AstroNote 2021-10

We report the discovery of ZTF21aabgpub / AT 2021ia, a transient reported by the ALeRCE broker (TNS #95604) using the ZTF public alert stream. The object is detected on a ZTF r-band difference image obtained on Jan 4th, 2021 at 12:55 UT with 18.32+-0.20 mag, with no previously reported non-detections in the ZTF public stream bands. Since then, there were two subsequent detections at 18.48 mag (g-band) and 18.20 mag (r-band) on Jan 5th, 2021 and Jan 7th, 2021 respectively. Its ZTF light curve can be explored in https://alerce.online/object/ZTF21aabgpub. ZTF21aabgpub is located towards 2M J1134-2103, a quadruply imaged quasar (Lucey et al. 2018).

A search for previous ZTF detections in the line of sight of the gravitationally lensed system revealed another transient, ZTF20aaehksw, detected on a ZTF r-band difference image obtained on Jan 3rd, 2020 at 11:31 UT with 19.05+-0.24 mag. This object shows a second detection at 19.01 mag on Jan 6th, 2020, and a third detection at 18.23 mag on Dec 21st, 2020 (350 days later), both in the r-band. Since then it has two r-band and three g-band subsequent detections, the last one on Jan 7th, 2021. Its ZTF light curve can be explored in https://alerce.online/object/ZTF20aaehksw.

The angular separation between the average coordinates of ZTF21aabgpub and ZTF20aaehksw is ~0.66 arcsec. Both objects lie towards the A quasar image, but show faint elongation extending toward images B and C of the lensed system (image naming scheme from Lucey et al. 2018). All quoted magnitudes are difference magnitudes. We encourage additional follow-up observations of this transient.