TITLE:   GCN CIRCULAR
NUMBER:  27799
SUBJECT: ZTF Discovery of ZTF20abbiixp: The Likely Optical Afterglow to GRB 200524A
DATE:    20/05/24 18:47:58 GMT
FROM:    Anna Ho at Caltech  <annayqho@gmail.com>

Anna Y. Q. Ho, Yuhan Yao (Caltech), Daniel A. Perley (LJMU) report on
behalf of the Zwicky Transient Facility collaboration:

The Zwicky Transient Facility (ZTF, Bellm et al. 2019, Graham et al. 2019)
discovered ZTF20abbiixp, a fast transient at
14h12m10.33s +60d54m19.0s (J2000)
213.043037 +60.905288 (J2000)
as part of the ZTF Uniform Depth Survey (Goldstein et al. in prep).

ZTF20abbiixp was first detected at r~17.4 mag on UT 2020-05-24T06:52:07 and
faded by 0.3 mag over the next 30 minutes, with a total of four
observations showing continuous fading. The last non-detection was 24 hours
prior (2020-05-23T06:43:03) with a limiting magnitude of r>20.4 mag. There
is no counterpart in ZTF reference images down to a limiting magnitude of
r>22.90.

We searched the GCN archive and determined that ZTF20abbiixp is in the
error circle of GRB 200524A, which was detected by Fermi-GBM and Fermi-LAT
(GCN 27797). The position of ZTF20abbiixp is 0.151 degrees offset from the
LAT position, within the 0.2 degree error radius (90% containment).

The time of GRB 200524A was 1.802 hours prior to the first detection of
ZTF20abbiixp. We conclude that ZTF20abbiixp is very likely to be the GRB
afterglow.

Follow-up observations are highly encouraged.

ZTF is a project led by PI S. R. Kulkarni at Caltech (see ATEL #11266), and
includes IPAC; WIS, Israel; OKC, Sweden; JSI/UMd, USA; UW,USA; DESY,
Germany; NRC, Taiwan; UW Milwaukee, USA and LANL USA. ZTF acknowledges the
generous support of the NSF under AST MSIP Grant No 1440341. Alert
distribution service provided by DIRAC@UW. Alert filtering is being
undertaken by the GROWTH marshal system, supported by NSF PIRE grant
1545949.