TITLE: GCN CIRCULAR NUMBER: 25199 SUBJECT: LIGO/Virgo S190728q: Optical Candidate from the Zwicky Transient Facility ZTF19abjethn DATE: 19/07/28 15:06:42 GMT FROM: Mansi M. Kasliwal at Caltech/Carnegie Mansi M. Kasliwal (Caltech), Leo P. Singer (NASA GSFC), Eric C. Bellm (UW), Michael W. Coughlin (Caltech), Daniel A. Perley (LJMU), Jesper Sollerman (OKC), Igor Andreoni (Caltech), Erik Kool (OKC), Danny Goldstein (Caltech), Rahul Biswas (OKC), Richard Walters (Caltech), Varun Bhalerao (IITB), G. C. Anupama (IIA), Shreya Anand (Caltech), Tomas Ahmuda (UMD), Brad Cenko (NASA GSFC), Sara Webb (Swinburne), Jeff Cooke (Swinburne), Kirsty Taggart (LJMU), Sudhanshu Barway (IIAP), Alessandra Corsi (TTU), Mattia Bulla (OKC), Albert Kong (Taiwan), Chris Copperwheat (LJMU) 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 gravitational wave trigger S190728q (GCN 25187) with the Palomar 48-inch telescope equipped with the 47 square degree ZTF camera (Bellm et al. 2019, Graham et al. 2019). A new tiling was automatically optimally determined and triggered using the GROWTH Target of Opportunity marshal (Coughlin et al. 2019a, Kasliwal et al. 2019b). We enclosed 64% of the probability and obtained a sequence of 300s exposures in g-band, r-band and g-band before sunrise. The images were processed through the ZTF reduction and image subtraction pipelines at IPAC to search for potential counterparts (Masci et al. 2019). We applied standard filtering criterion including rejecting stellar sources (Tachibana and Miller 2018) and moving objects and applying machine learning algorithms (Mahabal et al. 2019, Duev et al. 2019). A promising candidate with the first detection after the GW merger time and a fast intra-night rise in both filters is: ------------------------------------------------------------------------------------------------------------------- ZTF Name | RA (deg) | DEC (deg) | Filter | Mag | Magerr | Filter | Mag | Magerr -------------------------------------------------------------------------------------------------------------------- ZTF19abjethn | 326.395431 | 20.690590 | r | 19.65 | 0.11 | g | 19.82 | 0.16 -------------------------------------------------------------------------------------------------------------------- We reported this candidate to TNS and it was given the name AT2019lvs. The candidate is offset from a galaxy with SDSS phot-z estimate of 0.228 �� 0.0286. We serendipituously observed this field the previous night with ZTF and did not detect the candidate. Additional analysis and continued follow-up is in progress and encouraged. 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). Alert filtering and follow-up co-ordination is being undertaken by the GROWTH marshal system (Kasliwal et al. 2019).