TITLE:   GCN CIRCULAR
NUMBER:  28441
SUBJECT: IceCube-200916A: Two Candidate Counterparts from the Zwicky Transient Facility
DATE:    20/09/17 17:23:43 GMT
FROM:    Simeon Reusch at DESY  <simeon.reusch@desy.de>

Simeon Reusch, Robert Stein, Anna Franckowiak (DESY), Igor Andreoni (Caltech) and Michael Coughlin (UMN) 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-200916A (Blaufuss et. al, GCN 28433) 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 g-band and r-band beginning at 2020-09-17 11:23 UTC, approximately 14.7 hours after event time. We covered 3.6 sq deg, corresponding to >99% 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 candidates by our pipeline, all lying within the 90.0% localization of the skymap.

+----------------------------------------------------------------------------------------------------+
| ZTF Name     | IAU Name   | RA (deg)    | DEC (deg)   | Filter | Mag   | MagErr | MJD      | Notes |
+----------------------------------------------------------------------------------------------------+
| ZTF20acaapwk | AT2020tno  | 108.9968806 | +14.9039078 | r      | 19.24 | 0.06   | 59109.49 |  (a)  |
| ZTF20acaapwo | AT2020tnp  | 108.5820017 | +14.4452676 | r      | 20.41 | 0.14   | 59109.49 |  (a)  |
+---------------------------------------------------------------------------------+------------------+

(a) ZTF20acaapwk/AT2020tno and ZTF20acaapwo/AT2020tnp are both unclassified transients with a last upper limit from 20 days ago. 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).