AstroNote 2021-86

We draw attention to the unusual lightcurve of AT2021esi from the combined ZTF and ATLAS survey data and Pan-STARRS1 observations, and encourage rapid spectroscopic classification.  

AT2021esi was discovered by the Zwicky Transient Facility ZTF (Bellm et al. 2019 2019PASP..131a8002B) on 2021-03-07 (MJD=59280.15) with an internal name ZTF21aaoiaar. It is likely associated with the galaxy SDSS J065003.1+272909.9, locating 0.00'' N, 2.31'' E from the galaxy centre. 

The transient peaked at g =18.49, r = 18.65 on 59280.16, adopting a host photo-z=0.031 (taken from the SDSS DR16) implies the transient would be an absolute mag of Mg = -17.3 mag. 

After applying forced photometry (on the co-added 4 x 30sec nightly exposures) from the ATLAS survey (Smith et al. 2020, PASP, 132, 5002), we find ATLAS c > 20.7 mag (5 sigma limit) on 59277.32. Comparing this deep non-dection with the ZTF discovery, the object has a rapidly rising ~ 2 magnitudes within 3 days. 

The latest ZTF data stream processed through the Lasair broker (Smith, Williams, Young et al. 2019 2019RNAAS...3a..26S) show the g = 20.10 +/- 0.13 mag on 59290.21 and r = 19.41 +/- 0.11 mag on 59290.24. The rapid decline is confirmed by a forced detection in ATLAS of c = 20.09 +/- 0.18 on 59287.28. This indicates a fast decline (1.5 mag in 10 days) and SED cooling (g-r is blue at peak and has reddened significantly). 

https://lasair.roe.ac.uk/object/ZTF21aaoiaar/

We measured the following from Pan-STARRS1 (Chambers et al. 2016, arXiv:1612.05560), photometric observations on 2020-03-18 (59291.44) : 

g = 19.98 +/- 0.16,
r = 19.44 +/- 0.08,
i = 19.45 +/- 0.10, and 
z = 19.36 +/- 0.13 mag.

The object could be similar to the rapidly evolving SN2019bkc (Prentice et al. 2020, A&A 635, 186, Chen et al. 2020, ApJ, 889, LC) or AT2018kzr (McBrien et al. 2019, 885, L23 , Gillanders et al. 2020, MNRAS, 497, 246). It could also be the cooling tail of the shock breakout from a IIb supernova which are frequently detected (e.g. SN1993J, Richmond et al. 1994 AJ, 107, 1022  and SN2016gkg, Arcavi et al. 2017, ApJ, 837, L2). 

We compared the photometric evolution of AT2021esi  with the locus of viable kilonova models from Nicholl et al. (2021 arXiv:2102.02229). We placed the models onto a contour probability function of decline rate versus absolute magnitude by applying a Gaussian Kernel Density Estimation. The decline rate of AT2021esi is on par with the slowest declining kilonovae in the model set but is intrinsically more luminous.

Hence we encourage spectroscopic follow-up to determine the nature of this unusual object.