TITLE:   GCN CIRCULAR
NUMBER:  29938
SUBJECT: ZTF and LT observations of ZTF21aayokph (AT2021lfa), a fast-fading red transient
DATE:    21/05/05 02:18:12 GMT
FROM:    Yuhan Yao at Caltech  <yyao@astro.caltech.edu>

Yuhan Yao (Caltech), Daniel Perley (LJMU), Anna Ho (UC Berkeley)

We report the discovery of a fast-evolving red transient by the Zwicky Transient Facility (ZTF) public all-sky survey and the Liverpool Telescope (LT).

ZTF21aayokph (AT2021lfa) was discovered at the position (J2000) of:
ra = 12:32:48.725 (188.203019 deg)
dec = -01:29:22.56 (-1.489601 deg)
on 2021 May 4 by ZTF at r=18.60 +/- 0.08 mag (MJD=59338.2325) and g=18.80+/-0.11 mag (MJD=59338.3126). The last upper limits were on 2021 May 2 at g > 20.58 mag (MJD=59336.2569) and r > 20.23 mag (MJD=59336.3110). The rise rate (>0.85 mag/day) of AT2021lfa is therefore very fast. The Galactic extinction towards the direction of AT2021lfa is E(g-r)=0.02. The nearest counterpart in Legacy Survey deep imaging is 2.9 arcsec away (g=24.2 mag, r=23.5 mag).

The high Galactic latitude, red color, and lack of stellar counterpart or bright host galaxy motivated us to trigger follow-up observations.

Multi-band LT imaging at 15.5 hours and 19 hours after the first detection revealed rapid fading (1.9 mag/day in r-band) and colors consistent with a non-thermal SED. At the time of the most recent LT observation (at MJD 59339.04026) we measure r=20.12+/-0.04 mag and g=20.52 +/- 0.04 mag.

We did not identify any GRBs coincident with this position during the time window between the last non-detection and the first detection.

The fast rise, fast decay, red color, and lack of counterpart of AT2021lfa make it a strong candidate afterglow.

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.