TITLE:   GCN CIRCULAR
NUMBER:  33226
SUBJECT: ZTF and LT Observations of AT2023avj, a Candidate Optical Afterglow
DATE:    23/01/27 02:10:08 GMT
FROM:    Anna Ho at UC Berkeley  <annayqho@berkeley.edu>

Kailai Wang (Cornell), Anna Y. Q. Ho (Cornell), Daniel Perley (LJMU) on
behalf of the Zwicky Transient Facility (ZTF) collaboration:

We report the discovery of a fast-evolving red transient in Zwicky
Transient Facility (ZTF) partnership data and Liverpool Telescope (LT) data.

ZTF23aaarlti (AT2023avj) was discovered at the position (J2000) of:
RA = 09:39:20.82 (144.83673 deg)
Dec = +58:08:12.52 (58.13681 deg)

on 2023 January 22 by ZTF at i = 19.27 +/- 0.27 (MJD=59966.29) and g =
 20.02 +/- 0.20 (MJD=59966.32).  Forced photometry on P48 images revealed
an additional r-band detection (r=20.14+/-0.19; MJD 59966.39) as well as
limits the previous night of g > 20.18 mag (MJD 59965.37) and r > 21.14 mag
(MJD 59965.35). The rise rate (>1 mag/day in r-band) of AT2023avj was
therefore very fast.

The Galactic latitude of AT2023avj is 45 degrees, and the Galactic
extinction towards the direction of AT2023avj is low: E(B-V)=0.01 from
Schlafly & Finkbeiner (2011). There is no counterpart within several
arcseconds in Legacy Survey DR9 imaging.

The red color, fast rise rate, and lack of a stellar counterpart or bright
host galaxy motivated us to trigger follow-up observations.

LT griz imaging at 4.7 days after the first ZTF detection confirmed the red
colors. With a detection at r=23.08 +/- 0.25 (MJD=59971.05), the implied
average fading rate is 0.46 mag/day in r-band.

The fast rise, fast decay, red color, and lack of archival optical
counterpart of AT2023avj make it a strong candidate afterglow. However, we
did not identify any GRBs coincident with this position during the time
window between the last non-detection and the first detection.

ZTF is supported by the National Science Foundation under Grant No.
AST-2034437 and a collaboration including Caltech, IPAC, the Weizmann
Institute for Science, the Oskar Klein Center at Stockholm University, the
University of Maryland, Deutsches Elektronen-Synchrotron and Humboldt
University, the TANGO Consortium of Taiwan, the University of Wisconsin at
Milwaukee, Trinity College Dublin, Lawrence Livermore National
Laboratories, and IN2P3, France. Operations are conducted by COO, IPAC, and
UW.