TITLE:   GCN CIRCULAR
NUMBER:  25108
SUBJECT: LIGO/Virgo S190718y: ZTF pre-detection of MASTER candidate AT2019lly
DATE:    19/07/19 17:14:50 GMT
FROM:    Igor Andreoni at Caltech  <igor.andreoni@gmail.com>

Igor Andreoni (Caltech) reports on behalf of the Zwicky Transient Facility (ZTF) and Global Relay of Observatories Watching Transients Happen (GROWTH) collaborations


We searched for Zwicky Transient Facility (ZTF) detections associated with MASTER OT J203053.79+384020.1 / AT2019lly (Lipunov et al., GCN #25105) in the archive of ZTF alerts.  A point source (g=19.85 +- 0.03) is present in the ZTF reference image at the location of AT2019lly and an outburst was detected on 2019-07-15 08:00:18 UT at magnitude g=14.240 +- 0.027.

The ZTF detection occurred 3 days before the gravitational wave candidate S190718y (LIGO and Virgo Collaboration, GCN #25087, GCN #25107), excluding an association between MASTER OT J203053.79+384020.1 / AT2019lly and S190718y.  Our conclusion is in agreement with the Cataclysmic Variable scenario suggested by Lipunov et al. (GCN #25105). 


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; IIT-B, India; IIA, India; LJMU, UK; TTU 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).