AstroNote 2021-292

ePESSTO+, the advanced Public ESO Spectroscopic Survey for Transient Objects (Smartt et al. 2015 2015A&A...579A..40S), reports J-band imaging followup of the fading transient AT2021aeba (Fulton et al. AstroNote #2021-291)  discovered by the Pan-STARRS Search for Kilonovae. Observations were performed on the ESO New Technology Telescope (NTT) with the SOFI imager (J-band) on two epochs, exposures beginning 2021-11-12 07:13 UTC (MJD = 59530.30) and 2021-11-13 06:54 UTC (MJD = 59531.29).

The transient is significantly detected on both epochs at a position coincident with those reported by Pan-STARRS (Fulton et al. AstroNote #2021-291). We estimate J = 20 (Vega mag) calibrated against the 2MASS field stars with systematic uncertainty of +/- 0.5 mag due to the uncertainty in the background subtraction. This indicates that the object has a significant near-infrared excess.

We note that the transient is coincident with a knot of excess emission in the g and r-bands of the Pan-STARRS archival imaging which may indicate that the J-band magnitude estimate contains significant host galaxy contamination. However this likely starforming knot is not visible in the z and y-band images. 

This may indicate an unusual spectrum. The Pan-STARRS colours are blue but the J-band indicates an excess in the near-infrared which is a factor ~2 higher than what would be expected for a simpe black-body continuum fit (resulting in a blackbody temperature of 7260 +/- 730 K) to the Pan-STARRS photometry.  We conclude there is likely a near-infrared excess, but the level of excess is not well constrained. A J-band excess is not expected for a IIb shock cooling or a luminous LBV outburst. Kilonovae are expected to have multiple components.  

There is no evidence for significant fade, or flux increase, over the two nights but we caution that the host galaxy contamination precludes a definitive measurement. Further follow-up of this unusual transient is encouraged.