AstroNote 2025-123

AT 2021gqu (https://alerce.online/object/ZTF21aapriwm) was discovered in the ZTF public alert stream on March 18, 2021 at RA = 12:40:00.598, Dec = +01:09:25.32 (J2000), with a g-band difference magnitude 19.60+-0.10 mag; it was reported to TNS by ALeRCE (Forster et al. 2021). It rose to a peak g-band difference magnitude of 18.76+-0.15 mag, 12 days after discovery. Prior to the re-brightening or second event, the last detection in the public ZTF alert stream was on April 21, 2021 with a r-band difference magnitude 19.35+-0.15 mag. It had no detections in the public ZTF alert stream until 1460 days later (or 1494 days after discovery), on April 20, 2025 with a g-band difference magnitude of 18.90+-0.11 mag. Since then it has been rising, having its most recent detection on April 22, 2025 with a g-band difference magnitude 18.41+-0.07 mag.

We attach a figure that displays the AT 2021gqu / ZTF21aapriwm light curve, based on PSF forced photometry on ZTF difference images (Masci et al. 2023) at its mean location. Such data show no signs of fainter events between the two main ones or prior to the first event.

AT 2021gqu / ZTF21aapriwm is located towards SDSS J124000.57+010924.3, a galaxy that has spectroscopic redshift z = 0.07959 in SDSS DR16 (Ahumada et al. 2020). The location of this galaxy in a BPT diagram is listed by Mamon et al. (2020) as in the transition between curves of Kewley et al. (2001) and Kauffmann et al. (2003).

The first transient event was photometrically classified as SNIbc by Aleo et al. (2023) using ParSNIP on YSE DR1 data. In the ZTF public alert stream, detection coordinates for the first transient event lie at 0.9 arcsec from the host nucleus, while recent detections lie at 0.6 arcsec from the first event mean coordinates and are closer to the host nucleus. We encourage additional follow-up observations of this event.

This report is based on observations obtained with the Samuel Oschin 48-inch Telescope and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. The ZTF forced-photometry service was funded under the Heising-Simons Foundation grant #12540303 (PI: M. J. Graham). We acknowledge the use of the ALeRCE broker (Forster et al. 2021), in particular its Web Interface and the ZTF Forced Photometry Notebook.