TITLE:   GCN CIRCULAR
NUMBER:  28005
SUBJECT: IceCube-200620A: One Candidate Counterpart from the Zwicky Transient Facility
DATE:    20/06/21 19:16:19 GMT
FROM:    Simeon Reusch at DESY  <simeon.reusch@desy.de>

Simeon Reusch, Robert Stein and Anna Franckowiak (DESY) report:

On behalf of the Zwicky Transient Facility (ZTF) and Global Relay of
Observatories Watching Transients Happen (GROWTH) collaborations:

We observed the localization region of the neutrino event
IceCube-200620A (Santander et. al, GCN 27997) with the Palomar 48-inch
telescope, equipped with the 47 square degree ZTF camera (Bellm et al.
2019, Graham et al. 2019). We started observations in the r-band
beginning at 2020-06-21T04:49 UTC, approximately 25.8 hours after event
time. We covered 1.2 sq deg, corresponding to 100.0% of the reported
localization region (this estimate does not account for chip gaps). Each
exposure was 300s with a typical depth of 21.0 mag.

The images were processed in real-time through the ZTF reduction and
image subtraction pipelines at IPAC to search for potential counterparts
(Masci et al. 2019). AMPEL (Nordin et al. 2019) was used to search the
alerts database for candidates. We reject stellar sources (Tachibana and
Miller 2018) and moving objects, and apply machine learning algorithms
(Mahabal et al. 2019). We are left with the following high-significance
transient candidate by our pipeline, lying within the 90.0% localization
of the skymap.

+---------------------------------------------------------------------------------+
| ZTF Name     | IAU Name   | RA (deg)    | DEC (deg)   | Filter | Mag
 | MagErr |
+---------------------------------------------------------------------------------+
| ZTF20abgvabi | AT 2020ncr | 162.5306820 | +12.1462203 | r      | 20.67
| 0.10   |
+---------------------------------------------------------------------------------+

AT 2020ncr has a faint host with a SDSS photometric redshift estimate of
z=0.51 +/- 0.09. This would result in an estimated absolute magnitude of
-21.7. There have been optical pre-detections up to 120 days prior to
neutrino arrival time. As the nature of the transient is unclear,
spectroscopic follow-up is recommended.

ZTF and GROWTH are worldwide collaborations comprising Caltech, USA;
IPAC, USA, WIS, Israel; OKC, Sweden; JSI/UMd, USA; U Washington, USA;
DESY, Germany; MOST, Taiwan; UW Milwaukee, USA; LANL USA; Tokyo Tech,
Japan; IITB, India; IIA, India; LJMU, UK; TTU, USA; SDSU, USA and USyd,
Australia.
ZTF acknowledges the generous support of the NSF under AST MSIP Grant No
1440341.
GROWTH acknowledges generous support of the NSF under PIRE Grant No 1545949.
Alert distribution service provided by DIRAC@UW (Patterson et al. 2019).
Alert database searches are done by AMPEL (Nordin et al. 2019).