60292.165 (2023-12-14 03:57:36 UTC). It was registered by TNS Astronomical Transient Report No. 196347 (Fremling et al. 2023) with a discovery mag of m_g = 19.2 +/- 0.1. The Sherlock contextual classifier implemented in Lasair indicated the transient is most likely associated with host galaxy UGC 02805, which is at z = 0.0201 or d = 86 Mpc (from NED), hence a distance modulus of μ = 34.67 +/- 0.15. This implies a peak absolute magnitude of M_g = -16.3 +/- 0.1 and M_r = -16.1 +/- 0.1 for the g-band and r-band, respectively (after foreground extinction correction). 

We run the "Fastfinder" annotator on the Lasair broker, highlighting rapidly rising or declining transients in the ZTF public stream (and will be implemented in Lasair-LSST). The results are available on the Lasair public page https://lasair-ztf.lsst.ac.uk (under annotators). 

Fastfinder highlighted the fast-evolving nature of AT2023zzk on the 14th of December and again on the 16th of December, finding that the lightcurve is both fast rising and fast fading with a peak magnitude in the "SN, Faint-SN regime(s)".  Through model fitting, we measure a fade of dm_g/dt = 0.62 +/- 0.06 and dm_r/dt = 0.44 +/- 0.09 magnitudes per day in the g-band and r-band. There is considerable dust extinction (Ag = 0.82, Ar=0.57; Schafly et al. 2011, from NED) along the line of sight to the transient. After correction for foreground extinction, Fastfinder estimates g-r = 0.1 +/- 0.1 near the latest epoch. Fastfinder annotations are available publicly for Lasair users.

ATLAS (Tonry et al. 2018, Smith et al. 2020) observed the field 0.76 days before the ZTF discovery on MJD == 60291.41 (2023-12-13 09:53:45 UTC) in both filters c and o and measured 3-sigma upper limits of m > 20.6 in each filter. This implies a rapid rise to peak brightness. ATLAS also detected the transient right after ZTF on MJD=60292.42 at o = 19.1 +/- 0.1. 

The combined ATLAS and ZTF photometry are summarised below. The limits quoted are to 3σ and have not been corrected for foreground dust extinction. Where applicable, multiple detections within the same filter have been averaged on a nightly basis.

While the peak absolute magnitude is on the brighter end for kilonovae (e.g. Nicholl et al. 2021), the quick time scales and colour evolution are not unlike that predicted for a kilonova. A neutron star merger producing a kilonova at this distance of 86 Mpc would be a strong detection in the LIGO gravitational wave detector. However, we note that there was a ~3-hour observation gap (2023-12-13 19:50 UTC --> 2023-12-13 23:05 UTC) between the last ATLAS non-detection and the first ZTF detection where neither LIGO detector was observing.

We caution that similar characteristics are shared by shock cooling in type II and IIb supernovae and ultra-stripped supernovae, which cannot be ruled out at this stage. A foreground CV is also possible, in chance alignment with UGC 02805, although we find no evidence of historical outbursts at this position in ATLAS, Pan-STARRS or the Lasair history of ZTF. We plan further follow-up observations with Pan-STARRS. Spectroscopic classification is strongly encouraged.

The Asteroid Terrestrial-impact Last Alert System [ATLAS] project is primarily funded to search for Near-Earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from the survey area. This work was partially funded by Kepler/K2 grant J1944/80NSSC19K0112 and HST GO-15889, and STFC grants ST/T000198/1 and ST/S006109/1. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen's University Belfast, the Space Telescope Science Institute, the South African Astronomical Observatory, and The Millennium Institute of Astrophysics (MAS), Chile. 

Lasair is supported by the UKRI Science and Technology Facilities Council and is a collaboration between the University of Edinburgh (grant ST/N002512/1) and Queen’s University Belfast (grant ST/N002520/1) within the LSST:UK Science Consortium.