TITLE:   GCN CIRCULAR
NUMBER:  29508
SUBJECT: ZTF21aakruew/AT2021cwd: ZTF and LT discovery of a hostless, red, and rapidly fading optical transient
DATE:    21/02/14 00:31:34 GMT
FROM:    Anna Ho at UC Berkeley  <annayqho@berkeley.edu>

Yuhan Yao (Caltech), Daniel Perley (LJMU), Anna Ho (UCB) on behalf of the
Zwicky Transient Facility (ZTF) collaboration:

We report the discovery two nights ago of the fast optical transient
ZTF21aakruew/AT2021cwd in Zwicky Transient Facility (ZTF, Bellm et al.
2019, Graham et al. 2019) data at coordinates (J2000):

RA = 10:24:42.16 (156.17566 deg)
Dec = +11:36:40.98 (+11.61138 deg)

ZTF21aakruew was first detected at g=19.64+/-0.10 mag in an image obtained
at 2021-02-12 08:52.4 UT as part of the 2-day cadence public survey, and
passed a filter designed to find fast transients (Ho et al. 2020, ApJ, 905,
2). The most recent upper limit was one night prior: r > 20.93 mag on
2021-02-11 07:59.6 UT in an image obtained for reference construction.
Liverpool Telescope (LT) follow-up photometry showed that the source faded
by ~2.1 mag in r-band in the next 0.9 days. The transient was most recently
detected in an LT image on 2021-02-13 22:49 UT at r = 22.42 +/- 0.21 mag.
The color appears to be red, with g-r~0.9 on 2021-02-13 and low extinction
along the line of sight (E(B-V)=0.04 mag; Planck Collaboration et al.,
2015).

There do not appear to be any reported GRBs consistent with the position of
ZTF21aakruew during the interval between the last non-detection and the
first detection. No counterpart is visible in deep Legacy Survey
pre-imaging (>24 mag; Dey et al. 2019, ApJ, 157, 5).

We encourage spectroscopic classification and multi-wavelength follow-up of
ZTF21aakruew.

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. Alert distribution service provided by DIRAC@UW (Patterson et al.
2019). Alert database searches are performed using Kowalski (Duev et al.
2019).