AstroNote 2023-17

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

ZTF23aaarlti (AT2023avj) was discovered at the position (J2000) of:

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. From the most recent LT observation, we also measure i = 21.95 +/- 0.13 mag (MJD=59971.06), and g = 23.16 +/- 0.24 mag (MJD=59971.07). 

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 Wiscnsin at Milwaukee, Trinity College Dublin, Lawrence Livermore National Laboratories, and IN2P3, France. Operations are conducted by COO, IPAC, and UW.